VERSION 2 OF THIS "INFORMATION" > https://aip.scitation.org/doi/10.1063/1.4733534

"... Published Online: 28 June 2012 :: Commentary: JWST near-infrared detector degradation— finding the problem, fixing the problem, and moving forward ..."

 On this PAGE ( BY Susan) OVERVIEW Mikulski Archive for Space Telescopes (MAST) electromagnetic spectrum    photons  matter  Spectroscospy Key Physical Observations that Support Root Cause   ------ Path Forward (Resolution)    ------------ FIX NIRSPEC MICROSHUTTERS  -------------  THE EXCHANGE PROCESS --- Figure 82: Deployed observatory, back view: H H -------  LINKS (more) H --  Introduction of JWST spinoff technologies: TITLE:   17 November 2017   The NIRspec assembly integration and test status

The James Webb Space Telescope/observatory [JWST]
"...  "
Webb's" instruments are contained within the Integrated Science Instrument Module (ISIM) which is one of three major elements that comprise the James Webb Space Telescope Observatory flight system.
The others are the Optical Telescope Element (OTE) and the Spacecraft Element (Spacecraft Bus and Sunshield).  ..."

The Integrated Science Instrument Module ISIM Includes The Following Instruments:
1. Near-Infrared Camera, or NIRCam - provided by the University of Arizona
2. Near-Infrared Spectrograph, or NIRSpec - provided by ESA, with components provided by NASA/GSFC.
3. Mid-Infrared Instrument, or MIRI - provided by the European Consortium with the European Space Agency (ESA), and by the NASA Jet Propulsion Laboratory (JPL)
4. Fine Guidance Sensor/ Near InfraRed Imager and Slitless Spectrograph , or FGS/NIRISS - provided by the Canadian Space Agency.

JWST is the planned predecessor of the Nancy Grace Roman Telescope ( WFIRST ) .

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TITLE:   "...   NEW DETECTORS AND MICROSHUTTERS FOR NIRSPEC

25 February 2015

The past two months have seen a team of engineers engaged in the intricate activity of replacing key components of the Near InfraRed Spectrograph (NIRSpec) on the James Webb Space Telescope.

The instrument is now ready for the next series of extensive environmental tests devised to ensure that JWST's instruments can withstand the stresses and strains of launch and operation in space.

... In the summer of 2014, the James Webb Space Telescope (JWST) Integrated Science Instrument Module (ISIM), fitted with all four instruments (NIRSpecMIRI, NIRCam, and FGS/NIRISS), successfully completed cryogenic testing in a '24/7' campaign that lasted 116 days.

However, the positive outcome of this important test campaign did not mean that ISIM and the instruments were ready for integration onto JWST's telescope.

It has been known for over a year that additional work would be necessary to get some of the instruments into their final flight configuration. As a consequence, a period of a few months was allocated for these activities, immediately after the completion of the cryogenic test campaign.

In particular, NIRSpec needed to have its detectors, microshutter assembly and optical assembly cover replaced.

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Also, the NIRCAM and FGS/NIRISS teams had to exchange some components in their instruments. MIRI was the only instrument that remained integrated with the ISIM. However, MIRI's configuration was also updated by installing the flight model cooler Cold Head Assembly (CHA) and exchanging some of the cooler lines and their supports.

### NIRSPEC DETECTORS

The first generation of JWST's near-infrared detectors suffered from a design flaw that resulted in a progressive and unacceptable degradation of their performance.

"... James Webb Space Telescope (JWST) Detector Degradation Failure Review Board (DD-FRB) Executive Summary: Root Cause Determination   ..."   ( TABLE: DD-FRB Members... )

1 Context, Statement of the Problem, and Charter
The JWST science instrument "payload" contains four science instruments and a fine guidance sensor. Three of the science instruments and the fine guidance sensor utilize  [ mercury cadmium telluride] "HgCdTe detectors" that are designed to achieve high responsivity to light [photons] over the 0.6–5 micron spectrum. [ Spectral Range Of Interest ]

PHOTO SENSITIVE "devices" > https://en.wikipedia.org/wiki/Photodiode

PHOTO SENSITIVE MATERIALS : https://en.wikipedia.org/wiki/Film_speed#Digital_camera_ISO_speed_and_exposure_index ...
::  https://en.wikipedia.org/wiki/Infrared_detector

One instrument also utilizes HgCdTe detectors that are designed for the 0.6–2.5 micron spectrum.

Seven of the 5 micron cut-off detectors and 8 of the 2.5 micron cutoff detectors are required for flight as shown in Table 1. Flight model integration has begun on all of the instruments listed in Table 1. Teledyne Imaging Sensors produced all of the JWST HgCdTe detectors during the 2007-8 timeframe.

The JWST assembly and test sequence requires that the science instrument detectors have an ambient temperature shelf life of several years prior to launch and an operational life of at least 5.5 years after launch.

Instrument team test data obtained over the past year has revealed degradation of pixel operability impacting several of the 5 and 2.5 micron cut-off detectors.

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There is a strong concern that the degradation will continue with time and many of the flight arrays will be out of specification by the time of launch. The key detector degradation observed was an order of magnitude increase in the dark count rate of individual pixels to levels in the range of 0.1 to 60 electrons per pixel per second (e-/pix/sec).

FIX-NIRsPEC-01.JPG

< FIGURE 01

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Figure 1
shows an example of this increase in dark count rate for one pixel in a flight spare NIRSpec detector (S060). Other performance anomalies were also observed and are listed in Table 2 at the end of this summary. Figure 1. Example of increase in dark count rate for one pixel of a degraded detector. The blue data is for a good pixel and the red data is for the same pixel that has degraded with time.

Table 1: HgCdTe sensors in the JWST ISIM Instrument Agency Quantity: 5 um cut-off Quantity: 2.5 um cut-off NIRCam NASA 2 8 NIRSpec ESA 2 NA FGS-TF CSA 1 NA FGS-Guider CSA 2 NA Approved for Public Release, Distribution Unlimited Page 3

The JWST Project initiated a
Detector Degradation Failure Review Board (DD-FRB) to address the following items: (a) Determine the root cause of the detector degradation (b) Determine manufacturing and/or post-manufacture handling/process changes to avoid it (c) Define tests that are needed to screen-out degradation prone parts and ensure the continued integrity of flight parts (d) Define tests to determine whether the existing detectors are qualified for flight This Executive Summary addresses item (a) only. The DD-FRB will release additional Executive Summaries for items b-d as work progresses. We will write a comprehensive Final Report upon completion of the investigation. Distribution of summaries covering items (b), (c), and the Final Report will be subject to Teledyne proprietary and ITAR data restrictions. 2

2 Root Cause Determination
The DD-FRB finds that the detector degradation is caused by a design flaw in the barrier layer of the pixel interconnect structure. The flawed barrier layer design makes the detectors vulnerable to migration of indium from the indium bump interconnect into the detector structure, degrading its performance. The most obvious effect is the formation of an indium (In) gold (Au) intermetallic that is highly visible in Scanning Electron Microscopy (SEM) images taken during destructive physical analysis. The electrical data of degraded pixels reveal curved, “RC” shaped dark ramps that are indicative of parasitic capacitance, reactance, and shunting in the HgCdTe side of the interconnect. Typically a few hundred seconds after reset, true leakage currents become dominant. These effects cause pixels to fail to meet operability requirements.

< FIGURE 2   TOP & Page Contents list

Figure 2a shows a cross-section of the pixel contact structure design. In this sensor design, each HgCdTe pixel is connected via the In bump to a source-follower amplifier in a silicon Read-Out Integrated Circuit (ROIC). The critically important barrier layer is intended to prevent In bump material from reacting with the Au pad and Au contact material such that it can not diffuse into the HgCdTe detector material. Figures 2b and 2c show crosssectional micrographs obtained with SEM of a non-degraded pixel from a 2.5 micron NIRCam detector array (C105) and a degraded pixel from a 5 micron NIRCam detector array (C094). The cross-section of the pixel structure was generated by destructive physical analysis (DPA) using a focused ion beam (FIB) to cut through a line of pixels in the array. Figure 2c shows the formation of an AuIn2 intermetallic as well as a crack in the left corner of the pixel contact structure propagating into the HgCdTe detector. The Approved for Public Release, Distribution Unlimited Page 4 intermetallic expands upon formation and most likely created a pocket of stress in the pixel. Figure 2. a) Pixel contact structure; b) Scanning Electron Microscope (SEM) image of a nondegraded pixel in NIRCam detector C105; c) SEM of degraded pixel in NIRCam detector C094

FIX-NIRsPEC-03.JPG

< FIGURE 3  TOP & Page Contents list

Figure 3(a) shows a diagram depicting failure of the barrier layer. Poor sidewall coverage of the layers over the step of the passivation layer or porosity of the barrier layer can allow In to inter-diffuse with the Au contact and Au pad metals to create In-Au intermetallics. Figure 3(b) illustrates some potential degradation mechanisms; the intermetallic expansion may cause strain and lattice dislocation damage to the HgCdTe and/or enable In to diffuse into the p+ HgCdTe of the implanted junction layer. Apart from production of charge traps in the semiconductor band gap, dislocation damage can also allow In or Au to diffuse more rapidly into the HgCdTe resulting in a dark current performance degradation rate that can be non-linear and difficult to reliably estimate. Approved for Public Release, Distribution Unlimited Page 5 Figure 3. (a) Inadequate barrier layer coverage; (b) Potential degradation mechanisms

FIX-NIRsPEC-04.JPG

< FIGURE 4  TOP & Page Contents list

Figure 4 shows the flow diagram of the degradation mechanisms. Figure 4. Degradation process in a pixel due to inadequate barrier layer A degraded detector pixel can be modeled by an electrical circuit, which produces an integration ramp signal with an “RC”-like curvature early in the ramp (see Fig. 1). More extensive damage or indium diffusion will produce additional leakage currents through the photodiode. Although this circuit model approximately captures the essential behavior of degraded pixels (an “RC” at early times and leakage at later times), the actual circuit elements are far from ideal. Approved for Public Release, Distribution Unlimited Page 6

FIX-NIRsPEC-05.JPG

FIGURE 5 TOP & Page Contents list

Figure 5. This electrical circuit model of a degraded pixel accounts for the “RC”-like curvature of dark ramps (see Fig. 1). The red-highlighted components form in the HgCdTe immediately above the failed barrier layer. These cause the “RC”-like shape. This simple model does not attempt to explain the degradation in the photodiode that causes enhanced leakage current. Formation of the In-Au intermetallic was confirmed by Energy Dispersive x-ray Spectroscopy (EDS) to provide a direct measure of the elemental composition.

FIX-NIRsPEC-06.JPG

FIGURE 6  TOP & Page Contents list

Figure 6a shows a SEM image of a corner of another detector pixel in detector array C094 with a corresponding elemental map for Au, In, and the barrier layer in Figure 6b. For these samples, the cross-section was prepared by cutting through the sample with a wire saw followed by mechanical polishing. The data show the formation of the In-Au intermetallic with a break in the barrier layer at the sidewall of the contact opening. Figure 6. a) SEM of a pixel corner in NIRCam detector C094; b) X-ray elemental analysis (EDS) of the same area showing that Au and In have interdiffused to form an intermetallic compound (AuIn2) due to failure of the barrier layer Approved for Public Release, Distribution Unlimited Page 7 Additional EDS data was taken on another pixel in detector C094 as well as the Process Evaluation Chip (PEC) for C094.

FIX-NIRsPEC-07.JPG

< FIGURE 7   TOP & Page Contents list

Figure 7a shows the SEM and the x-ray analysis area (red box) from the PEC and Figure 7b shows the x-ray spectrum. Quantitative analysis of the weight percentage of the volume measured shows that the In-Au compound is AuIn2. Figure 7. a) X-ray analysis (EDS) of red box area in SEM image demonstrates the formation of an In-Au intermetallic (AuIn2)

FIX-NIRsPEC-08.JPG

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Figure 8 shows a SEM image and a backscatter electron image of a cross-section of a pixel in detector array C094. Combined with EDS analysis on the different regions, the results show that there is interdiffusion of both In and Au past the barrier layer with the formation of AuIn2 and AuIn intermetallics that expand in volume. Figure 8: SEM and Backscatter Secondary Electron (BSE) image of detector pixel in C094 Approved for Public Release, Distribution Unlimited Page 8

3 Key Physical Observations that Support Root Cause

To avoid focusing on a single aspect of the observed degradation, the
Detector Degradation Failure Review Board (DD-FRB) developed a list of key observations that any root cause analysis would have to explain.

This list began at 14 items and has since grown to 25 items, with each new observation adding or reinforcing the list (Table 2).

There are some common elements for all explanations:

- SOURCE: https://en.wikipedia.org/wiki/Schottky_barrier: "...  When a metal is put in direct contact with a semiconductor, a so called "Schottky barrier" can be formed, leading to a "rectifying behavior" of the electrical contact.

This happens both when the semiconductor is n-type and its work function is smaller than the work function of the metal, and when the semiconductor is p-type and the opposite relation between work functions holds.[3]

At the basis of the description of the Schottky barrier formation through the band diagram formalism, there are three main assumptions:[4]

1. The contact between the metal and the semiconductor must be intimate and without the presence of any other material layer (such as an oxide).
2. No interdiffusion of the metal and the semiconductor is taken into account.
3. There are no impurities at the interface between the two materials.

To a first approximation, the barrier between a metal and a semiconductor is predicted by the Schottky–Mott rule ...   ..."

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<  wHERE R = resistor and C = capacitor "RC circuit element"  ::    1) formation for an RC circuit element ( https://en.wikipedia.org/wiki/RC_circuit, most likely an n/p or Schottky barrier that completely intercepts the circuit after the contact; and 2) defects which increase the detector junction leakage current.

Common Elements

These "common elements" are likely caused by damage (dislocations, displaced ions) induced by the intermetallic formation itself due to an inadequate barrier layer.

The damage is further increased in its effect by enhanced diffusion of indium, now present at or in the HgCdTe from the proximate In-Au intermetallic.

Beyond this, every diode will have its own story, and there are millions of them in a detector array.

Further details of the physical mechanisms by which these various observations can arise will be provided in the final report of the DD-FRB.

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(Table 2) Key Physical Observations

DEFINITION:
"Warm pixel" ( "degraded pixel" ):
A pixel with a dark count rate 0.1 < rate < 60 e-/sec, where the count rate is measured using a linear 2-parameter fit to the up-the-ramp samples spanning 1000sec.

"Degraded detector" :  A detector that exhibits a statistically significant increase in the number of "warm pixels".

OBSERVATIONS:
1 The number of warm pixels increases with time in both the 2.5µm and 5µm cutoff detectors that show degradation.
2 In degraded detectors, some warm pixels get better at the same time as a larger number get worse.
3 The rate of degradation of the detectors varies from part to part and is not necessarily linear with time.
4 Although clustered, the new warm pixels do not form a contiguous group.
5 The spatial distribution of the warm pixels appears to be similar for all the NIRCam 5µm detectors.
In addition, there are similarities in the spatial distribution of warm pixels among the affected NIRSpec
detectors, but the distributions are different from those of the NIRCam parts. However, there is at least
one small area near the edge of the detectors with a higher density of warm pixels that is common to
both the NIRCam and NIRSpec parts.
6 No warm pixels have been observed in the reference pixels of any degraded detector, even though new
warm pixels are seen in the immediately adjacent regions of some degraded detectors.
7 Areas with an increased density of warm pixels also show a small decrease in flat field response relative to “good” regions.
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8 While some new warm pixels may be hot pixel neighbors, most new warm pixels are not related to hot pixels.
9 The regions with high densities of new warm pixels are preferentially found near the edges of the
detectors rather than at the centers. These regions are also where the stress-induced curvature of the
detectors is at a minimum.
10 A 12hr bake at 50C in a dry nitrogen environment resulted in an increased number of warm pixels,
indicating an increased rate of formation while at elevated temperature in one of the degraded NIRCam
5µm detectors (C094).
11 The new warm pixels that appeared after the 12hr-50C bake of C094 have a similar spatial distribution
and electrical properties (dark count rates, ramp shapes) as the pixels that had become warm during
ambient storage.
12 The character of the degradation of some WFC3 detectors at their operating temperature of 145K is
very similar to that of the JWST detectors at their ~40K operating temperature, despite the differences
in the long wavelength cut-off (1.7µm vs. 5µm), processing details, and subsequent storage and
handling. It is possible that the same physical processes are at work in both instances, while the details may differ.
13 Eight of the eleven tested 5µm detectors show degradation. However, only two out of thirteen 2.5µm
stored in ambient conditions for ~1 year less than the other JWST detectors.
14 The slope of the dark signal ramps for most (80-85%) new warm pixels shows statistically significant
curvature (RC-like behavior).
15 For a large fraction of the new warm pixels in NIRSpec detector S060 (5µm), the dark count rate is
approximately independent of temperature at low temperatures (T < 80K). However, at higher
temperatures (80-100K), a dependence of the dark count rate on temperature is observed, indicating
that a different mechanism is dominant in each of the two temperature regimes.
16 A change in temperature from 37.5K to 41K can result in some apparently good pixels becoming bad for S060.
17 Under the assumption of normal gain, the noise in some, or all, new warm pixels, while higher than for
good pixels, is lower than expected from shot noise associated with the measured signal.
18 For S060, the asymptotic value of the dark count rate is consistent with the noise enhancement in
degraded pixels. For this detector, the degradation manifests as a) the appearance of an “RC behavior”
shortly after reset, and b) real leakage current that dominates the “RC” after a few hundred seconds.
19 The two 2.5µm detectors (C038 & C041) that have exhibited an increase in warm pixels show an even
larger fraction of warm pixels (relative to the mean) when measured at higher temperatures (90K for
C038 and 85K for C041).
20 The region of C038 that exhibits an increased density of warm pixels (at both 39.5K and 90K) also
shows a decrease in well depth.
21 Most of the warm pixels in C041 become good when the detector is cooled to 23.4K.
22 Multiple labs have observed the same phenomena in different test sets.
23 The Scanning Electron Microscope (SEM) and Energy Dispersive x-ray Spectroscopy (EDS) analysis of
C094 shows that an In-Au intermetallic has formed in all 15 pixels examined to date. These include
examples of both degraded and non-degraded pixels. SEM analysis of the Process Evaluation Chip
(PEC) associated with this detector also shows the In-Au intermetallic in all pixels examined. The
major intermetallic formed is AuIn2. AuIn is also formed next to the AuIn2 where there was originally Au.
24 SEM analysis of the PEC associated with the good (i.e. showing no degradation) 2.5µm detector C105
shows no indication of In-Au intermetallic formation.
25 SEM analysis of the PEC associated with the 5µm detector S042 shows that an In-Au intermetallic has
formed, although the intermetallic volume appears to be less than in C094. This detector has shown no
degradation as of the most recent testing in Jan. 2010

4 Path Forward
Summary findings for charter items (b-d) above are in progress.           TOP & Page Contents list

At this juncture, the DDFRB believes that a specific and practical method for fabrication of a fully effective barrier layer is available at Teledyne Imaging Sensors to eliminate the above design flaw in newly manufactured detectors.

This design was developed for a higher background application than space astrophysics, and further testing is required to show that it can meet JWST performance requirements.

The Board anticipates recommendation of screening and accelerated life tests to verify the long-term effectiveness of this solution.

Finally, the Board anticipates recommendation of specific tests to assess the flight worthiness of JWST HgCdTe detectors that do not currently exhibit out-of-spec performance.

Once the origin of the problem had been identified and, more importantly, a solution found and verified, new detectors were manufactured for all three JWST near-infrared instruments (NIRSpec, NIRCam, and FGS/NIRISS).

At the end of 2013, the NIRSpec team selected two flight-quality detectors out of the pool of new detectors. These detectors were then integrated in a new focal-plane assembly (FPA). Fortunately, the performance of this new FPA is equivalent to or even better than that with the 'old design' detectors at the beginning of their life (i.e., before they started to degrade).

### FIX NIRSPEC MICROSHUTTERS - FIX

 NASA engineers inspect the new microshutter assembly. Credit: NASA/Chris Gunn

One of the defining, and pioneering, features of NIRSpec is its ability to record the spectra of many (more than 100) objects at the same time. This multi-object spectrography capability is made possible by the use of a Microshutter Assembly (MSA) consisting of just under a quarter of a million individually controlled microshutters, arranged into four quadrants each containing an array of 365×175 microshutters. These microshutters are a new technology that was developed for JWST and is provided by NASA.

At the end of 2012, it was discovered that, after acoustic testing of the instrument (replicating the conditions during launch), several thousand of the individual microshutters had become inoperable and could not be opened. This susceptibility to acoustic noise was not expected and had gone undetected because of the difficulty of reproducing the environment to which the microshutters are actually subjected in this instrument. As a result of this problem, the performance of the microshutters in NIRSpec was strongly degraded.

An Anomaly Review Board was established at NASA and many additional acoustic tests on the microshutters were performed in NASA's acoustic test facility in order to find the cause of the degradation. Tests were conducted both at array level, using a special test fixture, and at instrument level, using the NIRSpec Engineering Test Unit (ETU). The latter provided the most realistic test environment for the MSA. These various tests provided a wealth of information that helped NASA to identify the cause of the 'failed closed' shutters issue.

The design of the arrays was slightly altered and production of new arrays was started. Despite great efforts, NASA did not succeed in building all four necessary 'new design' arrays; this was related to yield issues in the manufacturing process. However, by successfully re-screening 'original design' microshutters that had already been produced, NASA engineers were able to identify a small pool of robust, flight-quality arrays. At the end of the process, the new MSA contains three 'original design' arrays and one 'new design' array. In addition to most arrays being pre-screened at array level, the complete new MSA flight model was acoustically tested in the NIRSpec ETU before it was installed in the flight version of NIRSpec.

In parallel, the lifetime of the mechanism used to move a magnet over the arrays to open the microshutters was greatly improved by the use of a 'super molybdenum disulphide (MoS2)' coating on some of its components. In addition, the material used for the rollers was changed to a harder alloy that, together with the newly applied coating, will minimise wear and potential electrical short-circuits in the MSA.

The overall performance of this new flight-spare MSA is much better than that of the flight MSA previously installed in NIRSpec. As a consequence, it was decided to use the opportunity of the detector exchange to also replace the current MSA with the new spare component.

### NIRSPEC OPTICAL ASSEMBLY MLI COVER

As part of this programme of hardware upgrades it had also been decided to replace the protective cover (multi-layer insulation, or MLI) for the NIRSpec optical assembly because it was showing some signs of wear. While this was not a serious problem, it was thought better to take the opportunity to replace the cover with one of exactly the same design as the original, to be sure that all elements of the instrument were in the best possible condition.

### THE EXCHANGE PROCESS

 Replacement of NIRSpec focal plane assembly. Credit: NASA/Chris Gunn

As the process of exchanging NIRSpec's MSA and FPA required the instrument cover to be opened, extreme care needed to be taken not to contaminate NIRSpec's interior. This concern was driven by the known extreme susceptibility of the MSA to particulate contamination, especially small polyester fibres that are found in many items, such as cleanroom garments, lacing cord and clean room wipes. The problem is that, during acoustic testing, these small fibres can enter the microshutters and prevent them from fully closing. These trapped fibres then lead to (partially) failed open shutters in the MSA that seriously affect the science results as spurious light can enter the NIRSpec spectrometer optics at these locations and project false spectra onto the detector.

In order to prevent such contamination, ESA and Airbus Defence and Space, supported by their NASA colleagues, implemented a very strict cleanliness regime during the exchange activities. As the NASA Spacecraft Systems Development and Integration Facility (SSDIF) cleanroom is 'only' certified to ISO Class 7 (Class 10 000) and the NIRSpec instrument was built in Germany under ISO class 5 conditions (Class 100), extra measures needed to be taken. The team managed to get the very cleanest location inside the NASA SSDIF cleanroom, this being the north-western corner, directly in front of the High-Efficiency Particulate Arrestance (HEPA) filter wall. The area was cordoned off and access was greatly restricted.

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During the activities, all team members had to wear special cleanroom garments that had previously been demonstrated in Germany to provide better cleanliness levels than that which can be achieved with standard cleanroom garb. This clothing included special garments under the normal cleanroom suits, special double mouth-masks and extra-long white cleanroom gloves. Implementing this new dressing procedure at NASA required a lot of coordination. The support offered by NASA in realising this was much appreciated.

In any cleanroom environment, humans remain the main source of contamination, even if extreme measures are taken to minimise this. For this reason, the NIRSpec exchange area needed to be 'screened and cleaned' on a daily basis. Every other day, the team had a dedicated early morning time slot where the SSDIF lights were switched off and, by using special 'Crime-lite' light sources, the complete area, scaffolding and instrument were screened for the presence of particulates, particularly fibres. During these time slots, the area and hardware were vacuum cleaned and wiped with special tissues. The recorded data, taken continuously throughout the two-month exchange period, confirmed that the team did an excellent job. Very high levels of cleanliness were maintained, occasionally rivalling what was achieved in Germany.

After demonstrating that the exchange area was very clean, the first task was to remove the NIRSpec instrument cover to gain access to the detectors and MSA. All the optical surfaces were then protected to prevent damage or contamination. The old detector system was removed and its replacement was installed, aligned and connected, after which a functional test was performed.

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The second stage of the exchange process involved removing the existing [Micro Shutter Array (MSA )and installing, connecting and aligning the new one.

Again, a functional test was performed, followed by an inspection to ensure cleanliness before installing the cover.

The exchange process was executed successfully during December 2014 and January 2015 by a team that included engineers from Airbus Defence and Space (the prime contractor for NIRSpec), ESA and NASA. NIRSpec is now ready to be installed on ISIM again. ... Later this year, the instruments on ISIM will begin an extensive programme of environmental tests to reproduce the conditions they will have to withstand during the launch and once they are in space. These tests will include vibration and acoustic testing as well as a third cryogenic test campaign. Throughout the course of the testing, the health of the MSA will be regularly checked by high magnification optical inspections with a specialised camera set-up.  ..."

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"Spectroscopy" is the study of the interaction between matter and electromagnetic radiation as a function of the wavelength or frequency of the radiation.

electromagnetic spectrum :  SOURCE: https://en.wikipedia.org/wiki/Electromagnetic_spectrum

Class
(Band names )
Wave-
length

{\displaystyle \lambda }
Freq-
uency

{\displaystyle f}
Energy per
photon

{\displaystyle E}
Ionizing
γ Gamma rays pm 300 EHz 1.24 MeV
10 pm 30 EHz 124 keV
HX Hard X-rays
100 pm 3 EHz 12.4 keV
SX Soft X-rays
nm 300 PHz 1.24 keV
10 nm 30 PHz 124 eV
EUV Extreme
ultraviolet
100 nm 3 PHz 12.4 eV
NUV Near ultraviolet,
visible
μm 300 THz 1.24 eV
NIR Near infrared
10 μm 30 THz 124 meV
MIR Mid infrared
100 μm 3 THz 12.4 meV
FIR Far infrared
mm 300 GHz 1.24 meV
Micro-
waves

and

waves
EHF Extremely high
frequency
cm 30 GHz 124 μeV
SHF Super high
frequency
dm 3 GHz 12.4 μeV
UHF Ultra high
frequency
m 300 MHz 1.24 μeV
VHF Very high
frequency
10 m 30 MHz 124 neV
HF High
frequency
100 m 3 MHz 12.4 neV
MF Medium
frequency
km 300 kHz 1.24 neV
LF Low
frequency
10 km 30 kHz 124 peV
VLF Very low
frequency
100 km 3 kHz 12.4 peV
ULF Ultra low
frequency
Mm 300 Hz 1.24 peV
SLF Super low
frequency
10 Mm 30 Hz 124 feV
ELF Extremely low
frequency
100 Mm 3 Hz 12.4 feV
Sources: File:Light spectrum.svg[1][2][3]

The electromagnetic spectrum is the range of frequencies (the spectrum) of electromagnetic radiation and their respective wavelengths and photon energies.

The electromagnetic spectrum covers electromagnetic waves with frequencies ranging from below one hertz to above 1025 hertz, corresponding to wavelengths from thousands of kilometers down to a fraction of the size of an atomic nucleus.

This frequency range is divided into separate bands, and the electromagnetic waves within each frequency band are called by different names; beginning at the low frequency (long wavelength) end of the spectrum these are: radio wavesmicrowavesinfraredvisible lightultravioletX-rays, and gamma rays at the high-frequency (short wavelength) end.

The electromagnetic waves in each of these bands have different characteristics, such as how they are produced, how they interact with matter, and their practical applications. There is no known limit for long wavelengths, while it is thought that the short wavelength limit is in the vicinity of the Planck length.[4] Extreme ultraviolet, soft X-rays, hard X-rays and gamma rays are classified as ionizing radiation as their photons have enough energy to ionize atoms, causing chemical reactions.

In most of the frequency bands above, a technique called spectroscopy can be used to physically separate waves of different frequencies, producing a spectrum showing the constituent frequencies. Spectroscopy is used to study the interactions of electromagnetic waves with matter.[5] Other technological uses are described under electromagnetic radiation.

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FIGURE:  A linearly polarized sinusoidal electromagnetic wave, propagating in the direction +z through a homogeneous, isotropic, dissipationless medium, such as vacuum.
The electric field (blue arrows) oscillates in the ±x-direction, and the orthogonal magnetic field (red arrows) oscillates in phase with the electric field, but in the ±y-direction.

In physicselectromagnetic radiation (EMR) consists of waves of the electromagnetic (EM) field, propagating through space, carrying electromagnetic radiant energy.[1]
It includes radio wavesmicrowavesinfrared(visible) lightultravioletX-rays, and gamma rays. All of these waves form part of the electromagnetic spectrum.[2]

Classically, electromagnetic radiation consists of electromagnetic waves, which are synchronized oscillations of electric and magnetic fields. Electromagnetic radiation or electromagnetic waves are created due to periodic change of electric or magnetic field. Depending on how this periodic change occurs and the power generated, different wavelengths of electromagnetic spectrum are produced. In a vacuum, electromagnetic waves travel at the speed of light, commonly denoted c. In homogeneous, isotropic media, the oscillations of the two fields are perpendicular to each other and perpendicular to the direction of energy and wave propagation, forming a transverse wave. The wavefront of electromagnetic waves emitted from a point source (such as a light bulb) is a sphere. The position of an electromagnetic wave within the electromagnetic spectrum can be characterized by either its frequency of oscillation or its wavelength. Electromagnetic waves of different frequency are called by different names since they have different sources and effects on matter. In order of increasing frequency and decreasing wavelength these are: radio waves, microwaves, infrared radiation, visible light, ultraviolet radiation, X-rays and gamma rays.[3]

en.wikipedia.org/wiki/Charged_particle  >  https://en.wikipedia.org/wiki/Charged_particle

Electromagnetic waves are emitted by electrically charged particles undergoing acceleration, and these waves can subsequently interact with other charged particles, exerting force on them. EM waves carry energy, momentum and angular momentum away from their source particle and can impart those quantities to matter with which they interact. Electromagnetic radiation is associated with those EM waves that are free to propagate themselves ("radiate") without the continuing influence of the moving charges that produced them, because they have achieved sufficient distance from those charges. Thus, EMR is sometimes referred to as the far field. In this language, the near field refers to EM fields near the charges and current that directly produced them, specifically electromagnetic induction and electrostatic induction phenomena.

In quantum mechanics, an alternate way of viewing EMR is that it consists of photons, uncharged elementary particles with zero rest mass which are the quanta of the electromagnetic field, responsible for all electromagnetic interactions.[6] Quantum electrodynamics is the theory of how EMR interacts with matter on an atomic level.[7] Quantum effects provide additional sources of EMR, such as the transition of electrons to lower energy levels in an atom and black-body radiation.[8] The energy of an individual photon is quantized and is greater for photons of higher frequency. This relationship is given by Planck's equation E = hf, where E is the energy per photon, f is the frequency of the photon, and h is Planck's constant. A single gamma ray photon, for example, might carry ~100,000 times the energy of a single photon of visible light.

The effects of EMR upon chemical compounds and biological organisms depend both upon the radiation's power and its frequency. EMR of visible or lower frequencies (i.e., visible light, infrared, microwaves, and radio waves) is called non-ionizing radiation, because its photons do not individually have enough energy to ionize atoms or molecules or break chemical bonds. The effects of these radiations on chemical systems and living tissue are caused primarily by heating effects from the combined energy transfer of many photons.

In contrast, high frequency ultraviolet, X-rays and gamma rays are called ionizing radiation, since individual photons of such high frequency have enough energy to ionize molecules or break chemical bonds. These radiations have the ability to cause chemical reactions and damage living cells beyond that resulting from simple heating, and can be a health hazard.

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SOURCE: http://umop.net/spectra/

hhhhhh Atomic Table Elements

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photons :  https://en.wikipedia.org/wiki/Photon

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: SOURCE: https://en.wikipedia.org/wiki/Spectroscopy

"...  Introduction

Spectroscopy is a branch of science concerned with the spectra [shown above ] of electromagnetic radiation as a function of its wavelength or frequency measured by spectrographic equipment, and other techniques, in order to obtain information concerning the structure and properties of matter.[7]

Specifically - HOW "PHOTONS" INTERACT WITH "MATTER".

Every known element ( periodic table of elements ) - has a "spectral fingerprint". SOURCE:  https://en.wikipedia.org/wiki/Emission_spectrum "...

The emission spectrum can be used to determine the composition of a material, since it is different for each element of the periodic table. One example is astronomical spectroscopy: identifying the composition of stars by analysing the received light. The emission spectrum characteristics of some elements are plainly visible to the naked eye when these elements are heated. For example, when platinum wire is dipped into a sodium nitrate solution and then inserted into a flame, the sodium atoms emit an amber yellow color. Similarly, when indium is inserted into a flame, the flame becomes blue. These definite characteristics allow elements to be identified by their atomic emission spectrum. Not all emitted lights are perceptible to the naked eye, as the spectrum also includes ultraviolet rays and infrared radiation. An emission spectrum is formed when an excited gas is viewed directly through a spectroscope.  ..."

Most spectroscopic analysis - in a laboratory. (  http://photobiology.info/Nonell_Viappiani.html  ) It starts with a "sample" to be analyzed. Then a "light source" is chosen from any desired range of the light spectrum.  The light goes through the sample to a  diffraction grating instrument and is captured by a photodiode.

"spectroscopic analysis" "video" > "Analysis of Iron in Spinach" >  https://www.youtube.com/watch?v=RUw5LV4p2Rs

For astronomical purposes,  telescopes equipped with light dispersion devices. (  https://en.wikipedia.org/wiki/Telescope  )

CONSIDER, IF YOU ARE IN A PLACE WHERE "LIGHT" IS ABSENT. THERE IS NO COLOR. No "black", "white" or "grey".  As humans - we experience "sight" - as a "reflection of "photons" - from a surface. The photons ( invisible, massless "particles" AND force carriers of the Electromagnetic spectrum) enter our eyes ... and "excite" special receptors ( "rods" and "cones" : https://askabiologist.asu.edu/rods-and-cones ) - which our brain translates to "visible" phenomenon.

Under the studies of James Clerk Maxwell [Spectroscopy] came to include the entire electromagnetic spectrum.[9]

Although color is involved in spectroscopy, it is not equated with the color of elements or objects which involve the absorption and reflection of certain electromagnetic waves to give objects a sense of color to our eyes.

Rather, spectroscopy involves the
splitting of light by a prism, diffraction grating, or similar instrument, to give off a particular discrete line pattern called a “spectrum” unique to each different type of element.

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... the spectral lines of the elements :

"...   Spectral lines of chemical elements

... The phrase "spectral lines", when not qualified, usually refers to lines having wavelengths in the visible band of the full electromagnetic spectrum. Many spectral lines occur at wavelengths outside this range. At shorter wavelengths, which correspond to higher energies, ultraviolet spectral lines include the Lyman series of hydrogen. At the much shorter wavelengths of X-rays, the lines are known as characteristic X-rays because they remain largely unchanged for a given chemical element, independent of their chemical environment. Longer wavelengths correspond to lower energies, where the infrared spectral lines include the Paschen series of hydrogen. At even longer wavelengths, the radio spectrum includes the 21-cm line used to detect neutral hydrogen throughout the cosmos.

### Visible light

For each element, the following table shows the spectral lines which appear in the visible spectrum at about 400-700 nm.   TOP & Page Contents list

..."

Spectral lines of the chemical elements

Element Z Symbol Spectral lines
hydrogen 1 H
helium 2 He
lithium 3 Li
beryllium 4 Be
boron 5 B
carbon 6 C
nitrogen 7 N
oxygen 8 O
fluorine 9 F
neon 10 Ne
sodium 11 Na
 magnesium 12 Mg aluminium 13 Al silicon 14 Si phosphorus 15 P sulfur 16 S chlorine 17 Cl argon 18 Ar potassium 19 K calcium 20 Ca scandium 21 Sc
 titanium 22 Ti vanadium 23 V chromium 24 Cr manganese 25 Mn iron 26 Fe cobalt 27 Co nickel 28 Ni copper 29 Cu zinc 30 Zn gallium 31 Ga
 germanium 32 Ge arsenic 33 As selenium 34 Se bromine 35 Br krypton 36 Kr rubidium 37 Rb strontium 38 Sr yttrium 39 Y zirconium 40 Zr niobium 41 Nb
 molybdenum 42 Mo technetium 43 Tc ruthenium 44 Ru rhodium 45 Rh palladium 46 Pd silver 47 Ag cadmium 48 Cd indium 49 In tin 50 Sn antimony 51 Sb tellurium 52 Te
 iodine 53 I xenon 54 Xe caesium 55 Cs barium 56 Ba lanthanum 57 La cerium 58 Ce praseodymium 59 Pr neodymium 60 Nd promethium 61 Pm samarium 62 Sm europium 63 Eu
 gadolinium 64 Gd terbium 65 Tb dysprosium 66 Dy holmium 67 Ho erbium 68 Er thulium 69 Tm ytterbium 70 Yb lutetium 71 Lu hafnium 72 Hf tantalum 73 Ta
 tungsten 74 W rhenium 75 Re osmium 76 Os iridium 77 Ir platinum 78 Pt gold 79 Au thallium 81 Tl lead 82 Pb bismuth 83 Bi polonium 84 Po

85?
h

87?
( https://www.webqc.org/periodictable-Francium-Fr.html )

FIGURE:  The difference between prism AND diffraction grating instruments - to "split" incoming light to line patterns called "sprectrum":

Figure 1: While dispersion prisms separate wavelengths through refraction (top), diffraction gratings instead separate wavelengths through diffraction because of their surface structure (bottom).  ..."

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METHODs:
Most elements are first put into a gaseous phase to allow the spectra to be examined - although today other methods can be used on different phases.

Each element - that is "diffracted" by a prism-like instrument displays either an absorption spectrum or an emission spectrum depending upon whether the element is being cooled or heated.[10]

Until recently all spectroscopy involved the study of line spectra and most spectroscopy still does.[11] Vibrational spectroscopy is the branch of spectroscopy that studies the spectra.[12]

However, the latest developments in spectroscopy can sometimes dispense with the dispersion technique. In biochemical spectroscopy, information can be gathered about biological tissue by absorption and light scattering techniques. Light scattering spectroscopy is a type of reflectance spectroscopy that determines tissue structures by examining elastic scattering.[13]   In such a case, it is the tissue that acts as a diffraction or dispersion mechanism.

Spectroscopic studies were central to the development of quantum mechanics; because, the first useful atomic models described the spectra of Hydrogen which models include the Bohr model, the Schrödinger equation, and Matrix mechanics which all can produce the spectral lines of Hydrogen, therefore, providing the basis for discrete quantum jumps to match the discrete hydrogen spectrum.

Also, Max Planck's explanation of blackbody radiation involved spectroscopy because he was comparing the wavelength of light using a photometer to the temperature of a Black Body.[14] Spectroscopy is used in physical and analytical chemistry because atoms and molecules have unique spectra. As a result, these spectra can be used to detect, identify and quantify information about the atoms and molecules. Spectroscopy is also used in astronomy and remote sensing on Earth. Most research telescopes have spectrographs. The measured spectra are used to determine the chemical composition and physical properties of astronomical objects (such as their temperature, density of elements in a star, velocityblack holes and more).[15] An important use for spectroscopy is in biochemistry. Molecular samples may be analyzed for species identification and energy content.[16]   ..."

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... The Near InfraRed Spectrograph (NIRSpec) is one of four instruments on The James Webb Space Telescope (JWST).

NIRSpec is a multi-object spectrograph

"...  A spectrograph is an instrument that separates incoming light by its wavelength or frequency and records the resulting spectrum in some kind of multichannel detector, like a photographic plate. Many astronomical observations use telescopes as, essentially, spectrographs.   "

The JWST is capable of observing more than 100 astronomical objects simultaneously. - AS INDIVIDUAL, CITABLE AND ADDRESSABLE OBJECTS.

Consider - the difference between a photo (of your entire school population - squeezed onto a school parking lot) - AND, a photo - of just your 4th grade class.

...[ The NIRSpec instrument]  will support JWST's four main science themes by providing low, medium and high-resolution spectroscopic observations.

NIRSpec was built by European industry to ESA's specifications and managed by the ESA JWST Project at ESTEC, the Netherlands. The prime contractor is Airbus Defence and Space GmbH (formerly EADS Astrium GmbH) in Ottobrunn, Germany. The NIRSpec detector and microshutter array (MSA) subsystems are provided by NASA's Goddard Space Flight Center.  ..."

"... The James Webb Space Telescope (JWST) is a space telescope ...
The telescope is named after James E. Webb, who ... played an integral role in the Apollo program.[10][11] It is intended to succeed the Hubble Space Telescope as NASA's flagship mission in astrophysics.
[ It [JWST] is the planned predecessor of the Nancy Grace Roman Telescope. [ NASA ] ]

... JWST was launched on 25 December 2021 on Ariane flight VA256.
... It is designed to provide improved infrared resolution and sensitivity over Hubble, viewing objects up to 100 times fainter than the faintest objects detectable by Hubble.[14]

This will enable a broad range of investigations across the fields of
astronomy  and cosmology, such as observations ... of some of the oldest and most distant objects and events in the Universe (including the first stars and the formation of the first galaxies), and detailed atmospheric characterization of potentially habitable exoplanets
...  NIRSpec (Near InfraRed Spectrograph) will ... perform spectroscopy ...
... It was built by the European Space Agency at ESTEC in Noordwijk, Netherlands. The leading development team includes members from Airbus Defence and Space, Ottobrunn and Friedrichshafen, Germany, and the Goddard Space Flight Center; with Pierre Ferruit (École normale supérieure de Lyon) as NIRSpec project scientist.
The NIRSpec design provides three observing modes:

1. a low-resolution mode using a prism, [ "JWST" "low-resolution" mode using a "prism" :
( https://jwst-docs.stsci.edu/jwst-near-infrared-spectrograph/nirspec-instrumentation/nirspec-dispersers-and-filters )

2. an R~1000 multi-object mode, [ "R~1000 multi-object mode" "NIRSpec"   ]

and 3. an R~2700 integral field unit or long-slit spectroscopy mode.[78]

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[ SOURCE: https://arquivo.pt/wayback/20090715053036/http://www.stsci.edu/jwst/instruments/detectors/
"... Rockwell Scientific H2RG will be used in NIRCam , NIRSpec , and the Guider system .

"...  The 2048×2048 pixel HAWAII-2RG™ (H2RG) is the state-of-the-art readout integrated circuit for visible and infrared astronomy in ground-based and space telescope applications.
(  https://en.wikipedia.org/wiki/Readout_integrated_circuit ... )  ... [  https://en.wikipedia.org/wiki/Sensor  ...  ::  https://en.wikipedia.org/wiki/Detector ::
"Sensors" redirects here. "Detector" redirects here. ]

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DESCRIPTION

The 2048×2048 pixel HAWAII-2RG™ (H2RG) is the state-of-the-art readout integrated circuit for visible and infrared astronomy in ground-based and space telescope applications.

Large (2048×2048 pixel) array with 18 μm pixel pitch.
Compatible with Teledyne Imaging Sensors (TIS) HgCdTe infrared (IR) and silicon PIN HyViSI™ visible detectors, providing sensing of any spectral band from soft X-ray to 5.5 μm.
Substrate-removed HgCdTe enhances the J-band QE, enables response into the visible spectrum (70% QE down to 400nm) and eliminates fluorescence from cosmic radiation absorbed in the sub
Reference rows and columns for common-mode noise rejection.
Guide window output – windowing with simultaneous science date acquisition of full array. Programmable window which may be read out at up to 5 MHz pixel rate for guiding. Readout is designed to allow interleaved readout of the guide window and the full frame science data.
Selectable number of outputs (1, 4, or 32) and user-selectable scan directions provide complete flexibility in data acquisition.
Built with modularity in mind – the array is 4-side-buttable to allow assembly of large mosaics of 2048×2048 H2RG modules, such as TIS’ 4096×4096 mosaic FPA and larger mosaics.
Fully compatible with the TIS SIDECAR™ ASIC Focal Plane Electronics.

# "... A Readout integrated circuit (ROIC) is an integrated circuit (IC) specifically used for reading detectors of a particular type. They are compatible with different types of detectors such as infrared and ultraviolet.  The primary purpose for ROICs is to accumulate the photocurrent from each pixel and then transfer the resultant signal onto output taps for readout. Conventional ROIC technology stores the signal charge at each pixel and then routes the signal onto output taps for readout. This requires storing large signal charge at each pixel site and maintaining signal-to-noise ratio (or dynamic range) as the signal is read out and digitized. [ https://en.wikipedia.org/wiki/Photocurrent ]

A ROIC has high-speed analog outputs to transmit pixel data outside of the integrated circuit. If digital outputs are implemented, the IC is referred to as a Digital Readout Integrated Circuit (DROIC).

Digital readout integrated circuit (DROIC) is a class of ROIC that uses on-chip analog-to-digital conversion (ADC) to digitize the accumulated photocurrent in each pixel of the imaging array. DROICs are easier to integrate into a system compared to ROICs as the package size and complexity are reduced, they are less sensitive to noise and have higher bandwidth compared to analog outputs.

Digital pixel readout integrated circuit (DPROIC) is a ROIC that uses on-chip analog-to-digital conversion (ADC) within each pixel (or small group of pixels) to digitize the accumulated photocurrent within the imaging array. DPROICs have an even higher bandwidth than DROICs and can significantly increase the well capacity and dynamic range of the device. ..."

Large (2048×2048 pixel) array with 18 μm pixel pitch. ( "pixel pitch" defined:  https://www.sciencedirect.com/topics/engineering/pixel-pitch )

- http://www.teledyne-si.com/products/Documents/H2RG%20Brochure%20-%20September%202017.pdf

Compatible with Teledyne Imaging Sensors (TIS) HgCdTe infrared (IR) and silicon PIN HyViSI™ visible detectors, providing sensing of any spectral band from soft X-ray to 5.5 μm. [ NOTE PSU ]

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Teledyne Imaging Sensors (TIS) >

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"Substrate-removed" WHY this would be desired Explained.  Substrate-Removed.pdf

Substrate-removed HgCdTe enhances the J-band quantum efficiency > QE [ https://en.wikipedia.org/wiki/Quantum_efficiency ], enables response into the visible spectrum (70% QE down to 400nm) and eliminates fluorescence from cosmic radiation absorbed in the sub Reference rows and columns for common-mode noise rejection.

Guide window output – windowing with simultaneous science date acquisition of full array. Programmable window which may be read out at up to 5 MHz pixel rate for guiding. Readout is designed to allow interleaved readout of the guide window and the full frame science data.

Selectable number of outputs (1, 4, or 32) and user-selectable scan directions provide complete flexibility in data acquisition.

Built with modularity in mind – the array is 4-side-buttable to allow assembly of large mosaics of 2048×2048 H2RG modules, such as TIS’ 4096×4096 mosaic FPA and larger mosaics.

Fully compatible with the TIS SIDECAR™ ASIC Focal Plane Electronics.  ..."

These are 2k x 2k HgCdTe arrays, sensitive through the 0.6-5 micron wavelength region. The development systems have reached dark currents of less than 0.001 electrons per second and less than 10 electrons rms read noise per pixel in 1000 sec total exposure. These detectors also have very low latency of less than 0.02%.

Ten of these detectors will be used in NIRCam, four in the guider.  ..."]

...Switching of the modes is done by operating a wavelength preselection mechanism called the Filter Wheel Assembly, and selecting a corresponding dispersive element (prism or grating) using the Grating Wheel Assembly mechanism.
[78]

... Both mechanisms are based on the successful ISOPHOT wheel mechanisms of the Infrared Space Observatory.

The multi-object mode relies on a complex micro-shutter mechanism to allow for simultaneous observations of hundreds of individual objects anywhere in NIRSpec's field of view. There are two sensors each of 4 megapixels.
The mechanisms and their optical elements were designed, integrated and tested by
Carl Zeiss Optronics GmbH of Oberkochen, Germany, under contract from Astrium.[78]  ..."

"... TITLE: "The grating and filter wheels for the JWST NIRSpec instrument" :

Authors:

#### Abstract :: The Near-Infrared Spectrograph (NIRSpec) ... can be reconfigured in space for astronomical observation in a range of filter bands as well as spectral resolutions.  This will be achieved using a Filter wheel (FWA) - which carries 7 transmission filters; and, a Grating wheel (GWA) - which carries six gratings and one prism.  The large temperature shift between warm launch and cryogenic operation ( 30K ) and high launch vibration loads on the one hand side and accurate positioning capability and minimum deformation of optical components on the other hand side must be consolidated into a single mechanical design which will be achieved using space-proven concepts derived from the successful ISO filter wheel mechanisms which were manufactured and tested by Carl Zeiss. ... Carl Zeiss Optronics has been selected by Astrium GmbH for the implementation of both NIRSpec wheel mechanisms. Austrian Aerospace and Max-Planck-Institut fur Astronomie Heidelberg (MPIA) will contribute major work shares to the project. The project was started in October 2005 and the preliminary designs have been finalized recently. Critical performance parameters are properly allocated to respective hardware components, procurements of long-lead items have been initiated and breadboard tests have started. ... Pub Date: June 2006 : DOI: 10.1117/12.670692  : Bibcode: 2006SPIE.6273E..23W  ..."

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SOURCEhttps://jwst.nasa.gov/content/observatory/instruments/nirspec.html ...
The NIRSpec is designed to observe 100 objects simultaneously. The NIRSpec will be the first spectrograph in space that has this remarkable multi-object capability.

# Range:

NIRSpec will operate over a wavelength range of 0.6 to 5 microns.  SPECTRAL RANGE

The Near InfraRed Spectrograph (NIRSpec) will operate over a wavelength range of 0.6 to 5 microns. A spectrograph (also sometimes called a spectrometer) is used to disperse light from an object into a spectrum. [detailed above].

Analyzing the spectrum of an object can tell us about its physical properties, including temperature, mass, and chemical composition. The atoms and molecules in the object actually imprint lines on its spectrum that uniquely fingerprint each chemical element present and can reveal a wealth of information about physical conditions in the object. Spectroscopy and spectrometry (the sciences of interpreting these lines) are among the sharpest tools in the shed for exploring the cosmos.

Many of the objects that the Webb will study, such as the first galaxies to form after the Big Bang, are so faint, that the Webb's giant mirror must stare at them [CAPTURE PHOTONS FROM THEM] for hundreds of hours in order to collect enough light to form a spectrum. In order to study thousands of galaxies during its 5 year mission, the NIRSpec is designed to observe 100 objects simultaneously. The NIRSpec will be the first spectrograph in space that has this remarkable multi-object capability. To make it possible, Goddard scientists and engineers had to invent a new technology microshutter system to control how light enters the NIRSpec.

### NIRSpec Innovations

One unique technology in the NIRSpec that enables it to obtain those 100 simultaneous spectra is a micro-electromechanical system called a "microshutter array".

NIRSpec's microshutter cells, each approximately as wide as a human hair, have lids that open and close when a magnetic field is applied.

Each cell can be controlled individually, allowing it to be opened or closed to view or block a portion of the sky.

It is this adjustability that allows the instrument to do spectroscopy on so many objects simultaneously. Because the objects NIRSpec will be looking at are so far away and so faint, the instrument needs a way to block out the light of nearer bright objects. Microshutters operate similarly to people squinting to focus on an object by blocking out interfering light. (Read more about NIRSpec's microshutter technology.)  jwst.nasa.gov/microshutters.html

... Microshutters are tiny windows with shutters that each measure 100 by 200 microns, or about the size of a bundle of only a few human hairs. The microshutter device can select many objects in one viewing for simultaneous high-resolution observation which means much more scientific investigation can get done in less time.

# MICROSHUTTERS

The entire microshutter device consists of approximately 250,000 individual windows with shutters arrayed in a waffle-like grid. Photo: NASA

The microshutters are a new technology that was developed for the James Webb Space Telescope mission. They are basically tiny windows with shutters that each measure 100 by 200 microns, or about the size of a bundle of only a few human hairs. Arrays of these tiny windows are a key component of one of Webb's instruments, the Near Infrared Spectrograph or NIRSpec.

NIRSpec will record the spectra of light from distant objects. (Spectroscopy is simply the science of measuring the intensity of light at different wavelengths. The graphical representations of these measurements are called spectra.) What is special about the microshutter device is that it can select many objects in one viewing for simultaneous observation and it is programmable for any field of objects in the sky.

Other spectroscopic instruments have flown in space before - but none have had the capability to enable high-resolution (spectroscopic) observation
- of up to 100 objects simultaneously, which means much more scientific investigating can get done in less time.

An array of microshutters on the James Webb Space Telescope's NIRSpec instrument. Photo: NASA

### Engineering Challenges

There were engineering challenges in developing these microshutters, including the fact that NIRSpec's operating temperature is cryogenic so the device has to also be able to operate at extremely cold temperatures. Another challenge was developing shutters that would be able to: open and close repeatedly without fatigue; open individually; and open wide enough to meet the science requirements of the instrument. (Silicon nitride was chosen for use in the microshutters, because of its high strength and resistance to fatigue.)

+

One of the four array quadrants of the microshutter device is about the size of a postage stamp. Each quadrant of the microshutter device consists of more than 62,000 individual windows with shutters arrayed in a waffle-like grid. Four of these arrays are butted together two-by-two into one microshutter device. Photo: NASA [  62,000 X 4 = 248, 000

Prior to an observation, each individual microshutter is opened or closed when a magnetic arm sweeps past, depending on whether or not it receives an electrical signal that tells it to be opened or closed. An open shutter lets light from a selected target in a particular part of the sky to pass through NIRSpec while a closed shutter blocks unwanted light from any objects that scientists don't want to observe. It is this programmable controllability that allows the instrument to do spectroscopy on so many different selected objects simultaneously from one viewing to the next.

"microshutter" "CONTROL" -  https://asd.gsfc.nasa.gov/ngmsa/ : "jwst" "microshutter" "CONTROL"
[ https://www.spiedigitallibrary.org/journals/journal-of-micro-nanolithography-mems-and-moems/volume-16/issue-2/025501/James-Webb-Space-Telescope-microshutter-arrays-and-beyond/10.1117/1.JMM.16.2.025501.short ::
https://jwst-docs.stsci.edu/jwst-near-infrared-spectrograph/nirspec-instrumentation/nirspec-micro-shutter-assembly
https://jwst-docs.stsci.edu/jwst-near-infrared-spectrograph/nirspec-instrumentation/nirspec-micro-shutter-assembly  ]

"To build a telescope that can peer farther than Hubble can, we needed brand new technology," said Murzy Jhabvala, chief engineer of Goddard's Instrument Technology and Systems Division. "We've worked on this design for over six years, opening and closing the tiny shutters tens of thousands of times in order to perfect the technology."

Harvey Moseley, the Microshutter Principal Investigator, adds, "The microshutters are a remarkable engineering feat that will have applications both in space and on the ground, even outside of astronomy in biotechnology, medicine and communications."

The microshutters were conceptualized and created at NASA's Goddard Space Flight Center.

This abstract image is a preview of the instrumental power that will be unleashed once the James Webb Space Telescope is in space.

The image was acquired during testing of the Near-InfraRed Spectrograph (NIRSpec) instrument, which is part of ESA's contribution to the international observatory. NIRSpec will be used to study astronomical objects focusing on very distant galaxies. It will do so by splitting their light into spectra - separating the light into components allows scientists to investigate what these objects are made of.

Created using one of the instrument's internal calibration lamps as the light source, the image shows many spectra as horizontal bands that were recorded by two detectors. The wavelengths are spread from left to right; the pattern of dark stripes, called absorption lines, is characteristic of the light source, much like a fingerprint.

The image was produced by sending commands to open over 100 of the instrument's microshutters - minuscule windows the width of a human hair - that will be used to study hundreds of celestial objects simultaneously. The thin strips in the upper and lower parts of the image are spectra created by light that passed through the micro-shutters, while the thicker bands at the centre of the images were produced by light that enters the instrument through five slits at the center.

Once in space, the microshutters will be opened or closed depending on the distribution of stars and galaxies in the sky.

This calibration image was obtained in 2017 during testing in the giant thermal vacuum chamber at NASA's Johnson Space Center in Houston, Texas. The tests demonstrated that the combined structure, comprising the Webb telescope and its four science instruments, operated flawlessly at temperatures of around -233°C, similar to those they will experience in space.

## JWST:

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Mikulski Archive for Space Telescopes

Mikulski Archive for Space Telescopes ::  https://en.wikipedia.org/wiki/Mikulski_Archive_for_Space_Telescopes

"...  The Mikulski Archive for Space Telescopes (MAST) is an astronomical "data archive". The archive brings together data from the visibleultraviolet, and near-infrared wavelength regimes. [  MAST home page  ]  The NASA funded project is located at the Space Telescope Science Institute (STScI) in Baltimore, Maryland and is one of the largest astronomical databases in the world.

The archive was named after Barbara Ann Mikulski, a long time champion of the Hubble and James Webb space telescopes, in 2012.... It is a component of NASA's distributed Space Science Data Services. The archive contains the data from a number of instruments like Pan-Starrs,[7][8] Kepler,[9] and TESS,[10] as well as data for the Hubble Space telescope[11] (HST) and James Webb Space Telescope (JWST).

In October 2020 the project released the largest and most detailed 3D maps of the Universe, the classification and photometric redshift catalog "PS1-STRM".

The data was created using neural networks and combines data from the Sloan Digital Sky Survey and others. [ https://en.wikipedia.org/wiki/Sloan_Digital_Sky_Survey ]
- Users can query the dataset online or download it in its entirety of ~300GB.[12][13][14]

## References

1. ^ "Hubble Archive, Supernova Named in Honor of Mikulski"SpaceNews.com.

2. ^ "Astronomy archive honors Sen. Mikulski"UPI. Retrieved 2018-05-26.

3. ^ "Barbara Mikulski Honored With Supernova"Huffington Post. 2012-04-05. Retrieved 2018-05-26.

4. ^ Dattaro, Laura. "Senator Barbara Mikulski, Supernova"citypaper.com. Retrieved 2018-05-26.

5. ^ Weaver, Dustin (2012-04-05). "Sen. Mikulski to have supernova named after her"TheHill. Retrieved 2018-05-26.

6. ^ "NASA Space Science Data Coordinated Archive".

7. ^ "Space Telescope Science Institute to Host Data from World's Largest Digital Sky Survey"Newswise.com.

8. ^ "The Biggest Digital Map of the Cosmos Ever Made"New York Times.

9. ^ "Tales from the Exoplanet Archive: How NASA Keeps Track of Alien Worlds"Space.com.

10. ^ "TESS Launches New Era of Exoplanet Discovery --"Zooming in on Alien Life""DailyGalaxy.com.

11. ^ "NASA extends Hubble Space Telescope science operations contract"Astronomy.com.

12. ^ "Astronomers produce largest 3-D catalog of galaxies"phys.org. Retrieved 9 November 2020.

13. ^ Williams, Matt (14 October 2020). "The Most Comprehensive 3D Map of Galaxies Has Been Released"Universe Today. Retrieved 9 November 2020.

14. ^ Szapudi, Istvan; Beck, Robert (2020). "PS1-STRM". [ https://archive.stsci.edu/hlsp/ps1-strm ]       MAST. STScI/MAST. doi:10.17909/t9-rnk7-gr88. Retrieved 9 November 2020.  Data available under CC BY 4.0.

## Maximizing the scientific accessibility & productivity of astronomical data.

##### The Mikulski Archive for Space Telescopes is an astronomical data archive focused on the optical, ultraviolet, and near-infrared.   MAST hosts data from over a dozen missions like Hubble, Kepler, TESS, and soon JWST. ... On This [THEIR] Page:

 H "Please try again or refer to our sitemap." Missions and Data Copernicus DSS & GSC EPOCh EUVE FUSE Gaia GALEX HSLA IUE JWST K2 Documents K2 Bulk Downloads Campaign Fields Kepler ORFEUS Pan-STARRS SwiftUVOT TESS Data Products VLA First XMM-OM NASA Data Centers HLSP   "... High Level Science Products are observations, catalogs, or models that complement, or are derived from, MAST-supported missions. These include Hubble (HST), James Webb (JWST), TESS, PanSTARRS, Kepler/K2, GALEX, Swift, XMM, and others. HLSPs can include images, spectra, light curves, maps, source catalogs, or simulations. They can include observations from other telescopes, or data that have been processed in a way that differs from what's available in the originating archive.  All HLSPs are public immediately with no proprietary periods.  Use the filters below to discover HLSP. Search HLSP by coordinates or filenames on MAST Classic. Or, see all HLSPs in a simplified, searchable table.  ..." Table of HLSPs AGNSEDATLAS "... AGNSEDATLAS presents the spectral energy distributions (SEDs) of 41 individual active galactic nuclei, derived from multiwavelength photometry and archival spectroscopy, including eight MAST-supported projects (HST, SWIFT-UVOT, GALEX, PanSTARRS, IUE, FUSE, HUT, WUPPE) plus at least nine other missions or observatories. In addition to these individual AGN SEDs, there are an additional 72 Seyfert SEDs produced by mixing the SEDs of the central regions of Seyferts with galaxy SEDs.  ..." asPIC CDIPS COS-GAL COSMOS-DASH CRATER DIAmante ELEANOR EVEREST FHUFD FIGS QLP REFERENCE-ATLASES STELLA STORM SUPERBORG TASOC TESS-COADD-CUTOUTS TESS-SPOC TICA ULLYSES UVESCAPE UVOT-OC UVUDF WD-GRID WFC3IR-FLUXCAL WFC3IR-FLUXCAL WFCJ H

Magnitude (astronomy) :: https://en.wikipedia.org/wiki/Magnitude_(astronomy ) "...  In astronomymagnitude is a unitless measure of the brightness of an object in a defined passband, often in the visible or infrared spectrum, but sometimes across all wavelengths. An imprecise but systematic determination of the magnitude of objects was introduced in ancient times by Hipparchus.

The scale is logarithmic and defined such that each step of one magnitude changes the brightness by a factor of the fifth root of 100, or approximately 2.512. For example, a magnitude 1 star is exactly 100 times brighter than a magnitude 6 star. The brighter an object appears, the lower the value of its magnitude, with the brightest objects reaching negative values.

Astronomers use two different definitions of magnitude: apparent magnitude and absolute magnitude. The apparent magnitude (m) is the brightness of an object as it appears in the night sky from Earth. Apparent magnitude depends on an object's intrinsic luminosity, its distance, and the extinction reducing its brightness. The absolute magnitude (M) describes the intrinsic luminosity emitted by an object and is defined to be equal to the apparent magnitude that the object would have if it were placed at a certain distance from Earth, 10 parsecs for stars. A more complex definition of absolute magnitude is used for planets and small Solar System bodies, based on its brightness at one astronomical unit from the observer and the Sun.

The Sun has an apparent magnitude of −27 and Sirius, the brightest visible star in the night sky, −1.46. Apparent magnitudes can also be assigned to artificial objects in Earth orbit with the International Space Station (ISS) sometimes reaching a magnitude of −6. ..."

ABOUT "Star Catalog"  > https://en.wikipedia.org/wiki/Guide_Star_Catalog ::

>  http://gsss.stsci.edu/Catalogs/Catalogs.htm :
"...  The Catalogs and Surveys Branch of the Space Telescope Science Institute has digitized the photographic Sky Survey plates from the Palomar and UK Schmidt telescopes to produce the "Digitized Sky Survey".

These images were then processed and calibrated to produce the Guide Star Catalogs in support of HST operations.

The Guide Star Catalog I (GSC-I) [ https://gsss.stsci.edu/Catalogs/GSC/GSC1/GSC1.htm ] "...

Description : The Catalogs and Surveys Branch of the Space Telescope Science Institute constructed the Guide Star Catalog I (GSC-I) to support the pointing and target acquisition for the Hubble Space Telescope. For the last decade the GSC-I has been used for numerous other purposes, for example, the observation planning for fiber optic spectrographs, the preparation of finding charts, and the operation of ground-based telescopes. ... The GSC-I catalog is an all-sky catalog of positions and magnitudes for approximately 19 million stars and other objects in the sixth to fifteenth magnitude range. The GSC is primarily based on an all-sky, single-epoch collection of Schmidt plates. For centers at +6° and north, a 1982 epoch "Quick V" survey was obtained from the Palomar Observatory, while for southern fields, materials from the UK SERC J survey (epoch = 1975) and its equatorial extension (epoch = 1982) were used. See "Sky Surveys" for more information on the plates.

All the plates were digitized at the Space Telescope Science Institute into 14000 X 14000 rasters at a 25 micron sampling interval using two modified PDS micro-densitometers.

IMPORTANT - HST observers should now use GSC 2 for observation planning

References :

The Guide Star Catalog I - Astronomical Foundations and Image ProcessingLasker et al 1990 AJ 99, 2019
The Guide Star Catalog II - Photometric and Astrometric Models and SolutionsRussell et al 1990 AJ 99, 2059
The Guide Star Catalog III - Production, Database Organization and Population StatisticsJenkner et al 1990 AJ 99, 2081
Some comments on the astrometric properties of the guide star catalogTaff et al 1990 ApJ 353, L45
GSC 1.2- An Astrometric Re-Reduction and other RefinementsMorrison et al 2001 AJ 121, 1752

Properties :  Summary of catalog properties and coded values

Data Access :
GSC 1.0 : not available
..."

The Guide Star Catalog II (GSC-II) [ gsss.stsci.edu/Catalogs/GSC/GSC2/GSC2.htm ]

"...  Description :

The Guide Star Catalog II (GSC-II) is an all-sky optical catalog based on 1" resolution scans of the photographic Sky Survey plates, at two epochs and three bandpasses, from the Palomar and UK Schmidt telescopes. This all-sky catalog will ultimately contains positions, proper motions, classifications, and magnitudes in multiple bandpasses for almost a billion objects down to approximately Jpg=21, Fpg=20. The GSC-II is currently used for HST Bright Object Protection and HST pointing. Looking ahead, the GSC-II will form the basis of the Guide Star Catalog for JWST. This was constructed in collaboration with ground-based observatories for use with the GEMINI, VLT and GALILEO telescopes

Note - always use the most recent version of GSC 2 (currently 2.3.4) for HST target coordinates!

Versions :

 Version Date Sky Coverage Mag limit Bands Ast Phot PM Delivered 2.0 January 2000 Science target fields none J,F TYC1 GSPC1 no Ops 2.1.0 April 2000 30% none J,F TYC1 GSCP1 no consortium 2.1.1 July 2000 50% none J,F TYC2 GSPC2.1 no consortium 2.2.0 June 2001 98% F=18.5 J,F TYC2 GSPC2.2 no Public release notes 2.2.1 August 2001 98% none J,F TYC2 GSPC2.2 no Ops 2.3.0 September 2003 100% none J,F,N TYC2 GSPC2.3 no consortium 2.3.1 October 2004 100% none J,F,N,+ TYC2 GSPC2.3 yes consortium 2.3.2 October 2005 100% none J,F,N,+ TYC2 GSPC2.3 yes/no* Ops / Public release notes 2.3.3 October 2009 100% none J,F,N,+ TYC2 GSPC2.3 yes/no* Ops / Public release notes

+ the additional passbands are primarily from the POSSI-E, the POSSI-O and Palomar Quick-V surveys so only cover the northern heimpshere

* the computed proper motions are not yet publically available because we discovered a 10mas/year systematic error for the southern hemisphere that we are still investigating.

References : The Second Generation Guide Star Catalog : Description and Properties
Lasker et al 2008 AJ 136,735

Properties :   Summary of catalog properties and coded values

Data Access :
GSC 2.0 : not available
GSC 2.1 : not available
GSC 2.2 : superseded by v2.3
..."

The Guide Star Photometric Catalog I (GSPC-I) [ gsss.stsci.edu/Catalogs/GSPC/GSPC1/GSPC1.htm ]
"...
Description :

The Guide Star Photometric Catalog I (GSPC-I) is a all-sky catalog of secondary "standard-stars" that were used to perform the photometric calibration of the GSC-I. The GSPC-I consists of approximately 6 stars between 9-15th magnitude near the center of each of the Palomar and UKSTU survey fields. These were all observed using a photo-electric photometer.

References :

The Guide Star Photometric Catalog I

Properties :

Data Access :

The Guide Star Photometric Catalog II (GSPC-II)
"...
Description :

The Guide Star Photometric Catalog II (GSPC-II) is an all-sky catalog of secondary "standard-stars" that were used to perform the photometric calibration of the GSC-II. The GSPC-II typically consists of stars between 14-19th magnitude near the center of each of the Palomar and UKSTU survey fields. These were all observed using CCD detectors from a number of different observatories.

References :

An all-sky set of (B)-V-R photometric calibrators for Schmidt surveys. GSPC2.1: First release
Bucciarelli et al 2001 A&A 368, 335

Properties :

Data Access :
..."

These catalogs support ground and space-based telescope operations and provide a valuable scientific resource to the astronomical community. ..."

" tHE SECOND-GENERATION GUIDE STAR CATALOG: DESCRIPTION AND PROPERTIES  "

- https://jwst-docs.stsci.edu/jwst-observatory-characteristics/jwst-guide-stars "...

# JWST Guide Stars  ... JWST uses a single guide star in one of the FGS fields for fine guiding during a given visit.

Fine guiding is provided by a single guide star.
Roll control is provided separately by the spacecraft star trackers.

It is not the user's responsibility to pick specific guide stars to be used for their observations. JWST guide stars are selected from the Guide Star Catalog [GSC] 2.4 catalog by the guide star selection system (GSSS) based on several factors related to telescope pointing and suitability of the star.
Tools and reports are available to visualize the availability of guide stars for a target.

# Guide Star Catalog

The JWST proposal planning system currently uses Guide Star Catalog (GSC) version 2.4.2 for the selection of guide stars and reference stars. GSC 2.4, released for JWST use in November 2017, was a major update to the GSC used for many years for HST operations. GSC 2.4 was a merger of GSC 2.3 with data from 2MASS, Sloan Digital Sky Survey (SDSS) DR13, Gaia DR1, and VISTA Hemisphere Survey (VHS) DR4. SDSS and VHS stellar data improve the quality of the GSC, particularly at the fainter magnitudes (J > 17), which significantly improves the JWST guide star availability at higher Galactic latitudes. GSC 2.4.2 has been updated to GAIA DR2 astrometry and includes new sources added from other catalogs (GAIA, PanSTARRS, SDSS, Skymapper, etc.).

Improvements in the catalog that have resulted in increased guide star availability include:

1. Guide star catalog object classifications as stellar or non-stellar have been improved. Analysis has shown that SDSS and VHS are particularly helpful in this regard, especially for J > 17. Guiding on slightly extended sources can reduce guiding accuracy.

2. Additional photometric information, especially in the near-infrared, has been made available. In order to be considered a valid JWST guide star candidate, a GSC object must have photometry available in 2 or more bandpasses so interpolations or extrapolations can be made as needed. If a given star has more photometry bands, its total observed FGS count rate can be more accurately predicted, making it possible to determine if it’s a guide star that will provide successful acquisition, tracking, and fine guidance functions.

3. The Gaia DR1 catalog provides significant astrometric improvement (0.001” vs >0.1”) for essentially all stars near the galactic plane, and for about half the stars at higher galactic latitudes.

4. Coverage gaps in GSC 2.3 have been filled, for example, in areas around very bright stars and in regions of very high extinction.

Visible and Infrared Survey Telescope for Astronomy   ..."  [  "VISTA Hemisphere Survey Home Page"  ]

- https://archive.stsci.edu/missions-and-data/dss--gsc ::  [ " DSS & GSC " ]

This date: 2-3-2022

Dear  Mikulski Archive for Space Telescopes STAFF  [ https://en.wikipedia.org/wiki/Mikulski_Archive_for_Space_Telescopes ]

MAST  Catalogs and Surveys Group
CASG
MAST
STScI
Data Centers
Missions
VO
-
Help
TUTORIAL

https://www.projectpluto.com/gsc_act.htm

"...  This is a "webform" which allows a user to retrieve Guide Star Catalog 2  (GSC2) data. It is only necessary to enter ONE of the following input conditions.

[ "GSC2" HST ID  : https://arxiv.org/pdf/0807.2522.pdf ] ::

https://archive.stsci.edu/hst/hsc/ <  Version 3.1 of the Hubble Source Catalog  >  Five things you should know about the HSC  1. Detailed Use Cases ( https://archive.stsci.edu/hst/hsc/help/HSC_faq.html#use_case )
and Videos ( https://archive.stsci.edu/hst/hsc/help/use_case_3_v2.html  )  are available.

https://archive.stsci.edu/hst/hsc/help/use_case_1_v3.html  < USE CASE EXAMPLE :::          MAIN PORTAL >  https://mast.stsci.edu/portal/Mashup/Clients/Mast/Portal.html

• HST ID - the 10 digit ID (begins with N or S)
• GSC1 ID - the original GSC1 ID of an object if available
• Free form RA,DEC - text boxes to enter RA & DEC in either decimal degrees or sexagesimal
• Sexagesimal RA,DEC - text boxes to specify RA & DEC in sexagesimal format (deprecated)

If a position is entered one need to specify the size/type of the search area

• Cone Search radius in arcmin with the option of specifying an inner radius to return an annulus
• Box search with side length in arcminutes

When returning data to the user it is possible to select a number of different formats

• HTML - webpage format for browser viewing
• CSV - comma separated variables for analysis
• TSV - tab separated variables for analysis
• JSON - JavaScript Object Notation for data interchange
• VOTABLE - Virtual Observatory standard for data interchange
• TEXT - plain text format to support service calls (deprecated)

There are also optional parameters to specify bright/faint magnitude limits and the maximum number of objects to return (sorted by mag)

It is also possible to programatically call this webservice using the endpoint
gsssdev.stsci.edu/webservices/GSC2/GSC2DataReturn.aspx?  ..."

FAQ

Column 01
Sky Surveys
Catalogs
Software
HST Support
Related Sites

"GSC2 Catalog Access - Please fill out one of the Position/ID fields"

HST ID    field  10 digit ID (beginning with N or S)
GSC1 ID    field  5 digit region + 5 digit number
RA,DEC    field, field  (hh:mm:ss.s +/-dd:mm:ss.s or decimal deg)
RA field field field    hms
DEC  field dropDown field field field    + +/-dms

Format    HTML Version  GSC 2.4.2 (Current HST Ops)

Optional Parameters
Bright Mag  field
Faint Mag     field
Max Obj    field

FORM FOOTER

Acknowledgments
Email Us
Contacts
-

... "

Authors:   S. H. Moseley*,a, R. Arendta,b, R. A. Boucaruta , M. Jhabvalaa , T. Kinga , G. Kletetschkaa,c, A. S. Kutyreva,b, M. Lia , S. Meyera , D. Rapchuna,d, and R. F. Silverberga a Goddard Space Flight Center, Greenbelt, Md. 20771 b Science Systems and Applications, Inc. ,10210 Greenbelt Rd., Suite 600 Lanham, MD 20706 c Catholic University of America, Washington, D.C d Global Science & Technology, Inc.,7855 Walker Drive, Suite 200, Greenbelt, MD 20770 .

ABSTRACT
The Near Infrared Spectrograph (NIRSpec) for the James Webb Space Telescope (JWST) is a multi-object spectrograph operating in the 0.6-5.0 µm spectral range.  ( ElectroMagnetic Spectrum :  https://en.wikipedia.org/wiki/Electromagnetic_spectrum )

Spectral Range Of Interest  FOR WEBB
"0.6 µm" spectral range >

"5.0 µm" spectral range >

One of the primary scientific objectives of THE jwst (this instrument) is to measure the number and density evolution of galaxies following the epoch of initial formation.

- "Big Bang" epoch of "initial formation" : [ https://universe-review.ca/F05-galaxy07.htm ]
( SOURCE: https://universe-review.ca/I05-14-galaxyevolution.jpg )

## Galaxies

### Formation and Evolution of Galaxy

The most accepted view on the formation and evolution of large scale structure is that it was formed as a consequence of the growth of primordial fluctuations by gravitational instability. Galaxies can form in a "bottom up" process in which smaller units merge and form larger units. It is referred to as the "Inside-out Theory" or "Merger" as shown in the upper half of Figure 05-08a. In the present epoch, large concentrations of galaxies (clusters of galaxies) are still in the process of assembling. The opposing view is the "top down" process in which large clump breaks up into smaller units. It is referred to as the "Outside-in Theory" as shown in the lower half of Figure 05-08a. The figure also shows the kind of objects the NGST (Next Generation Space Telescope) will detect according to the two opposing theories.

#### Figure 05-08b BH, Supermassive

( https://universe-review.ca/I05-14-superbh.jpg ) < LARGER
 The "bottom up" theory have been given a boost in 2008 by the first ever detection of the infant protogalaxies with an unprecedented 92-hour session on the European Southern Observatory's Very Large Telescope. These protogalaxies were irregularly shaped and with low star-formtion rates, but the stars that did form were massive and consequently exploded as supernovae. The image on the left shows a group of protogalaxies in the process of merging. (See a movie of "Galaxy Formation") ( https://universe-review.ca/I05-14-protogalaxies2.jpg ) < LARGER

By 2013 the bottom up theory becomes less sustainable with the discovery that supermassive black hole with mass over billion Msun is common at an age about 750 - 900 million years after the Big Bang. It just did not have enough time to amass for such behemoth. Various schemes have been proposed to resolve the discrepancy, but each one has its own problem (see Figure 05-08b).

The difference between the "bottom up" (inside-out) and "top down" (outside-in) point of view is related to whether the universe is composed with cold dark matter (CDM, slow moving) or hot dark matter (HDM, fast moving). In the former scenario there is fluctuation in the power spectrum over a wide range of physical scales as shown in Figure 05-08c. Smaller size have larger fluctuation, therefore structure formed first with small objects, which then merge to form ever larger structures. This is called bottom up'' structure formation. The observations strongly favour this scenario over its competitor: "top down" structure formation. The proto-typical top down'' scenario is structure formation in a universe dominated by hot dark matter. Hot dark matter cannot support fluctuations on small length scales - they are washed out with the rapid motion of the particles. Thus only large scale fluctuations survive to the present epoch. Structure forms first large scale objects which fragment into smaller objects.

#### Figure 05-08c Density Fluctuations in Three Models

... The early universe was a barren wasteland of hydrogen, helium, and a touch of lithium, containing none of the elements necessary for life as we know it.  ..." ]

"epoch of initial formation"...

1. https://ntrs.nasa.gov/citations/20040095910
3. https://rwoconne.github.io/rwoclass/astr511/moseley-microshutters-JWST.pdf

NIRSpec is designed to allow simultaneous observation of a large number of sources, vastly increasing the capability of JWST to carry out its objectives.

A critical element of the instrument is the programmable field selector, the "Microshutter Array". The system consists of four 175 x 384 close packed arrays of individually operable shutters, each element subtending 0.2” x 0.4”on the sky. This device allows simultaneous selection of over 200 candidates for study over the 3.6’ x 3.6’ field of the NIRSpec, dramatically increasing its efficiency for a wide range of investigations.

< END >

hh

The image shows a central portion of the Hubble Deep Field, created from exposures taken in 1995. The Hubble Deep Field covers a piece of sky about 1/13th the diameter of the full Moon.

Credits: NASA

> You're Doing What?
> A Core Sample of the Universe
> Hubble Deep Field South
> Hubble Ultra Deep Field
> Hubble Ultra Deep Field-Infrared
> Hubble eXtreme Deep Field
> Ultra Deep Field 2012
> Frontier Fields
The Hubble Space Telescope has made over 1.5 million observations since its launch in 1990, capturing stunning subjects such as the Eagle Nebula and producing data that has been  featured in almost 18,000 scientific articles. But no image has revolutionized the way we understand the universe as much as the Hubble Deep Field.

Taken over the course of 10 days in 1995, the Hubble Deep Field captured roughly 3,000 distant galaxies varying in their stages of evolution.

( "Hubble Space Telescope" "launch" in "1990" :  https://en.wikipedia.org/wiki/Hubble_Space_Telescope   )

https://youtu.be/ZGoDK18b3LE    Photons time and distance "experience"

### You’re Doing What?

When Hubble first launched, many members of the astronomy community doubted its ability to observe distant galaxies. John Bahcall, one of the world’s top astrophysicists and a Hubble advocate, co-wrote a Science paper in 1990 arguing that Hubble wouldn’t reveal any galaxies not already visible from ground-based methods.

History played out a little differently. In 1994, Robert Williams, director of the Space Telescope Science Institute in Baltimore, Maryland, developed an interest in Hubble’s ability to observe distant objects when he witnessed exposures taken in May and June by Wide Field and Planetary Camera 2 (WFPC2), a high-resolution camera capable of capturing images over a wide field of view and wavelengths. The data showed a “cosmic zoo” of objects, suggesting that another deep exposure could reveal unknown parts of the universe.

Williams controlled 10 percent of Hubble’s observation time, known as director’s discretionary time. He decided to use some of that time to take a long exposure that would be made immediately available, instead of waiting for a similar proposal to come in, go through a lengthy approvals process, and be restricted on when the data could be made public. After his decision, he created a team at the Space Telescope Science Institute for nearly a year of preparation, working to find the right location in the sky and taking test exposures to confirm the area they chose was free of large galaxy clusters.

The next step was choosing a spot in the sky on which to train Hubble’s camera. Williams and his team needed a dark area with no nearby stars or other objects, whose glow would drown out the fainter galaxies he hoped to find with the exposure. That meant the area couldn’t be near the plane of the Milky Way, which was bright with cosmic objects. The area also needed to be a continuous viewing zone, or an area without interference from the Earth, Sun, or Moon. The team ultimately chose a location within Ursa Major, near the handle of the Big Dipper, that looked relatively empty. That emptiness would provide a gateway to see much farther back in time.

In the end, the patch of sky was about the equivalent of holding a pinhead at arm’s length.

### A Core Sample of the Universe

Light from far-distant objects must cross unimaginably vast expanses of space over millions to billions of years to reach us. This means the farthest objects we can see look to us like they appeared in the early universe, when their light began traveling.

Credits: NASA

The Hubble Deep Field image holds 342 separate exposures taken between December 18 and 28, 1995. The picture we see was assembled from blue, red, and infrared light. The combination of these images allows astronomers to infer the distance, age, and composition of the galaxies photographed. Bluer objects, for example, contain young stars or could be relatively close. Redder objects contain older stars or could be farther away.

Most of the galaxies are so faint ― four billion times fainter than the human eye can see ― that they had never been observed before, even by the largest telescopes.

“As the images have come up on our screens, we have not been able to keep from wondering if we might somehow be seeing our own origins in all of this,” Williams said at the time. “These past 10 days have been an unbelievable experience.”

The “deep” in Hubble Deep Field refers to the telescope’s ability to look at some of these far, faint objects. Looking at far-away objects in space is like seeing back in time. Light moves at tremendous speed, but it still takes time to travel across the vastness of space. Even the light from our own Sun needs eight minutes and 20 seconds to reach Earth, so when we look at the Sun, we see it as it was a little more than eight minutes earlier. The farther away the object, the younger it appears in Hubble’s gaze. The Deep Field was like a core sample of space, showing galaxies at different and earlier stages of development the deeper they appeared in the image.

Researchers from the State University of New York at Stony Brook analyzed the photo and chose several dozen candidates that could be more distant than any galaxies seen up to that point. They identified the galaxies based on their color, because more distant galaxies appear redder as the light reaches us. This happens because the light stretches as it travels through the universe, transforming into infrared wavelengths, which are redder.

A 1998 follow-up infrared image taken with Hubble’s Near Infrared Camera and Multi-Object Spectrometer discovered galaxies believed to be over 12 billion light-years away, even farther than those seen in the Hubble Deep Field.

This photo of a portion of the Hubble Deep Field South captures galaxies in both visible and infrared light. The bluish galaxies are in visible light, while the reddish galaxies are in infrared light. Some of the brightest objects in the photo are in the “foreground,” or located within the Milky Way.

Credits: NASA

### Hubble Deep Field South

After the success of the original Hubble Deep Field, astronomers sought new ways to increase our understanding of the universe. Since it would take 900,000 years for astronomers to observe the whole sky, they knew they would have to rely on more samples like  the Hubble Deep Field to infer what the entire universe looks like.

The Hubble Deep Field South focused on a region in the constellation Tucana, near the south celestial pole, and doubled the number of distant galaxies available to astronomers. Williams and a team of 50 astronomers and technicians at the Institute and at Goddard Space Flight Center in Greenbelt, Maryland, carried out the 10-day-long observation in October 1998.

### Hubble Ultra Deep Field

The Hubble Ultra Deep Field

Credits: NASA

In 2004, Hubble captured a million-second-long exposure that contained 10,000 galaxies. This new image, the Hubble Ultra Deep Field, observed the first galaxies to emerge from the “dark ages,” a time just after the Big Bang.

A servicing mission in 2002 had installed a new camera, called the Advanced Camera for Surveys. That camera had twice the field of view and a higher sensitivity than WFPC2, the camera that captured the original Deep Field. The final Ultra Deep Field photo is actually combined from an ACS image and an image from Hubble’s Near-Infrared Camera and Multi-object Spectrometer.

“Hubble takes us to within a stone’s throw of the Big Bang itself,” said Massimo Stiavelli, an instrument scientist for Hubble at the Space Telescope Science Institute.

From ground-based telescopes, the location of the Ultra Deep Field in the constellation Fornax ― right below the constellation Orion ― looked mostly empty, much like the other Deep Field locations, allowing for more distant observations to take place.

The Ultra Deep Field image contained several odd galaxies, such as one shaped like a toothpick and another shaped like a bracelet link. Such galaxies come from a more chaotic time before the development of structured galaxies like the Milky Way.

Ultra Deep Field data also taught astronomers that black holes at the center of galaxies likely grew over time, that large galaxies build up gradually as others merge and collide, and that some of the earliest galaxies were much smaller than our current Milky Way.

This video (at the SOURCE)  shows the discoveries made with Hubble Ultra Deep Field.

Credits: NASA

### Hubble Ultra Deep Field-Infrared

These four candidate galaxies might have emitted their light when the universe was just 750 million years old. The bottom row shows the the various wavelengths that each galaxy was measured in, and the right-hand column displays the galaxy’s redshift, or how the expansion of the universe stretched light as it traveled long distances to reach us. The candidates were found using the Hubble Ultra Deep Field and the Great Observatory Origins Deep Survey observations.

Credits: NASA

In 2009, Hubble captured near-infrared light wavelengths in the same region as the Ultra Deep Field, revealing galaxies formed just 600 million years after the Big Bang.

The light from one object, called UDFj-39546284, traveled 13.2 billion light-years to reach Earth. It’s a compact galaxy made up of blue stars, and astronomers found that the rate of star formation grew by a factor of 10 in just over 200 million years ― that may sound like a long time to us, but it’s tiny for the universe.

The eXtreme Deep Field.

Credits: NASA

### Hubble eXtreme Deep Field

In 2012, Hubble took it to the extreme. Astronomers combined 10 years of photographs taken of a region in the center of the original Ultra Deep Field. Even with its smaller view, the eXtreme Deep Field still showed 5,500 galaxies.

The faintest galaxies visible in this image are one ten-billionth of what the human eye can see, and most of the galaxies shown are from when they were young and small, often colliding and merging together.

### Ultra Deep Field 2012

After observations made over six weeks in August and September 2012, a team of astronomers discovered a population of seven primitive galaxies formed when the universe was just 3% of its present age. The observations supported the idea that galaxies may have provided enough energy to reheat the universe after the Big Bang.

### Frontier Fields

NASA’s Great Observatories ― Hubble, Spitzer, and Chandra ― teamed up in 2013 for the Frontier Fields, a bold multi-year campaign to provide critical data to aid investigations of dark matter and how galaxies change over time, among others.

Abell 370 is a cluster with several hundred galaxies at its core. It was one of the first clusters where astronomers observed gravitational lensing and part of the Frontier Fields project.

Credits: NASA, ESA, R. Bouwens and G. Illingworth (University of California, Santa Cruz)

The campaign provided 12 new deep field images, and astronomers were able to detect galaxies 100 times fainter than those they observed in the Hubble Ultra Deep Field. Focusing on high-redshift galaxies and gravitational lensing, or the natural distortion of light from massive galaxy clusters, the team worked to detect galaxies too faint to be seen by Hubble alone. Such an undertaking propelled our understanding of the universe in ways that could only be achieved with all the Great Observatories working together. The campaign ended in 2017, and now astronomers can use the dataset to continue exploring the early universe.

Not only did the Hubble Deep Field change how we understand the universe, it also changed how we share findings.

“This coming together of the community to generate a shared, nonproprietary dataset was essentially unprecedented but has since become the model for the majority of large astronomical projects,” wrote University of Washington astronomer Julianne Dalcanton. “This new mode of operating has democratized astronomy.”

Hubble’s data was compiled for the Legacy Field, a combination of nearly 7,500 Hubble exposures. It represents 16 years of observations, 265,000 galaxies, and 13.3 billion years, making it the largest collection of galaxies documented by Hubble.

The role of exploring the early universe further will fall to the James Webb Space Telescope, expected to launch in late 2021. Designed to see even farther back than Hubble because of its powerful infrared vision, Webb promises exciting observations and new discoveries. But our evolving understanding began with Hubble, and a team not afraid to explore what looked like nothing.

National Aeronautics and Space AdministrationPage Last Updated: Oct 29, 2021Page Editor: Michelle BellevilleNASA Official: Brian Dunbar

 Authors: Alexander Kutyrev University of Maryland, College Park , Mary Li NASA .  Samuel Moseley   - Quantum Circuits, Inc. .  David A. Rapchun .  Stephen Snodgrass ,  David W. Sohl ,  Leroy Sparr https://www.researchgate.net/institution/NASA  https://www.researchgate.net/profile/Robert-Silverberg https://www.researchgate.net/profile/Richard-Arendt  https://www.researchgate.net/profile/David-Franz-2 https://www.researchgate.net/profile/Gunther-Kletetschka https://www.researchgate.net/institution/Charles_University_in_Prague  https://www.researchgate.net/profile/Mary-Li-3  https://www.researchgate.net/profile/Samuel-Moseley  https://quantumcircuits.com/about https://www.researchgate.net/scientific-contributions/D-Rapchun-11349869 https://www.researchgate.net/scientific-contributions/Stephen-Snodgrass-2025654053 https://www.researchgate.net/scientific-contributions/D-Sohl-2004180468  https://www.researchgate.net/scientific-contributions/Leroy-M-Sparr-34717859

11 authors, including: Some of the authors of this publication are also working on these related projects:
Younger Dryas Impact Hypothesis View project Transients View project
Robert F. Silverberg NASA 232 PUBLICATIONS   10,220 CITATIONS    SEE PROFILE
Richard G. Arendt NASA 251 PUBLICATIONS   10,328 CITATIONS    SEE PROFILE
David E. Franz NASA 34 PUBLICATIONS   262 CITATIONS    SEE PROFILE
Gunther Kletetschka Charles University in Prague ( 243 PUBLICATIONS   2,785 CITATIONS    SEE PROFILE )
( The user has requested enhancement of the downloaded file). A Microshutter-based Field Selector for JWST’s Multi-Object Near Infrared Spectrograph Robert F. Silverberga, Richard Arendtb, David E. Franza, Gunther Kletetschkac, Alexander Kutyrevb, Mary J. Lia, S. Harvey Moseleya, David A. Rapchund, Stephen Snodgrasse, David W. Sohla
and Leroy Sparra and "the Microshutter Team"
a  NASA/Goddard Space Flight Center, Greenbelt, MD USA;
b  University of Maryland, College Park, MD USA
c  Catholic University, Washington, DC USA
d  Global Science and Technology, Greenbelt, MD USA
e  MEI Technologies, Seabrook, MD USA

"Kutyrev" "A microshutter-based field selector for JWST's multi-object near infrared spectrograph"

2007, Alexander Kutyrev  - https://asd.gsfc.nasa.gov/ngmsa/docs/

- https://www.spiedigitallibrary.org/conference-proceedings-of-spie/4561/0000/Magnetically-actuated-microshutter-arrays/10.1117/12.443109.short

"...  2 October 2001, Magnetically actuated microshutter arrays ::  David Brent Mott, Shahid Aslam, Kenneth A. Blumenstock, Rainer K. Fettig, David E. Franz, Alexander S. Kutyrev, Mary J. Li, Carlos J. Monroy, Samuel Harvey Moseley Jr., David Scott Schwinger
Author Affiliations  ... Proceedings Volume 4561, MOEMS and Miniaturized Systems II; (2001) https://doi.org/10.1117/12.443109
Event: Micromachining and Microfabrication, 2001, San Francisco, CA, United States

Abstract :: Two-dimensional microshutter arrays are being developed at NASA Goddard Space Flight Center (GSFC) for the Next Generation Space Telescope (NGST) for use in the near-infrared region. Functioning as focal plane object selection devices, the microshutter arrays are 2-D programmable masks with high efficiency and high contrast. The NGST environment requires cryogenic operation at 45 K. Arrays are close-packed silicon nitride membranes with a unit cell size of 100x100 micrometer. Individual shutters are patterned with a torsion flexure permitting shutters to open 90 degrees with minimized mechanical stress concentration. The mechanical shutter arrays are fabricated with MEMS technologies. The processing includes a RIE front-etch to form shutters out of the nitride membrane, an anisotropic back-etch for wafer thinning, and a deep RIE (DRIE) back-etch down to the nitride shutter membrane to form frames and to relieve the shutters from the silicon substrate. A layer of magnetic material is deposited onto each shutter. Onto the side-wall of the support structure a metal layer is deposited that acts as a vertical hold electrode. Shutters are rotated into the support structure by means of an external magnet that is swept across the shutter array for opening. Addressing is performed through a scheme using row and column address lines on each chip and external addressing electronics.
:::  David Brent Mott, Shahid Aslam, Kenneth A. Blumenstock, Rainer K. Fettig, David E. Franz, Alexander S. Kutyrev, Mary J. Li, Carlos J. Monroy, Samuel Harvey Moseley Jr., and David Scott Schwinger "Magnetically actuated microshutter arrays", Proc. SPIE 4561, MOEMS and Miniaturized Systems II, (2 October 2001); https://doi.org/10.1117/12.443109 ..."

- https://femci.gsfc.nasa.gov/workshop/2002/presentations/loughlin/Loughlin_MEMS_Shutter.pdf
"...  Structural Analysis of a Magnetically Actuated Silicon Nitride Micro-Shutter for Space Applications ..."

--  https://ieeexplore.ieee.org/document/9609553

"...  High-Performance Silicon Photonics Using Heterogeneous Integration

Publisher: IEEE

aUTHORS: Chao Xiang; Warren Jin; Duanni Huang; Minh A. Tran; Joel Guo; Yating Wan; Weiqiang Xie; Geza Kurczveil; Andrew M. Netherton; Di Liang; Haisheng Rong; John E. Bowers

Abstract

Document Sections
I.
Introduction
II.
Silicon Photonic Devices
III.
Silicon Photonic Integrated Circuits
IV.
Commercializing Heterogeneous Silicon Photonics
V.
Summary and Outlook

Abstract:  ..."

HH

Abstract:

The performance of silicon photonic components and integrated circuits has improved dramatically in recent years. As a key enabler, heterogeneous integration not only provides the optical gain which is absent from native Si substrates and enables complete photonic functionalities on chip, but also lays the foundation of versatile integrated photonic device performance engineering. This paper reviews recent progress of high-performance silicon photonics using heterogeneous integration, with emphasis on ultra-low-loss waveguides, single-wavelength lasers, comb lasers, and photonic integrated circuits including optical phased arrays for LiDAR and optical transceivers for datacenter interconnects.

Published in: IEEE Journal of Selected Topics in Quantum Electronics ( Volume: 28, Issue: 3, May-June 2022)

Article Sequence Number: 8200515

Date of Publication: 15 November 2021

ISSN Information:

Publisher: IEEE

Funding Agency:

CCBY - IEEE is not the copyright holder of this material. Please follow the instructions via https://creativecommons.org/licenses/by/4.0/ to obtain full-text articles and stipulations in the API documentation.

SECTION I.

## Introduction

For the past fifty years, most telecom, datacom and sensor systems have relied on individual optical components, such as lasers, modulators and photodetectors [1]. Recently, integrated photonics has been commercialized due to its advantages in terms of size, weight, cost and power consumption [2][3]. The performance of photonic integrated circuits (PICs) is largely determined by the selection of the integration platform. Silicon photonics leverages mature CMOS facilities for high-yield, low-cost manufacturing of photonic components and may have an intrinsic advantage over the III-V-based devices due to the availability of low loss and compact waveguides. Silicon photonic modulators, photodetectors and passive devices are available through monolithic silicon-on-insulator (SOI) waveguide based structures. Heterogeneous integration using bonded III-V active materials onto SOI provide optical gain and thus enables lasers and amplifiers, expanding the complexity and improving the performance of PICs [4].

Heterogeneous integration allows the selection of optical materials for the best achievable on-chip performance. For example, in addition to quantum wells (QWs), III-V active materials are now supplemented with quantum dots (QDs) for wide gain bandwidth and high temperature operation [5]. On the passive waveguide side, ultra-low loss silicon nitride (Si3N4) waveguides are being integrated for a wide variety of applications and heterogeneous device performance optimization [6][7][8]. As an example, semiconductor laser frequency noise can be reduced by 50 dB by self injection locking (SIL) the laser to ultra-low loss Si3N4 waveguides [9]. A wide range of available materials from group IV, III-V and II-VI semiconductors to dielectric, ferroelectric, piezoelectric, and magnetic materials can be integrated [10][11]. The integration approach can be die-to-wafer, wafer-to-wafer bonding [12] or micro-transfer-printing [13]. It can also be combined with heteroepitaxial growth to further reduce the cost and improve the integration versatility [14][15].

Photonic integrated circuits with dense integration of lasers, modulators and other devices are evolving rapidly from proof-of-concept demonstrations to products shipping million units per year [16]. The elimination of chip-to-chip coupling loss and availability of on-chip gain from heterogeneous integration expanded its application scenarios from optical transceivers to sensors, such as gyroscopes and LiDAR [17][18]. The high integration level also results in high energy efficiency. Heterogeneous integration with co-packaged silicon photonics and electronics is the key towards future petabyte-per-second bandwidth network switches in datacenters [19]. The scalability of heterogeneous integration is naturally compatible with silicon photonics using CMOS facilities. Industry-standard 300-mm-diameter or 200-mm-diameter Si substrates promise the scalable production of heterogeneous PICs with similar performance, but at orders of magnitude lower cost and higher manufacturing scale [16][20].

A comparison of the wafer sizes is shown in Fig. 1(a). Industry-standard 300-mm-diameter SOI wafers offer a high device throughput. Indium phosphide (InP) substrate PICs are commonly restricted to 4-inch or smaller. Gallium arsenide (GaAs) substrates PICs are making a transition from small size (2-inch, 3-inch) to 150-mm substrates, but its average material cost is still significantly higher than 300-mm Si substrates. The integration level of monolithic and heterogeneous silicon photonics is growing at similar pace with increasing number of components integrated on a single PIC (Fig. 1(b)[19]. With the addition of on-chip lasers and amplifiers, the integration level and device scale of heterogeneous Si PICs are projected to exceed that of monolithic Si PICs without lasers.

Fig. 1.

(a) Wafer size comparison of a 300-mm SOI wafer with smaller-sized GaAs and SOI wafers. (b) The number of photonic components integrated on a single PIC over time for three photonic integration platforms [19].

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This paper is organized as follows. We start with introducing the most recent results in high-performance silicon photonic devices using heterogeneous integration (Section II). This section starts with the developments of ultra-low loss silicon and silicon nitride waveguides, which is a key motivation of silicon photonics and also forms the basis of several key performance optimizations for heterogeneous integrated devices. This section then discusses progress in lasers with the unique silicon photonic devices enabled by heterogeneous integration, including single-wavelength lasers and comb lasers. We highlight the developments in both gain materials and laser passive cavity. We then showcase the recent representative photonic integrated circuits, showing the improving integration scale and future integration perspectives with electronics (Section III). In Section IV, we discuss the advantages of commercializing heterogeneous silicon photonic circuits in datacenters for co-packaged optics and present an exemplary heterogeneous silicon photonic transceiver from an Intel 300-mm CMOS fab, which achieved a total of 260 Gbps data rate on a single wavelength. Finally in Section V, we provide a summary and future perspectives on heterogeneous integration in silicon photonics.

SECTION II.

## Silicon Photonic Devices

Heterogeneous integration allows performance optimization for both passive and active devices. This section focuses on the recent progress of ultra-low loss waveguides and heterogeneous integrated lasers on Si.

### A. Ultra-Low Loss Waveguides

The availability of low-loss integrated photonic waveguides is critical, not only for reducing component insertion losses as the scale and complexity of integrated photonic circuits increases, but also for enabling emerging microresonator-based technologies, such as as on-chip soliton microcombs [23] and ultra-narrow linewidth integrated lasers [24], whose performance improves with increasing cavity Q-factor [25][26] (i.e decreasing propagation loss).

Thanks to decades-long development of CMOS technology for the microelectronics industry, silicon, silicon dioxide, and silicon nitride thin films are widely available in foundries for use as high quality waveguide core and cladding material on wafers up to 300 mm in diameter. Fig. 2 compares the achieved loss in waveguide platforms compatible with heterogeneous integration. Compared to >1 dB/cm waveguide loss in GaAs or InP waveguides, typical silicon waveguide loss is on the order of 0.1 dB/cm to 1 dB/cm. Silicon waveguide losses are largely dominated by sidewall scattering losses and can be reduced by reducing etch depth and increasing waveguide width. Indeed, 5 dB/m loss was achieved in a 500 nm thick device layer at 56 nm etch depth and 8 μm width in multi-mode silicon rib waveguides [27]. It needs to be noted that the selection of 56 nm is based on the equipped etch depth monitor at UCSB Nanofab facility which gives an accurate etch depth control. The etch depth can be varied as long as the shallow etch depth is precisely controlled and supports highly confined mode with reduced mode interaction with waveguide sidewalls. The 500-nm thickness of Si instead of 220 nm that is widely used for SOI wafers is favored here for both low loss and also possible integration with heterogeneous III-V lasers. Additional improvements of photoresist reflow and optimized etch chemistry enabled 3 dB/m propagation loss at a deeper etch depth of 190 nm for the same device layer thickness and waveguide width [22]. Moreover, silicon waveguides can be efficiently coupled to silicon nitride waveguides on the same platform [28]. Recent progress of silicon nitride waveguides have demonstrated waveguides loss ranging from 0.1 dB/cm to 0.001 dB/cm (0.1 dB/m), using CMOS-ready processing techniques. A recent publication has demonstrated as low as 0.1 dB/m loss using high-temperature processing within a CMOS foundry [9]. However, reducing the thermal budget for back-end processing (high temperature annealing to reduce hydrogen absorption) is desirable to facilitate integration with III-V materials. For this case, a low temperature, deuterated, silicon dioxide top cladding was developed with demonstrated loss of 3 dB/m [21] and has been successfully integrated into heterogeneous lasers [8][29].

Fig. 2.

Comparison of achieved waveguide loss for waveguide platforms compatible with heterogeneous integration. Propagation loss of 0.1 dB/m has been achieved in Si3N4 subject to high temperature annealing [9], while around 3 dB/m has been demonstrated within the thermal budget of heterogeneous integration using either a deuterated, low-temperature top-oxide-cladded single-mode Si3N4 waveguides [21], or multi-mode silicon waveguides [22].

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### B. Single-Wavelength Lasers

Single wavelength lasers are important in applications including optical interconnects, optical communications, sensing etc. Single-wavelength lasers require wavelength-selective filters. Heterogeneous integration provides versatile flexibility of using silicon-based cavity for the single-wavelength selection. Grating-based DFB (distributed feedback) or DBR (distributed Bragg reflector) lasers offer the ease of operation, while ring-resonator-based lasers can enable a widely tunable range and greater laser noise reduction.

An advantage of heterogeneous III-V/Si lasers is the availability of on-chip Si mirrors or Si facets as the laser mirror. The degradation of cleaved facets in monolithic III-V lasers can thus be eliminated due to the excellent robustness of passive Si-based mirrors. Fig. 3(a) shows a scanning electron microscope (SEM) image of heterogeneous III-V/Si lasers with on-chip Si loop mirrors as the laser reflectors. III-V/Si monitor photodetectors (PDs) can be included to enable and expedite wafer-scale laser screening and testing.

Fig. 3.

(a) SEM image showing the III-V/Si gain section and its transition to passive Si loop mirror-based on-chip reflectors. Image also shows III-V/Si monitor PDs and Si edge-couplers for facet coupling. (b) Optical spectrum of a III-V/Si DFB laser with over 67 dB SMSR. Insets show the DFB array picture and SEM image of a Si λ/4-shift Bragg grating. (c) Device pictures and frequency noise comparison of heterogeneous III-V/Si E-DBR lasers and a hybrid RSOA-FBG laser [30].

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#### 1) Grating-Based III-V/Si Lasers

Si waveguide Bragg grating can offer excellent single-wavelength filtering, thus enabling III-V/Si-based DFB lasers and DBR lasers. A narrow-band grating response together with high side lobe suppression ratio is required for high-performance grating-based single-wavelength lasers, since it directly impacts the laser linewidth and laser side mode suppression ratio (SMSR) [2]. Si offers excellent low loss waveguide which can form high-Q and narrow-band Si waveguide Bragg filters. In applications where high-power and narrow linewidth is preferred, e.g externally modulated lasers and sensing applications, the heterogeneous III-V/Si DFB or DBR lasers offer excellent performance.

For III-V/Si DFB lasers, the grating is etched on the Si layer under the III-V gain mesa area. The DFB grating can be etched on the top, on the sidewall of the rib waveguide core, or on the rib section of the Si rib waveguide. For a long III-V/Si DFB laser targeting high output power, the grating κ needs to be kept small to lower the grating κLg value and consequently, a narrow filter bandwidth for single mode selectivity. The SEM image of the side grating etched on the Si waveguide rib and an image of the InP/Si QW DFB laser array are shown in the inset Fig. 3(b).

The maximum output power of a 1.2-mm long InP/Si DFB laser is over 20 mW. The optical spectrum in Fig. 3(b) shows over 67 dB SMSR due to the excellent mode selectivity provided by the side hole Si rib grating. This laser alone, or integrated together with a semiconductor optical amplifier, could be the pump source for on-chip nonlinear applications through heterogeneous integration with nonlinear materials including AlGaAs, LiNbO 3 (lithium niobate) or Si3 N4.

Another type of Bragg grating-based laser is a DBR laser. While DFB lasers are limited in grating length by the active III-V section length, DBR lasers have the potential to include an ultra-long external cavity. Low-loss Si could enable a long external cavity without significantly increasing the cavity loss. As a result, it offers more potential for laser performance engineering.

Fig. 3(c) shows an image of heterogeneous III-V/Si extended DBR (E-DBR) laser array that contains twelve lasers [31]. The gratings are based on shallow-etched (56 nm etch depth) Si rib waveguide. The grating κ is controlled through the gap distance between the etch holes and Si rib waveguide core. For ultra small κ values, a long grating length is possible for a fixed target κLg value. A 15-mm long Bragg grating is designed to maximize the linewidth reduction ratio through passive-active cavity integration. The fundamental linewidth of such external cavity based laser is Δυ=Δυ0/(1+A+B)2, where Δυ0 is the laser Schawlow-Townes linewidth without external feedback, A is the ratio of external passive Si cavity length to the active III-V/Si gain section length, and B includes the detuned loading effect [32].

The maximum output power of this laser is over 30 mW in the Si waveguide and the fundamental linewidth is down to 240 Hz. The fundamental linewidth value is derived from the white-noise-limited frequency noise floor of the frequency noise spectrum. Fig. 3(c) shows its frequency noise spectrum comparison with a commercially-available hybrid integrated RSOA-FBG (reflective semiconductor optical amplifier- fiber Bragg grating) laser [30]. The heterogeneously integrated E-DBR laser frequency noise performance is approaching that of the hybrid-integrated RSOA-FBG laser, while it holds advantages on the device scalability and lithographically-defined grating uniformity. With further reduced waveguide loss, the frequency noise performance should be comparable with the much larger RSOA-FBG laser. These high-power, low-noise lasers are suitable for applications including coherent optical communications, microwave photonics and LiDAR.

#### 2) Ring-Resonator-Based Tunable III-V/Si Lasers

Wavelength-tunable lasers add additional flexibility in these applications, and find further utilization in wavelength division multiplexed (WDM) communications and sensing systems, such as spectroscopy and optical frequency domain reflectometry (OFDR) [33]. Heterogeneous silicon photonic integration not only allows for lithographic alignment and manufacturing scalability, but also capitalizes on silicon waveguides as a superior low-loss platform over monolithic III-V-based photonic platforms. These low loss waveguides catalyze a new suite of functionalities, resulting in tunability up to 118 nm and instantaneous linewidths below 100 Hz [34].

By incorporating thermally-sensitive silicon waveguides in the laser cavity, heterogeneous integration allows for widely tunable lasers while maintaining low round-trip losses. Resistive heaters can be placed in close proximity to the silicon waveguides to provide a thermally induced change in refractive index, thus changing the cavity length and lasing wavelength. The cladding thickness between the metal heaters and silicon waveguide offers a trade-off between tuning efficiency and absorptive losses [35]. One way of achieving wide tunability is by integrating thermally tunable ring resonators into the laser cavity. One or more ring resonators can be incorporated in a Sagnac reflector to form a laser mirror. Connecting two or more ring resonators with bus waveguides on the drop ports creates a wider effective free spectral range (FSR) due to the Vernier effect, resulting in a broader tuning range than that of a single ring resonator. These results have been reviewed in [24][33][36].

An alternate strategy mimicking external cavity lasers employs multiple ultra-low-loss, large radii ring resonators to enable even wider tunability and further reduced instantaneous linewidth [24][37][38]. These high-Q ring resonators are facilitated by ultra-shallow-etch (56 nm) silicon waveguides, allowing for the optical mode to be weakly guided [27]. This has two major benefits. First, the modal overlap with the waveguide sidewalls is reduced, which mitigates sidewall roughness induced scattering. Second, the cross-sectional modal area is expanded, which distributes the optical power, mitigating the effects of intensity-dependent nonlinear loss, particularly two-photon absorption (TPA) and TPA-induced free carrier absorption (FCA) [39]. The cost of the ultra-shallow etch and weak guiding is bending loss, therefore larger bending radii are necessary. Due to the smaller FSRs of these larger rings, more than two rings are necessary to suppress close-in side modes and facilitate single mode operation. The ring dimensions and coupling coefficients are thus tailored to balance wide tunability with mode selectivity.

Fig. 4(a) shows the schematic design diagram and an image of such a device with three ultra-shallow etch, large-radii ring resonators forming the back mirror in the laser cavity. This design is supplemented by MZI-based thermally tunable couplers, allowing the coupling ratio and thus the reflectivity to be adjusted across the wavelength tuning range. These tunable reflectors can be optimized to tap more power from the front mirror or ASE-filtered light from the ring resonator mirror. A stitched microscope image of a widely-tunable III-V/Si laser shown in also Fig. 4(a), featuring a III-V/Si gain section, Si MZI-based tunable coupler, Si triple-ring mirror and phase tuner.

Fig. 4.

(a) Device schematic design (top) and picture (bottom) of an 3-ring-based III-V/Si widely tunable laser. (b) Stacked optical spectra of the 3-ring-based III-V/Si widely tunable laser operating at different wavelengths (top). Bottom plots show the wavelength tuning map and associated laser SMSR map corresponding to different heater power values applied on the two Vernier rings [36].

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Fig. 4(b) displays the wide tuning range of these devices, from 1490 nm to 1600 nm, with each spectrum exhibiting over 45 dB SMSR over the entire tuning range. These lasing spectra were taken by systematically stepping the heater powers in each of the rings across the full Vernier FSR of 110 nm, while also optimizing the phase section power to maximize output power. This effective FSR is wide enough that the InAlGaAs multiple quantum well (MQW) gain bandwidth becomes the limiting factor for laser oscillation, as evidenced by the drop in power at the edges of the tuning range.

This passive cavity architecture not only features tunable, narrowband filtering, but also enables significant reduction in the quantum-limited white frequency noise. This is due to the resonantly enhanced fraction of the cavity contributing to filtering (i.e. the optical mode in the ring resonators) vs. the portion contributing to phase noise (i.e. the gain section generating ASE). The linewidth narrowing effects of this process are quantified by the ‘A’ parameter in the modified Schawlow-Townes linewidth equation, as discussed before [24][36]. The Schawlow-Townes (also instantaneous, fundamental, intrinsic, or quantum-limited) linewidth can be further narrowed by detuning the lasing frequency to the red side of the dispersive mirror, taking advantage of an optical negative feedback loop. This detuned loading effect (the ‘B’ factor), relies on the coupling between phase and amplitude unique to semiconductor lasers, which is described by αH, the linewidth enhancement factor [40].

#### 3) Heterogeneous Integrated III-V/Si/Si3N4 Lasers

As discussed in the previous section, Si3N4 waveguide loss can be orders of magnitude lower than that of Si. Further noise reduction through laser active-passive integration can be achieved if Si3N4 can be integrated within the laser cavity. There has been extensive research into hybrid integrated lasers with single-wavelength or ring-based wavelength-tunable Si3N4 cavity [32][41][42][43][44]. Heterogeneous integration of a silicon nitride cavity eliminates the time-consuming process of coupling and packaging the laser gain chip and Si3N4 chip, and increases the device robustness. Additionally, it enables wafer-scale production of lasers with integrated Si3N4 cavities.

Due to the requirement of high temperature processing for low-loss Si3N4 films, heterogeneous integration with Si3N4 usually requires the low-loss Si3N4 to be processed at the front-end, followed by wafer bonding process which limits the overall temperature budget of the remaining process. Another issue with a direct III-V/Si3N4 heterogeneous structure is the index mismatch between the normal III-V epi stack (∼2 μm thick) with Si3N4 waveguide preventing efficient evanescent mode coupling and transitions in between. A multilayer III-V/Si/Si3N4 structure is employed to bridge the index mismatch using an intermediate Si layer [8]Fig. 5(a) shows the integration approach that includes multiple wafer bonding and oxide deposition steps for the multilayer structure.

Fig. 5.

(a) Schematic of the multilayer heterogeneous integration process. (b) Device design illustration of a III-V/Si/Si3N4 laser with Si3N4-based spiral grating. The inset IR images shows lasing within the Si3N4 cavity. (c) Cross-sectional SEM images showing the InP/Si gain and Si/Si3N4 section. (d) A comparison of temperature-dependent wavelength shift of the III-V/Si/Si3N4 laser and III-V/Si E-DBR laser [8].

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The III-V/Si/Si3N4 laser leverages the Si3N4 spiral shaped Bragg grating as the external cavity (Fig. 5(b)). The grating length is 20 mm, packed in a footprint of only 3.5 mm × 3.6 mm due to the spiral shape. It provides single-wavelength feedback around 1548 nm. In order to achieve high coupling efficiency between the InP layer and Si3N4 layer, an InP-Si taper and Si-Si3N4 dual-level taper are used to evanescently transiting the corresponding optical modes [8].

Lasing from the Si3N4 cavity is verified using the infrared camera image captured during measurement (Fig. 5(b) inset). The laser multilayer cross sections including InP/Si gain and Si/Si3N4 are shown in Fig. 5(c). The laser wavelength follows the grating response and is stable in terms of temperature change due to the low thermo-optic coefficient of Si3N4 and SiO2. The wavelength drift dependence on temperature is 10.46 pm/o C, which is 7x smaller than that of an InP/Si E-DBR lasers with similar cavity length (Fig. 5(d)). We anticipate that these lasers, together with Si3N4 based arrayed waveguide gratings (AWGs) could enable DWDM (dense wavelength division multiplexing) applications instead of CWDM (coarse wavelength division multiplexing) in datacenter interconnects as the lasing wavelengths are much more stable against temperature drift, which minimizes the inter-channel crosstalk. The output power and laser linewidth of the first-generation of these lasers are still limited by the Si3N4 waveguide loss. A new generation of III-V/Si/Si3N4 E-DBR lasers have over 20 mW output powers with Lorentzian linewidth down to 400 Hz [45]. Another approach to achieve such a multilayer structure is to use an α-Si layer for the immediate index bridging. Heterogeneously integrated SOAs and lasers on SiN are thus demonstrated using micro-transfer printing techniques [46].

#### 4) Heterogeneous Integrated QD/Si Lasers

QD lasers offer new opportunities for improved performance towards lower threshold current density, narrower linewidth, higher temperature stability, better immunity to epitaxy and fabrication defects, and reduced linewidth enhancement factor (αH) [5]. These features make QD lasers particularly attractive in data communication applications, where laser drive current and thermoelectric cooling can make a substantial impact on the overall energy efficiency. On the other hand, reflection insensitivity in single-wavelength QD lasers can significantly reduce packaging complexities and enhance integration density associated with isolator integration. These favorable material merits make it ideal to serve as the optical gain medium on silicon in both heterogeneous and monolithic fashions. For heterogeneous QD lasers in O-band, similar wafer bonding and device process are implemented and GaAs-based QD epitaxial material can easily replace InP-based QW epitaxial material to build different device designs. The first demonstration of light coupling between QD lasers and Si waveguides was achieved by B. Jang et al. [47] and G. Kurczveil et al. [48]. Since then, various types of heterogeneously integrated QD lasers have been demonstrated including DFB lasers [49][50][51], colliding pulse mode-locked lasers [52], microring lasers [53], and tunable lasers [54]. Since the light is evanescently coupled between the QD active region and the low-loss Si waveguides, the laser cavity design and the gain active region can be separately optimized (Fig. 6(a)). The GaAs-based QD epitaxial structure typically has five to eight stacked QD layers. Due to the enhanced carrier localization in QDs, the non-radiative recombination impact to the laser operation and reliability at the etch-exposed QD active region are greatly reduced. This makes it feasible to realize mesa widths on the order of 2 μm without imposing an obvious penalty on the lasing threshold. The narrow mesa width laterally confines both the current channel and the optical mode, efficiently suppressing high order modes as well as avoiding current spreading issues that are beneficial for higher laser efficiencies. Therefore, proton implantation to define the current channel is not necessary.

Fig. 6.

(a) Device schematic of a GaAs/Si QD DFB laser in different configurations (I-III) and device cross-sectional SEM image and transmission electron microscopy image of the MOS (metal oxide semiconductor) capacitor, MOSCAP (IV-V). (b) Temperature-dependent light-current-voltage characteristic of the GaAs/Si QD DFB laser shown in a, and its spectral map up to 50 mA injection current at 20°C and 60°C, respectively [50]. (c) Small-signal modulation responses (left) and frequency noise spectrum (right) of a high-speed GaAs/Si QD DFB laser [51].

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By engineering the QD epi stack and Si grating designs, largely improved performance is shown in heterogeneously integrated QD DFB lasers (Figs. 6(b) and (c)): up to 100 oC lasing in CW mode [49], a demonstrated CW threshold current density as low as 134 A/cm2, up to 9.5% wall plug efficiency, and a single-wavelength operation with SMSR up to 61 dB in broad spectral and injection current ranges based on a shallow etched first-order grating [50], an SMSR of 60 dB and a Lorentzian linewidth of 26 kHz based on a first order side-hole gratings [51]. This Lorentzian linewidth of tens of kHz is two orders of magnitude lower compared to that of the typical solitary QW laser linewidth of several megahertz, attributed to the much lower αH of QDs and lower loss of Si waveguide. In the latest design, DFB gratings are formed by patterning a surface corrugation on top of the Si waveguides, and the widths are tailored to accommodate a suitable κ, low radiation loss, and reliable fabrication process.

The QD/Si heterogeneous DFB lasers find applications in data interconnects when combined with suitable modulation methods. Direct modulation of QD lasers are usually limited by the finite intraband relaxation time and gain saturation effect. However, the latest results (Fig. 6(c)) show 3-dB modulation bandwidth (f3dB) of 13 GHz attained from a directly-modulated QD/Si heterogeneous DFB laser [51]. The results can be further improved by applying bandwidth enhancement techniques including adding appropriate facet coatings, depositing the metals on lower-capacitance dielectrics and so on. Another approach is to utilize external modulation for the QD/Si heterogeneous DFB laser [50]. 25 Gb/s data-links isolator-free modulation with a metal-oxide semiconductor capacitor microring modulator has been achieved. Their narrow linewidth characteristic makes them attractive for coherent communications as well.

As previously discussed, heterogeneous integration allows gain material to be combined with high-performance passive silicon circuits, enabling widely tunable and narrow linewidth lasers. In the same way, QD gain material can be integrated together with low-loss silicon waveguides to form a laser cavity, but with the added advantages of QD over QW lasers. Tunable lasers with QDs as the gain material have been implemented together with Si ring-resonator-based external cavities [54]. For a laser based on an MZI and 2-ring mirror, the demonstrated tuning range is 52 nm in the O-band with at least 45 dB SMSR and Lorentzian linewidth below 10 kHz.

### C. Comb Lasers

Comb lasers are ideal for multi-wavelength data interconnects due to the natural fit with WDM systems. Contrary to single-wavelength laser arrays that are deployed in conventional long-haul WDM systems, comb lasers provide a solution that dramatically increases the number of multiplexed signal channels without the need to rely on multiple laser sources. Since the intrinsic energy consumed by a laser to reach threshold does not scale with the number of channels for comb lasers, the approach of using one laser instead of multiple lasers for multiple channels is critically important to lower the overall power consumption.

Two types of comb lasers are now being deployed into system demonstrations, including mode-locked lasers and nonlinear Kerr frequency combs. In both scenarios, integrated photonics plays an important role. There has been dramatic progress in the past decade developing integrated semiconductor mode-locked lasers on silicon [57][58] and nonlinear Kerr frequency comb generation on various platforms built on silicon substrates [59][60][61][62][63][64][65][66].

Heterogeneously integrated mode-locked lasers on silicon serve as multi-wavelength sources capable of integration with other devices in silicon photonic circuits. Recent developments of heterogeneous laser integration with Kerr optical frequency combs brings low-noise comb sources one step closer to advanced chip-scale deployment with mature silicon photonic functional devices [29].

#### 1) InP/Si QW Mode-Locked Lasers

Heterogeneously integrated III-V/Si mode-locked lasers using integrated mirrors do not rely on cleaved or polished facets for laser feedback, and hence the mode spacing can be controlled quite accurately, such that with slight tuning of the gain current, a desired spacing can be achieved with high yield [67]. Saturable absorbers can be implemented directly using the laser gain material by reverse biasing.

Fig. 7(a) shows a schematic illustration of a III-V/Si colliding pulse mode-locked laser [55]. The on-chip reflector is based on a Si loop mirror. For better performance, the loop mirror is designed to be spline shaped reflector to minimize the overall loss [58]. A saturable absorber is placed at the middle of the structure. To reduce back-reflection from the uncoated Si facet, the output waveguide is tilted at 7 degrees. Mode locking is achieved with certain bias current and reverse bias on the saturable absorber. Wide mode locking range is obtained with gain currents ranging from 60 to 200 mA and SA reverse bias from 2.5 V to 5 V. Stable mode locking is verified by the RF beating tone, which has over 60 dB signal-to-noise ratio. The minimum pulse width is 1.08 ps with passive mode locking and is further reduced to 890 fs by hybrid mode locking using an external RF source. An additional advantage of heterogeneous mode-locked lasers is the low-loss Si waveguides or SiN, which can be utilized for higher performance, including dispersion compensation, phase noise reduction and so on [68][69]. Recent demonstrations include record-low repetition rate mode-locked-lasers with 1-GHz and 755-MHz mode spacing, using low loss Si and SiN waveguides respectively [70][71].

Fig. 7.

(a) Schematic of an InP/Si QW mode-locked laser (left), pulse width mapping as a function of the gain section forward bias current and SA section reverse voltage under passive mode locking state (middle) and single pulse autocorrelation trace with sech2 fit (right) [55]. (b) Optical spectrum of QD-based III-V/Si comb laser (left). The flat spectrum reduces the need for channel equalization at the receiver; Eye diagram and BER of each channel from the device in a after external modulation at 10 Gb/s (right) [56].

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#### 2) GaAs/Si QD Comb Lasers

Quantum dot material features an inhomogeneously broadened gain spectrum and ultrafast carrier dynamics, which are ideal for comb laser operation [5][72]. QD-based III-V/Si comb lasers have demonstrated remarkably flat optical spectra, which distinguish them from traditional mode-locked lasers, as shown in Fig. 7(b) (left) [56]. The device consists of a 1.2-mm-long gain section, and a 120-μm-long saturable absorber which are centered between the mirrors. The front and back mirrors are formed by multimode interferometers and loop mirrors having reflectivities of 50% and 100% respectively. An external cavity was formed between the front mirror and the grating-based output coupler, which resulted in a channel spacing of 101 GHz. While the device contained a saturable absorber, the optical output is CW (continuous wave, no pulses are observed), which reduces unwanted nonlinearities in the link and potentially increases the device reliability. Filtering out one comb line at a time and modulating it externally yields eye diagrams and bit-error-rate (BER) values shown in Fig. 7(b) (right). We note open eye diagrams and BER values or 10−12 or lower in 14 of the 15 channels. The width of the comb can likely be broadened by tuning the total cavity dispersion (by using an appropriately chirped grating for example) and spatial hole burning (by offsetting the gain section from the cavity center [73]).

#### 3) Laser Soliton Microcombs

Optical frequency combs generated in on-chip microresonators, termed as ‘microcombs’ have been extensively studied and deployed in many applications in the past decade [23][26][74]. Emerging platforms for microcomb generation are mostly built on silicon substrates. This is largely due to the requirement for compact and tight mode confinement for microcomb generation and the benefits of high index contrast from Si-based platforms.

Integrated photonics enables miniaturization of frequency comb generation on a chip scale. However, in most applications lasers and amplifiers are still based on bulky and expensive table-top equipment. Semiconductor lasers are normally too noisy for soliton generation. Recent development of integrated laser-comb systems utilizing self-injection locking eliminates the need for an optical isolator as well as a narrow-linewidth pump laser, since the laser-resonator system itself relies on the resonator feedback signal to simultaneously reduce the laser linewidth and generate optical frequency combs [75][76].

Heterogeneous integration allows this laser-resonator to be integrated on a monolithic substrate to enable high-performance multi-wavelength laser sources directly on chip. III-V materials and low-loss nonlinear materials are combined through wafer bonding. The first demonstration of such devices is illustrated in Fig. 8(a). A high-power InP/Si DFB single-wavelength laser outputs the pump light with λ around 1550 nm, entering the nonlinear microresonator for comb generation. The back-scattered signal from the high-Q Si3N4 microresonator is fed back to the laser. When the relative phase between the forward and backward signal is matched, the resonant feedback pulls the laser frequency into the resonator mode, injection locking the laser. Different frequency comb states are generated depending on the laser frequency detuning with respect to the Si3N4 microresonator resonance. The wafer picture and multiple dies of devices are shown in Fig. 8(b).

Fig. 8.

(a) Device schematic and (b) images of laser soliton microcomb devices based on InP/Si/Si3N4 multilayer structure. (c) Optical spectrum of the generated 100 GHz-repetition rate single soliton state [29].

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The entire device with multilayer structures is heterogeneously integrated on a silicon substrate with wafer-scale processing. Similar to the III-V/Si/Si3N4 lasers discussed in the previous section, silicon is used as the index matching layer between the III-V gain and ultra-low loss Si3N4. Similarly, a thermo-optic resistive heater on the Si waveguide is used to tune the phase between the laser and nonlinear resonator, thus allowing electrical control of comb state generation.

The fabrication process of the laser soliton microcomb devices is based on multilayer heterogeneous integration, compromising two wafer bonding steps and several planarization steps [29]. The device can output a single soliton state with 100-GHz repetition rate (Fig. 8(c)) and soliton crystal states with larger comb spacing. Due to the laser self-injection locking to the high-Q microresonator, the DFB laser frequency noise is reduced by 10 to 30 dB depending on the offset frequency range. The Lorentzian linewidth is 25 Hz for the self-injection locked pump line in the single-soliton state and the low frequency noise is also transferred to the other comb lines. Further laser linewidth reduction can be achieved with resonators of higher Q and larger mode volume.

The demonstration of heterogeneously integrated soliton microcombs provides another solution for on-chip multi-wavelength sources with low noise performance that is unattainable from semiconductor mode-locked lasers. Heterogeneous integration of gain materials with ultra-low loss photonic circuits is the key and can be expanded to many other material platforms that have unique properties. For example, III-V integrated with LiNbO3 which has a strong electro-optic effect and second order nonlinearity can offer the generation and modulation of the soliton microcomb signals, fully-integrated E-O combs, second/third-harmonic-generated light or ultra-high-capacity optical transceivers.

SECTION III.

## Silicon Photonic Integrated Circuits

In additional to individual devices, heterogeneous integration also shows advantages in a series of photonic integrated circuits and this section will cover some of the most recent results.

### A. Optical Phased Arrays

Heterogeneous III-V/Si devices can be regarded more than as just stand-alone function units which are usually spaced at more than 100 μm. Heterogeneous silicon photonic PICs see tremendous progress in both the device scale and device performance. Recently, Si photonics has attracted tremendous attention in chip-based fully solid-state LiDAR applications, e.g. optical phased arrays (OPAs). III-V/Si heterogeneous integration can offer high-performance active building blocks like high-efficiency phase shifters with low power consumption in addition to laser sources, providing a promising solution for complete on-chip LiDAR systems. On the other hand, PICs for LiDAR (e.g. OPA-based system) involves thousands of components in a limited reticle area and thus requires large-scale and dense integration.

To make use of efficient electro-optic properties in III-V, heterogeneous integration technology has been further extended for high-density integrated III-V/Si PICs at large scales. Fig. 9(a) shows an OPA design with a III-V/Si phase shifter array with a pitch of only 4 μm. The cross-section view shows the density of the array. Fig. 9(b) shows the fabricated 240-channel and 32-channel OPAs. The number (N) of phase shifters are scalable in practice. These phase shifters exhibit low residual amplitude modulation (∼0.15 dB in C-band), low static power consumption (a few nW), low operating voltage (about −1 V in C-band), wide optical bandwidth (200+ nm) and high operating speed (>1 GHz), suitable for OPA applications. Notably, they exhibited uniform performance, proving the reliability of the heterogeneous III-V/Si platform for larger-scale integration. In a tested 32-channel OPA, 2D steerable far-field beams with a beam width of 0.78° × 0.02° and a field of view of 22° × 28° were demonstrated through both phase (Ψ-axis) and wavelength tuning (θ-axis with wavelength from 1450 nm to 1650 nm), respectively, as shown in Fig. 9(c). The beam divergence can be further improved by scaling up the element number of OPA and optimizing the optical antenna design. Combined with previously realized III-V/Si high-power lasers, amplifiers, and detectors on the same heterogeneous integration platform, high-power and high-quality steerable beams out of a single chip become achievable. Related systems for applications such as monolithic chip-fashioned LiDAR can be thus realized.

Fig. 9.

(a) Configuration of a III-V/Si OPA system with a 1×N star coupler, III-V/Si phase shifter array with a pitch of 4 μm, and grating emitter array. Right panel: schematic cross-section of the III-V/Si phase shifter array with locally opened n/p contact for probing one shifter on top. (b) Images of a fully fabricated 240-channel (upper panel) and 32-channel (lower panel) OPAs with enlarged SEM images for different parts of the OPA. (c) Measured beam profiles at 1550 nm wavelength with the beam steering across the field of view in the Ψ-axis at a 2.8° increment in the 32-channel OPA. The right inset shows the IR images of the main beam when steered by phase in Ψ-axis. (d) Measured beam profiles along θ axis (Ψ = 0) with the beam steering when tuning the wavelength from 1450 nm to 1650 nm at a 10 nm increment. The top inset shows the IR images of the main beam when tuning wavelength in θ-axis [77].

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### B. Integrating Silicon PIC and EIC

Silicon photonics allows for high-confinement, low-footprint devices. Ring modulators and demultiplexers with as-fabricated resonant offsets and driver electronics flipped on top are a compact, energy-efficient means of achieving high capacity communication links (>1 Tb/s). The comb sources discussed above are ideal to be integrated with linear arrays of rings to achieve an ultra-dense, high-capacity WDM link with sub-pJ/bit power consumption [72]Fig. 10(a) shows a 1 Tbps transmitter utilizing 20 wavelengths at 25 Gbps [67]Fig. 10(b) shows a picture of 3-D packaged PIC and EIC (electronic integrated circuit) with copper pillar bumps to minimize the electrical link length and associated high device capacitance. The copper pillar bumps are closely spaced with 36 um pitch to achieve high bandwidth density (> 1 Tb/s/mm2). Wire bonds to ring tuning electronics are used as shown in Fig. 10(b) and could be replaced by embedded PIC and control electronics in a silicon interposer to further lower the electrical power consumption.

Fig. 10.

(a) Architecture of a low power consumption transceiver using QD-MLL sources and integrated linear ring modulator arrays for an 1-Tb/s link. (b) Image showing the 3D flip-chip bonded EIC and PIC (left). The SEM image on the right shows the densely-spaced copper pillar bumps for the 3D EIC-PIC integration [67].

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Heterogeneous photonic integration refers to wafer bonding technology that integrates multiple photonic material groups on silicon, analogy to how electronics illustrates the capability to integrate separately manufactured components into a higher level assembly (System-in-Package, SiP). Heterogeneous integrated silicon photonic chiplets, with its CMOS-compatible nature, are ideal for dense electronic-photonic integration to solve the electronic and energy consumption bottleneck problems of electrical ICs [19].

SECTION IV.

## Commercializing Heterogeneous Silicon Photonics

Heterogeneous integration of the semiconductor lasers with silicon optical modulators to form highly functional transmitter PICs has tremendous benefits in terms of lower cost, lower packaging complexity, and high-volume scalability. When compared to alternative techniques such as butt-coupling of lasers directly to silicon PICs, or using fiber to bridge the two chips together, heterogeneous integration allows for the entire transceiver to be fabricated and tested at a 300 mm wafer level [16]. In addition to these economic and scaling benefits, integration of the laser and modulator also has multiple performance benefits stemming from the higher level of integration. Removing a fiber-to-chip optical coupler between the laser and modulator leads to less overall loss, which can improve the link budget. The emitted light from a semiconductor laser is also highly polarized, eliminating any polarization-based ambiguity at the input to the modulator. Intel has demonstrated the success of large-scale, high-volume heterogeneous silicon photonics through its silicon photonics platform and products, which are shipping millions of units per year [16][19][78].

Looking forward, as bandwidth requirements continue to increase beyond 400 G and 800 G due to the explosion in data center traffic, the size and power consumption of PICs need to decrease, analogous to Moore’s Law. One such approach is to utilize microring modulators (MRMs), which are hundreds of times smaller than conventional Mach-Zehnder modulators. In addition to the footprint reduction, MRMs also have higher modulation efficiency, lower power consumption, and can be engineered to have large electro-optic (EO) bandwidth. Intel’s previous work on MRMs showed EO bandwidths of more than 50 GHz and PAM4 eyes at 128 Gbps [79]. More recently, optimizations to the p-n junction design have allowed for the data rate to be increased to 192 Gbps PAM4 and 128 Gbps NRZ [80], making the MRM device suitable for 800 Gb/s optical interconnects and beyond.

Due to the resonant nature of MRMs, they require active control of the resonance condition to compensate for changes in the ambient temperature. This is typically done through integrated doped silicon or metal heaters placed near the MRM to align it to the laser wavelength. When the laser is integrated with the MRM on the same chip, the walk-off between the MRM resonance and laser wavelength due to temperature change is reduced when compared to the case where the laser is on a separate chip at a fixed temperature. This is another benefit of heterogeneous integration, and a fully integrated transmitter with laser, MRM, co-packaged CMOS driver and thermal control is shown in [81]. The scalability of heterogeneously integrated lasers with MRMs in Intel’s silicon photonic platform is shown, in which 800 gigabits per second are transmitted across 8 parallel optical channels, each operating at 106.25 Gbps PAM4 [82]. Eight transmitters with integrated laser, MRMs, and thermal control are simultaneously operational and characterized. The full PIC and package have a total of 16 optical channels, with a full capability of 1.6 Tbps, while maintaining a smaller footprint than a 100 G pluggable QSFP28 module.

As discussed in the previous section, WDM is an effective technique to further scale the bandwidth, especially on a per fiber basis. MRMs are perfect candidates for WDM, as the ring modulators can be easily cascaded along a single waveguide to increase the total bandwidth per fiber/waveguide. A recent example is provided in [83], in which error-free transmission is demonstrated in a PIC with 4 WDM channels with accompanying CMOS IC at 50 Gbps per channel. Other recent examples with varying levels of integration are presented in [84][85]. In such dense WDM systems, the wavelength spacing of the lasers and the MRMs need to be tightly controlled. For heterogeneously integrated lasers on silicon, the laser wavelength can be defined by gratings patterned on the silicon. The superior lithography and tight process control in a 300 mm CMOS fab is critical to achieve this task. Across an entire 300 mm wafer, the wavelength of the laser is controlled with a standard deviation of 0.2 nm [86]. Considering that lasers are in proximity to each other, such as those within a single PIC, the wavelength control can be even tighter. This allows for the lasers and rings in a WDM transmitter to be nearly aligned to a predetermined wavelength grid as fabricated. Some further fine-tuning of the wavelength may still be required and can be accounted for using promising techniques such as germanium trimming of ring resonators post-fabrication [87], which was demonstrated to permanently align all the ring resonators across a 300 mm wafer within 32 pm of a target wavelength.

Yet another vector to scale bandwidth is to utilize polarization division multiplexing (PDM), in which data is further encoded in the transverse electric (TE) or transverse magnetic (TM) polarization of light, effectively doubling the data rate of a transmitter when implemented. This approach is widely used in long-haul coherent links, but not as much within short reach data centers links. A fully integrated dual-polarization heterogeneous silicon transmitter is shown in Fig. 11 [88]. Here, power from a single DFB laser is split between two independent MRMs operating at 130 Gbps each, and then combined into orthogonal polarizations using a polarization rotator and combiner to achieve a total of 260 Gbps data rate for a single wavelength. Since the laser is integrated, there is no need for active polarization control or polarization maintaining fiber for the transmitter. The data is transmitted across a short length (∼3 m) of single-mode fiber, and then de-multiplexed on a separate chip using an integrated polarization controller consisting of a polarization splitter rotator, and Mach-Zehnder interferometer [89]. This polarization demux chip compensates for any polarization rotation in the fiber link, and recovers the two initial data streams, which are measured on a digital communication analyzer (DCA) and shown below. For each of the two eyes at 130 Gbps, the transmitter dispersion eye closure (TDECQ) is 3.0 dB, and the difference in TDECQ between an integrated and off-chip polarization demux is only 0.4 dB, showing that the integrated polarization controller is effective for this link over short distances.

Fig. 11.

(a) Micrograph and (b) schematic of a dual-polarization transmitter with integrated laser, MRM, polarization rotator (PR), polarization beam combiner (PBC) and optical coupler (OC). The polarization demux chip consists of a polarization splitter, a rotator (PSR), and several phase tuners (PT). (c) The received 2 × 130 Gbps PAM4 eyes are measured on the DCA, with a TDECQ of 3.0 dB. (d) A comparison is made between the integrated polarization demux chip and off-chip fiber polarization components.

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SECTION V.

## Summary and Outlook

We have presented recent progress of heterogeneous silicon photonic devices and integrated circuits. High device performance and high integration level are enabling and extending its applications in the fields of communications, interconnects and sensors. Key metrics of heterogeneous integration are approaching or exceeding monolithic integration or hybrid integration. Heterogeneous integration with optimized actives and passives opens up new opportunities for a whole new class of devices with superior performance to discrete optical components. Narrow-linewidth lasers are a prime example to demonstrate such superiority. Fig. 12(a) summarizes the laser linewidth progress over the past 30 years. For monolithic lasers, the linewidth enhancement factor αH is the deciding factor and as a result, QD-based lasers can achieve narrower linewidth than QW-based lasers. Hybrid integration and heterogeneous integration, which utilizes the separately-optimized actives and passives can achieve ultra-narrow linewidth. In this scenario, ultra-low-loss SiN passive waveguides are dominating the best performances. Multiple approaches including extended grating-based external cavities, ring-resonator-based external cavities and self-injection locking with ultra-high-Qs offer different laser operations and contrasting laser linewidth reduction ratios. The design-optimized heterogeneous III-V/Si widely tunable laser linewidth is already performing better than commercial external-cavity diode lasers (ECDLs) using bulky external cavities (Fig. 12(b)). Further laser noise reduction is expected with integrated ultra-high-Q SiN ring resonators, similar to hybrid integration. The heterogeneous integration performance is only 2-3 years behind the state-of-the-art hybrid integration.

Fig. 12.

(a) Semiconductor laser linewidth progression for three different integration approaches [19]. (b) Frequency noise comparison between a commercial ECDL (turquoise), a heterogeneous III-V/Si widely tunable laser (green) and a hybrid-integrated III-V/SiN laser based on SIL (blue). The green and blue curves correspond to the Si-Ring and SiN-SIL data shown in a, respectively [9][34]. SIL stands for self-injection locking.

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While most of the contents covered in this paper is about heterogeneous integration for O-band and C-band applications, it is important to note that heterogeneous integrated silicon photonics is also playing an important role in applications for extended wavelength ranges, including visible, mid-infrared (mid-IR) and so on [90][91][92][93]. For visible applications, InGaN-based gain materials can be integrated with TiO2 or SiN waveguides on Si, as the optical refractive index (InGaN and TiO2) are similar to facilitate efficient evanescent coupling between the active and passive sections [94]. High-Q SiN resonators have been demonstrated in the blue and violet regions [95], and self-injection locked InGaN lasers to these resonators should revolutionize the performance of visible lasers for a variety of display, sensor and optical clock applications. For the mid-IR range, there has been extensive research investigating heterogeneously integrated quantum cascade lasers (QCLs) on silicon [96], interband cascade lasers (ICLs) [97], modulators [98] and photodetectors [99], etc. Other heterogeneous integrated silicon photonics devices that are not discussed in this paper including directly-modulated membrane lasers [100], MOS capacitor-based modulators [101][102] and avalanche photodiodes [103] are also seeing improved performance. These should help improve optical interconnect link budgets and reduce energy consumption further. Moreover, heterogeneous integration also provides exciting opportunities for novel applications including quantum information processing [104], photonic neural networks [105] and bio-sensing for life sciences [106].

Overall, the commercialization of heterogeneous integrated silicon photonic circuits is evolving rapidly. Close integration with electronics is the key enabler for next-generation high bandwidth datacenter network switches. Reliable, narrow-linewidth and high channel-count heterogeneous laser sources are likely to play a critical role in the next phase, which demonstrates one significant advantage of heterogeneous integrated silicon photonics over other platforms.

### ACKNOWLEDGMENT

The authors would like to thank Gordon Keeler, Tin Komljenovic, Songtao Liu, Paul Morton, Lin Chang, Paolo Pintus, Kaiyin Feng, and Alan Liu for useful discussions and thank DARPA MTO for funding most of the research at UCSB reported here.

"... High-Performance Silicon Photonics Using Heterogeneous Integration  ..."

TOP & Page Contents list

ABSTRACT [ The JWST is designed to  "multi-band image" and "collect spectroscopic data" in the "near infrared" portion of the EM spectrum for large numbers of very faint galaxies.

"multi-band image" : https://homepages.inf.ed.ac.uk/rbf/HIPR2/mulimage.htm

One of the James Webb Space Telescope’s (JWST) primary science goals is to characterize the epoch of galaxy formation in the universe and observe the first galaxies and clusters of galaxies. This goal requires multi-band imaging and spectroscopic data in the near infrared portion of the spectrum for large numbers of very faint galaxies.   Because such objects are sparse on the sky at the JWST resolution, a multi-object spectrograph is necessary to efficiently carry out the required observations.

We have developed a
fully programmable array of microshutters that will be used as the "field selector" for the multi-object Near Infrared Spectrograph (NIRSpec) on JWST.

This device allows apertures to be opened at the locations of selected galaxies in the field of view while blocking other unwanted light from the sky background and bright sources.

In practice, greater than 100 objects within the "field of view" can be observed simultaneously. This "field selection" capability greatly improves the sensitivity and efficiency of NIRSpec. In this paper, we describe the microshutter arrays, their development, characteristics, fabrication, testing, and progress toward delivery of a flight-qualified field selection subsystem to the NIRSpec instrument team.   ( Keywords: Field selector, JWST, Spectrograph, Near Infrared )

telescope "field of view" :  - https://jwst-docs.stsci.edu/jwst-observatory-characteristics/jwst-field-of-view

continued from above

... The James Webb Space Telescope (JWST) will be a cold 6.5 meter segmented telescope in space optimized for observations at infrared wavelengths.

Its purpose is to significantly advance our understanding of stars, planetary systems, and the formation and evolution of galaxies from the time of the first luminous objects in the universe.

To accomplish these goals, JWST has a complement of instruments for imaging and spectroscopy.

The Near-Infrared Spectrograph (NIRSpec), a multi-object spectrograph ( MOS ), will provide spectral information on selected objects at wavelengths in the range 0.6 - 5 µm. NIRSpec can operate with resolutions, R∼100 over its entire operating range and R∼1000 and ∼2700 from 1-5 µm.

" SPECTRAL resolution"  (  https://en.wikipedia.org/wiki/Spectral_resolution :  https://www.nrcan.gc.ca/maps-tools-and-publications/satellite-imagery-and-air-photos/tutorial-fundamentals-remote-sensing/satellites-and-sensors/spectral-resolution/9393 : http://www.vikdhillon.staff.shef.ac.uk/teaching/phy217/instruments/phy217_inst_dispersion.html )

NIRSpec has two 2Kx2K HgCdTe detector arrays in the focal plane built by Rockwell Science Center and a fully programmable field selector in the form of an array of small shutters (microshutters). The European Space Agency (ESA), will deliver the instrument with detectors and the field selector, a fully programmable two dimensional MEMS aperture mask, coming from NASA/Goddard Space Flight Center (GSFC).

A complete description of JWST, its mission, and scientific objectives is given in Gardner et al.1
Details of the NIRSpec design and its capabilities can be found in Posselt et al.2

( GOOGLE >   "Posselt" “NIRSpec- Near Infrared Spectrograph for the JWST" 2004 NO LUCK  )

- source:  https://www.spiedigitallibrary.org/conference-proceedings-of-spie/5487/0000/NIRSpec-near-infrared-spectrograph-for-the-JWST/10.1117/12.555659.short?SSO=1

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Figure 1 shows an artist’s concept of the NIRSpec instrument.

A large field of view (multi-object spectrograph) MOS provides a highly efficient means of simultaneously surveying the spectral properties of multiple objects.

To achieve this goal, the spectrographs require a field selection device to isolate the objects of interest from the surrounding objects and sky background.
Ground-based spectrographs have provided "field selection" by a variety of methods – customized aperture plates have been drilled for each object of interest 3 and fiber optic robots 4 have been designed to accurately position fibers to collect the light from only target objects.

A large field of view MOS provides a highly efficient means of simultaneously surveying the spectral properties of multiple objects.

To achieve this goal, the spectrographs require a field selection device to isolate the objects of interest from the surrounding objects and sky background. Ground-based spectrographs have provided field selection by a variety of methods– customized aperture plates have been drilled for each object of interest3 and fiber optic robots4 have been designed to accurately position fibers to collect the light from only target objects.

However, drilled aperture plates and the Further author information: (Send correspondence to Robert F. Silverberg) R. F. S.: E-mail: robert.silverberg@nasa.gov, Telephone: 1 301 286 7468 Infrared Spaceborne Remote Sensing and Instrumentation XV, edited by Marija Strojnik-Scholl, Proc. of SPIE Vol. 6678, 66780Q, (2007) · 0277-786X/07/$18 · doi: 10.1117/12.736118 Proc. of SPIE Vol. 6678 66780Q-1 Collimator Camera FPA Grating wheel IFU Refocus mechanism Fore optics Calibration assembly Micro-shutter assembly / Coupling optics Filterwheel —\ / Figure 1. Artist’s Concept of the NIRSpec Instrument from Posselt et al.2 The location of the microshutter subsystem is shown by the heavy line. mechanically complex and massive fiber robots are impractical for use in the space flight environment. Here we describe the development and current status of a fully programmable 2-dimensional array of microshutters. This MicroShutter Subsystem (MSS) is to be used as the field selector for NIRSpec. In this particular implementation of the technology, the devices are designed for use at near infrared wavelengths in a space-borne application, however, similar devices may be useful in other applications where their compact size, low power requirements, and great flexibility may enable enhanced performance. In the following sections, we discuss the motivation for using microshutters in this instrument, their design, characteristics, fabrication methods, testing and progress toward delivery of flight-qualified devices and an entire field selection subsystem. ... However, drilled aperture plates and the Further author information: (Send correspondence to: Robert F. Silverberg) R. F. S.: E-mail: robert.silverberg@nasa.gov, Telephone: 1 301 286 7468 Infrared Spaceborne Remote Sensing and Instrumentation XV, edited by Marija Strojnik-Scholl, Proc. of SPIE Vol. 6678, 66780Q, (2007) · 0277-786X/07/$18 · doi: 10.1117/12.736118

Proc. of SPIE Vol. 6678 66780Q-1 Collimator Camera FPA Grating wheel IFU Refocus mechanism Fore optics Calibration assembly Micro-shutter assembly / Coupling optics Filterwheel —\ /

Figure 1. Artist’s Concept of the NIRSpec Instrument from Posselt et al.2 The location of the microshutter subsystem is shown by the heavy line. mechanically complex and massive fiber robots are impractical for use in the space flight environment. Here we describe the development and current status of a fully programmable 2-dimensional array of microshutters. This MicroShutter Subsystem (MSS) is to be used as the field selector for NIRSpec. In this particular implementation of the technology, the devices are designed for use at near infrared wavelengths in a space-borne application, however, similar devices may be useful in other applications where their compact size, low power requirements, and great flexibility may enable enhanced performance. In the following sections, we discuss the motivation for using microshutters in this instrument, their design, characteristics, fabrication methods, testing and progress toward delivery of flight-qualified devices and an entire field selection subsystem.

2. DESIGN OF MICROSHUTTER ARRAY

2.1 Requirements NIRSpec’s field of view is designed to be large enough for there to be large numbers of high redshift galaxies at each JWST pointing. To meet its scientific requirements, NIRSpec must determine the spectra of many (∼100) galaxies simultaneously in each field of view. The aperture mask that selects the objects to be studied must provide high transmission in the region of the objects selected for study and be highly efficient in blocking the light from masked off regions– high contrast– to avoid contaminating spectra with data from foreground objects . In addition, the presence of the field selector should not greatly reduce the sky coverage in the field of view – apertures should have a high filling factor in the focal plane. The device must also be fully "addressable", withstand the rigors of launch and perform in the space flight environment. We summarize the requirements in Table 1.

2.2 Microshutter Array Construction A unit cell for a microshutter is shown in Fig. 2. The microshutter is suspended via a torsion bar hinge, which acts like a torsion spring. The microshutter blade is fabricated on 0.5µm thick high strength material and its static position is in ...

Proc. of SPIE Vol. 6678 66780Q-2 /

Table 1. Microshutter design requirements.
Microshutter System Format 342x730
Unit Cell Size ∼100 µm x ∼200 µm
Contrast Ratio >2000 (>10000 goal)
Operating Environment Vacuum
Operating Temperature ∼ 35K
Full Subsystem Mass ≤10 Kg
Average Power 40 mW

the plane of the ”front” side of the microshutter array (MSA) as seen in the figure 2.

The microshutter blade surface is coated with thin stripes of CoFe (∼90% Iron and 10% Cobalt) to provide high magnetic permeability to the microshutter blades for "actuation", the process of moving them to their latched position.

This magnetic material is required for proper operation of the addressing scheme which we describe in section 2.3.

In addition, the microshutter blades are covered with a thin layer of electrically conductive material, a metal nitride. This material serves three purposes; 1) to block passage of light through the very thin silicon nitride microshutter blades when they are closed and 2) provide electrical contact for addressing and 3) to improve flatness at cryogenic temperatures.

The "flatness" requirements are discussed below. A light shield surrounds the ”front” side of each microshutter blade (see fig. 3) that blocks the passage of light around the microshutter when the microshutter blade is in the closed position.

The entire array of microshutters is supported by the thick (∼120 µm) silicon ”egg crate” structure (fig. 4).

The NIRSpec requirements call for a field of view of about 3.6 INCHES?  x 3.6 INCHES? . The microshutter subsystem must cover this area with 342 microshutters in the imaging direction and 730 microshutters in the spectral dispersion direction. With the plate scale in the NIRSpec, this corresponds to a physical region approximately 75 mm on a side. In addition, the MSS layout must provide space for some fixed apertures. To satisfy this requirement, the flight MSS will be assembled from four sub-array assemblies, each of which contains a MSA in a 171x365 configuration.

MSA-FIELD-SELECTOR-02.JPG

Figure 2. SEM Image of microshutter. The figure shows an SEM image of a portion of a wafer during processing. At this stage of processing there is no light shield so the microshutter blade is visible. The torsion hinge can be seen as well as the CoFe stripes that give the microshutter blades their magnetic susceptibility. Light shields, fabricated later in the processing, will cover all gaps around the periphery of the rectangular microshutter blade so high contrast can be achieved.

In addition, the MSS layout must provide space for some fixed apertures. To satisfy this requirement, the flight MSS will be assembled from four sub-array assemblies, each of which contains a MSA in a 171x365 configuration.

The fabrication of microshutter arrays is done using silicon micromachining techniques in the GSFC Detector Development Laboratory.

Silicon nitride was found to be the material of choice for the microshutters due to its superior strength and excellent mechanical and thermo-mechanical properties.

Using this material, the microshutter blade is removed entirely from the optical path by a rotation of 90◦. In testing of early material samples, this large rotation has been repeated up to 109 times before signs of fatigue appeared. This is well beyond the requirements needed for the NIRSpec planned in orbit operations.

Figure 3. Front side of a MSA. The figure shows two images of the front side of an array. The inset shows the details of the light shields.

MSA-FIELD-SELECTOR-04.JPG

Figure 4. Back side of a MSA. The figure shows three images of the backside of an array, each at greater magnification. At the highest magnification the details of the individual shutters and their construction can be seen. Micromirror MEMS devices have stringent flatness requirements because they are used in reflection. For a microshutter device, these requirements are more relaxed, but significant deviations from flatness or tilting of the microshutter blades can compromise the ability of the microshutter to close properly and dramatically reduce contrast. Although some deviation

Proc. of SPIE Vol. 6678 66780Q-4 —U ? FEO k.
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from flatness can be tolerated at room temperature, a tighter specification (∼2 µm) is required at the operating temperature of ∼35K. To help meet the flatness requirements at low temperatures, the thickness of the metal nitride used for the microshutter blade electrical contacts is carefully controlled so a balance between the metal nitride film stress and the silicon nitride stress can be achieved. The results is a microshutter blade that is slightly bowed at room temperature, but becomes nearly flat at 35K. The microshutters are controlled by a cross addressing scheme (see section 2.3). This technique is used because it requires no active electronics in the unit cell and all addressing electronics is external to the MSA chip. Having no onchip electronics leads to simpler fabrication procedures. With no unit cell electronics to hide, the microshutter blades can occupy a large fraction of the unit cell area, allowing higher fill factor for the array. The cross addressing technique requires deposition of electrodes connected to the vertical walls on the torsion hinge side of each unit cell where the microshutter blades can be latched. The metal nitride coating on the microshutter blades themselves provides the electrode for the blades. Aluminum is used for the vertical wall electrodes and is deposited by angle deposition. To stop the light leaking through the gaps between each microshutter blade and the surrounding silicon support structure and torsion hinge , an aluminum light shield is fabricated which overhangs the gaps. A larger overhang will improve contrast, but results in lower filling factor due to the larger area that the light shield occupies around the edges of each unit cell. A more complete description of the development, fabrication, and integration of microshutter arrays can be found in Li et al.5 2.3 Operation The microshutters arrays are magnetically actuated, electrically addressed and electrostatically latched. Early tests showed that while purely electrostatic actuation might be possible, unacceptably high voltages were required and are not practical in the space environment. Instead a magnetic actuation combined with electrostatic latching is used. A narrow strong quadrupole magnet is swept across the array; in synchronism with the sweep, electrical signals are applied to the microshutter blades and to the vertical walls to cause all the microshutters to open. On the return sweep of the magnet, the desired pattern of open/closed shutters is achieved by allowing shutters to close will leaving open ones in the desired configuration to remain open. Figure 5 shows a schematic depiction of the array configuration process. In practice, the microshutter release electronics signals are also synchronized with magnet position as it returns to its home position. Because released shutters are captured by the magnetic field, they approach their light shields gently rather than with all the momentum created by the torsion hinge force. This technique avoids the microshutter blades slapping into the light shields each time a microshutter blade is released. This synchronized release technique greatly improves MSA lifetime.

MSA-FIELD-SELECTOR-05.JPG

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Figure 5. Cartoon showing the configuring of a MSA. The diagram shows the sequence required to configure the array to an arbitrary pattern of open/closed shutters. a) All the shutters are in their static closed position at the outset. b)Voltages are applied to all rows and all columns and the magnet sweeps across the array and forces the shutters close to the sidewall. Because voltages of opposite polarity are present on the sidewall and the microshutter blade, a strong electrostatic force results and the microshutter blades all latch electrostatically resulting in a fully open array. c) Next, the voltage on the columns in the row that is being configured is set to zero and the voltages on the column in that row that are to be closed are turned off. Microshutters that were open continue to be held open by the column voltage alone and microshutters that are to be closed release from the side wall because there is neither voltage on the sidewall nor microshutter blade electrodes. d) After cycling through all the rows, the desired configuration is achieved. 2.4 Packaging Once the 171x365 MSA chips are fabricated, they must be integrated into the MSA quadrant assembly. This assembly combines the control electronics with the MSA chip and provides positional reference and mounting points for mechanical

Proc. of SPIE Vol. 6678 66780Q-5 dispersion direction 365 shutters and optical alignment of the assembly (see Fig. 6. Four of the quadrants must be mounted in a 2x2 mosaic to produce the full Microshutter Subsystem (MSS). This layout of the MSS is shown schematically on the right side of fig. 6. Note that the NIRSpec focal plane contains several fixed slits and an aperture for the integral field unit (IFU). This MSS design offers some flexibility as each MSA quadrant is manufactured and tested independently before integration into the MSS. The MSA chips are mounted on quadrant substrates and are made from 2mm thick single crystal silicon. Since this silicon material is the same as the 120 µm thick support structure in the microshutter chip, there are minimal thermal stresses when the unit is cooled to the operating temperature of ∼35K. A clear aperture is cut in the quadrant substrate where the MSA chip will be mounted. There are more than 1000 electrical connections around the periphery of this aperture for making the required electrical contacts to the MSA chip. These connections are made with both bump bonding and wire bonding techniques. The MSA quadrant assembly also contains the electronic chips and other required connectors used for control and monitoring of the MSA quadrant assembly. 2.5 Electronics The electronics on the MSA quadrant assembly consists of five high voltage 128-channel serial to parallel converters and shift registers. These devices each contain four 32 bit shift registers and are customized radiation-hardened versions of a commercial device manufactured by Supertex Inc. In this application they must operate at the focal plane temperature of ∼35 K. These devices are controlled by external electronics operating at a higher temperature. The entire microshutter package including the magnet transport mechanism must operate with an average power of less then 40 mW. 2.6 Optical Performance Microshutters in the array must meet critical optical performance requirements. These include high transmission for open microshutters to let in as much light as possible from the faint objects being studied and very low transmission at closed microshutters to block out the light of the sky and any bright foreground objects in the large field of view of NIRSpec. Significant light leakage through closed microshutters results in excess photon noise from the background light and possible contamination of the spectrum of the selected object. Transmission losses are primarily set by the unit cell design and diffraction effects. As long as the microshutter blade is latched to the vertical wall and is totally behind the light shield, there is the inevitable geometrical loss due to the area of the unit cell blocked by the light shield. For the devices discussed here, the filling factor due to the design geometry is ∼67%. Diffraction losses are, of course, wavelength dependent.

MSA-FIELD-SELECTOR-06.JPG

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Figure 6. Fully Assembled MSA Quadrant. At the left side of the figure there is a picture of a fully assembled quadrant. The microshutter array is in the upper right. Around the periphery are the electronics. On the right side of the figure the optical locations of the four MSA quadrants are shown relative to the detector array. The locations of the IFU aperture and the fixed slits are also shown.

Proc. of SPIE Vol. 6678 66780Q-6 F-S Light Baffle Mosaic Integration Plate linkage / Daughter Flexures Launch Connectors with Board Lock Bracket llamess Mechanism Iso-grid

MSA-FIELD-SELECTOR-07.JPG

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Figure 7. Artist’s Concept of the Entire Microshutter Subsystem. The integration of the four quadrant assemblies can be seen in the lower right of the figure. On the left, is the magnet transport mechanism. A single magnet is used for all four quadrants. Since the microshutters are designed to move against the sidewalls, they should be hidden under the light shields when they are latched to the walls. Thus the transmission characteristics are primarily set by the light shield design and the wall thickness in the microshutter array chip. For JWST purposes, contrast is the ratio of the power at the detector with light coming through the microshutter aperture when open to the power at the detector when the microshutter blade is in the closed position. The leakage through the microshutter when it is closed is primarily determined by the flatness of the microshutter, the light shield design and the wavelength of the light. Our tests have shown that a high contrast closure can be achieved as long as the gap between the microshutter blade and the light shield does not exceed a few microns. A detailed discussion of the optical performance, modelling of the light leakage and testing of the microshutters is given in Kutyrev et al.6 2.7 MSS Integration The MSA quadrants must be integrated into the MSS. In addition to the four MSA quadrants in correct optical locations (see fig.7), this configuration contains the magnet transport assembly and light baffles. The MSS mount also contains provision for proper optical alignment in NIRSpec. Electronics for magnet transport control is not contained in the MSS, but in external microshutter control electronics. 2.8 Flight Qualification Prototype MSAs were subjected to the rigors of launch and space flight-like conditions. Prototype MSA quadrant assemblies were subjected to acoustic and vibrational testing at qualification levels relevant to the JWST’s Arianne launch vehicle. In addition to those tests, a prototype array was subjected to life testing as it accumulated 100,000 cycles– approximately 2.5 times the NIRSpec requirement. Electronic components on the quadrant assembly were subjected to ∼ 60-100 krad of ionizing radiation from a Co60 gamma ray source. Although the MEMS microshutter devices were not expected to be particularly sensitive to ionizing radiation, a MSA was also tested using the gamma ray source. A microshutter that fails in the open configuration is a more serious problem than a microshutter that fails in the closed configuration. An open microshutter in a row means that some light will fall on the detector from that open microshutter and contaminate the spectrum of the intended object. Because most NIRSpec sources are very faint galaxies, this contamination is troublesome. A row with a single failed open microshutter may not permit NIRSpec to perform up to its full requirement on a object in that row. On the other hand, a microshutter that is failed closed cannot be used for an object at that position;

Proc. of SPIE Vol. 6678 66780Q-7 since NIRSpec is generally in a source-rich environment, the loss of an opportunity to observe a few objects due to failed closed microshutters has little overall impact on achieving the scientific objectives. Because of this situation, acceptable MSAs can have up to 3% failed closed microshutters at the beginning of their life (BOL) but no more than 3% of the rows may have one or more failed open microshutters. At the end of life, up to 20% of the microshutters may fail closed, but only 6% of the rows may have one or more failed open microshutters. In our testing to date, we have not experienced such a high rate of failures of either type. 2.9 Summary Large scale two dimensional arrays of MEMS-based microshutters have been developed. These fully addressable devices are simpler, less massive, and lower power than some current methods for field selection used in ground-based applications. The devices are magnetically actuated, electrically addressed and electrostatically latched. Some of the prototype devices we are currently fabricating, assembling and integrating into fully functional assemblies have been subjected to acoustic, vibrational and environmental conditions simulating the rigors of launch and operation in space. Lifetime testing has shown that the devices can meet NIRSpec requirements. At the conclusion of these tests the devices continued to meet specifications. We expect that a completed MSS will be provided to ESA for use in NIRSpec. ACKNOWLEDGMENTS The development of these MEMS represents the work of a great many colleagues and support staff at the Goddard Space Flight Center over many years. We greatly appreciate their efforts in bringing the microshutter project to its current state of readiness. The microshutter effort is supported by the European Space Agency which is responsible for the NIRSpec instrument.

MSA-FIELD-SELECTOR-REFERENCES.JPG

REFERENCES

1. J. P. Gardner, J. C. Mather, M. Clampin, R. Doyon, M. A. Greenhouse, H. B. Hammel, J. B. Hutchings, P. Jakobsen, S. J. Lilly, K. S. Long, J. I. Lunine, M. J. McCaughrean, M. Mountain, J. Nella, G. H. Rieke, M. J. Rieke, H. Rix, E. P. Smith, G. Sonneborn, M. Stiavelli, H. S. Stockman, R. A. Windhorst, and G. S. Wright, “The James Webb Space Telescope,” Space Science Reviews 123, pp. 485–606, 2006.

2. W. Posselt, W. Holota, E. Kilinyak, G. Kling, T. Kutscheid, O. L. Fevre, E. Prieto, and P. Ferruit, “NIRSpec- Near Infrared Spectrograph for the JWST,” Proc. SPIE 5487, pp. 688–697, 2004.

3. D. Bottini, B. Garilli, D. Maccagni, L. Tresse, V. Le Brun, O. Le Fevre, J. P. Picat, R. Scaramella, M. Scodeggio, G. Vettolani, A. Zanichelli, C. Adami, M. Arnaboldi, S. Arnouts, S. Bardelli, M. Bolzonella, A. Cappi, S. Charlot, P. Ciliegi, T. Contini, S. Foucaud, P. Franzetti, L. Guzzo, O. Ilbert, A. Iovino, H. J. McCracken, B. Marano, C. Marinoni, G. Mathez, A. Mazure, B. Meneux, R. Merighi, S. Paltani, A. Pollo, L. Pozzetti, M. Radovich, G. Zamorani, and E. Zucca, “The Very Large Telescope Visible Multi-Object Spectrograph Mask Preparation Software,” PASP 117, pp. 996–1103, Sept. 2005.

4. D. Fabricant, R. Fata, J. Roll, E. Hertz, N. Caldwell, T. Gauron, J. Geary, B. McLeod, A. Szentgyorgyi, J. Zajac, M. Kurtz, J. Barberis, H. Bergner, W. Brown, M. Conroy, R. Eng, M. Geller, R. Goddard, M. Honsa, M. Mueller, D. Mink, M. Ordway, S. Tokarz, D. Woods, and W. Wyatt, “Hectospec, the MMT’s 300 Optical Fiber-Fed Spectrograph,” PASP 117, pp. 1411–1434, 2005.

5. M. J. Li, T. Adachi, C. Allen, S. Babu, S. Bajikar, M. Beamesderfer, R. Bradley, K. Denis, N. Costen, A. Ewin, D. Franz, L. Hess, R. Hu, K. Jackson, M. Jhabvala, D. Kelly, T. King, G. Kletetschka, A. Kutyrev, B. Lynch, T. Miller, H. Moseley, V. Mikula, B. Mott, L. Oh, J. Pontious, D. Rapchun, C. Ray, K. Ray, E. Schulte, S. Schwinger, P. Shu, R. Silverberg, W. Smith, S. Snodgrass, D. Sohl, L. Sparr, R. Steptoe-Jackson, V. Veronica, L. Wang, Y. Zheng, and C. Zincke, “Complex MEMS device: Microshutter Array System for Space Applications,” in Micro (MEMS) and Nanotechnologies for Defense and Security, T. George and Z. Cheng, eds., Proc. SPIE 6556, pp. 716–731, 2007.

6. A. Kutyrev, R. Arendt, S. H. Moseley, R. Boucarut, T. Hadjimichael, M. Jhabvala, T. King, M. J. Li, J. Loughlin, D. Rapchun, D. Schwinger, and R. F. Silverberg, “Programmable Microshutter Arrays for the JWST NIRSpec: Optical Performance,” IEEE Journal of Selected Topics in Quantum Electronics 10, pp. 652–661, 2004.

source:    https://www.researchgate.net/publication/253204493_A_microshutter-based_field_selector_for_JWST's_multi-object_near_infrared_spectrograph_-_art_no_66780Q

https://www.researchgate.net/publication/253204493_A_microshutter-based_field_selector_for_JWST's_multi-object_near_infrared_spectrograph_-_art_no_66780Q

NIRSpec Optics

NIRSpec is an all-reflective instrument with 14 mirrors, seven interchangeable filters and an opaque shutter that can be combined with seven interchangeable dispersers and a plane mirror.

JWST instruments share a focal plane, shown in Figure 1. NIRSpec is on the left, with an aperture position angle rotated approximately 138° counter-clockwise relative to the +V3 axis. JWST's other near-infrared instruments have coordinate axes that are roughly aligned with the V2 and V3 coordinates (shown at center right).

..."

Microshutters Arrays for the JWST Near Infrared Spectrograph

SOURCE:  https://studylib.net/doc/14294837/microshutters-arrays-for-the-jwst-near-infrared-spectrograph

"...  Microshutters Arrays for the JWST Near Infrared Spectrograph

S. H. Moseley*,a, R. Arendta,b, R. A. Boucaruta, M. Jhabvalaa, T. Kinga,  G. Kletetschkaa,c,A. S. Kutyreva,b,  M. Lia, S. Meyera, D. Rapchuna,d, and R. F. Silverbergaa Goddard Space Flight Center, Greenbelt, Md. 20771b Science Systems and Applications, Inc. ,10210 Greenbelt Rd., Suite 600 Lanham, MD 20706cCatholic University of America, Washington, D.Cd Global Science & Technology, Inc.,7855 Walker Drive, Suite 200, Greenbelt, MD 20770.ABSTRACTThe Near Infrared Spectrograph (NIRSpec) for the James Webb Space Telescope (JWST) is a multi-object spectrographoperating in the 0.6-5.0 μm spectral range. One of the primary scientific objectives of this instrument is to measure thenumber and density evolution of galaxies following the epoch of initial formation. NIRSpec is designed to allowsimultaneous observation of a large number of sources, vastly increasing the capability of JWST to carry out itsobjectives. A critical element of the instrument is the programmable field selector, the Microshutter Array.  The systemconsists of four 175 x 384 close packed arrays of individually operable shutters, each element subtending 0.2” x 0.4”onthe sky. This device allows simultaneous selection of over 200 candidates for study over the 3.6’ x 3.6’ field of theNIRSpec, dramatically increasing its efficiency for a wide range of investigations.  Here, we describe the development,production, and test of this critical element of the NIRSpec.Keywords:  Microshutter Array, MEMS, JWST, Infrared Spectroscopy, Early Universe, Galaxy Formation1.INTRODUCTION  ..."

Here, we describe the development, production, and test of this critical element of the NIRSpec.

Keywords: Microshutter Array, MEMS, JWST, Infrared Spectroscopy, Early Universe, Galaxy Formation

1. INTRODUCTION We are developing a large format microshutter array for use on the Near Infrared Spectrograph (NIRSpec) on the James Webb Space Telescope (JWST)1, 2.

It functions as an adaptive input mask for the multi-object instrument. Given an image of an area on the sky, the microshutter array can be programmed to admit light from an ensemble of selected objects, providing a capability for simultaneously observing a large number of discrete objects. The device consists of a two-dimensional array of closely packed slits, each with an independently selectable shutter element. The pattern of shutters to be opened for a given observation is determined by the distribution of objects of interest on the sky and the requirement that their spectra should not overlap on the detector. In cases where there are large numbers of candidate objects in the field, such as the study of high redshift galaxies3 , the shutters can be configured to make highly efficient use of nearly the entire detector array, with hundreds of spectra being obtained simultaneously. A successful aperture mask must provide high transmission efficiency in regions where it is open, excellent blocking in regions where it is closed, and must extend over the 3.6’ x 3.6’ field of view of the spectrograph. In the design we are developing for JWST, the slits are made on a thin aluminized silicon wafer (Fig. 1). The shutter in each unit cell is a silicon nitride vane metallized with aluminum and overcoated with magnetic stripes. The shutter is supported on a silicon nitride torsion hinge. The shutter is opened by scanning a magnetic across the array, and can be latched electrostatically in the open position. Selection is accomplished by a crosspoint addressing system in which all shutters are released except those required for the particular observation (Fig. 2). Following an observation, the system can be rapidly reconfigured for the next field under study. * Harvey.Moseley@nasa.gov, phone 301 286-2347, fax 301 286-1617 We have demonstrated the required operation and performance for the NIRSpec on prototype arrays. Here we describe the operation and optical performance of the devices, and discuss progress on the final steps in development, the demonstration of required lifetime and the scaling of the prototype arrays to the required format.

2. TECHNICAL APPROACH In the development of our concept for the field selector for the NIRSpec, there were several important decisions, which defined the path we chose. The most fundamental decision was to produce a device that operated in transmission rather than in reflection as existing micromirror arrays do. There were three motivations for this choice; first, with appropriate baffling, a transmissive microshutter can achieve very high contrast between open and closed state and high transmission in the open state. Second, a transmissive solution could be interchangeable with a mechanical slit alternative, providing an effective backup for the instrument in case of failure in development. Finally, in a reflective system such as with mirrors, the flatness of the reflectors is critical; there is no corresponding requirement for a transmissive system. Adding complexity to the development was the requirement that the microshutter array operate at the low temperature of the NIRSpec, around 37 K. In order to achieve a high filling factor for an array of shutters on the required scale ( ~ 100 µm x 200 µm), silicon micromaching was the obvious choice for the thin walled shutter support structure. Silicon nitride was a good choice for the torsion flexure and shutter vane because of its high strength and resistance to fatigue ( Fig. 1). Initial experiments were done with thin silicon, which also worked well, but silicon nitride was ultimately chosen because of its higher strength and lower cost. Achieving 90˚ rotation of the shutter on its torsion flexure in a system requiring large fractional open area was a challenging problem. While it could be achieved using electrostatic fields, the very high voltages required presented difficult systems problems. We ultimately chose to deposit a magnetically permeable material on the shutters, and to scan a magnet across the array to open the shutters. A tripole magnetic field is used, so that as it scans across, the shutter experiences a magnetic field which rotates through 180˚ during the scan, rotating it into contact with the wall of the support frame, where it can be electrostatically captured to the vertical electrode (Fig.2) with a reasonably low voltage. After configuring the array to the desired configuration (actuation), the shutters are held in the open position by an electrostatic field created by the voltage difference between the shutter vane and the vertical electrode. In the array design, the vertical electrodes are connected to rows, while the shutter vanes are connected to the columns (Fig. 2). During actuation and capture of the shutters, a positive voltage is applied to the shutters, and an equal negative voltage applied to the vertical electrode. The voltages are chosen so that after capture, either the row voltage or the column voltage is sufficient to hold the shutter open. If this is the case, a shutter will only be released if the row voltage and the column voltage are set to zero. In this manner, each shutter in the entire array can be individually released as required. This addressing scheme is particularly appealing, because it requires no active electronic elements on the array; all shutters can be individually controlled using an external crosspoint address scheme. ,

Figure 1. This schematic cross section (side view) of a microshutter unit cell shows the key elements required for shutter operation. A 100 µm thick Si grid with 7 µm wide walls supports the shutter. The shutter is connected to the grid through a torsion bar made of silicon nitride (left). The shutter is metallized to provide both optical opacity and electrical conductivity. Stripes of a highly permeable material (CoFe) allow the shutter to be opened to 90˚ (against the vertical electrode) using a scanning magnet. Electrical connections are made to the shutter blade on the front side of the wafer and the vertical electrode on the support structure wall. Cantilevered light shields minimize light leakage in the closed position, providing high open/closed contrast for the shutters. Each unit cell is 100 µm x 200µm. All Closed Actuate and Latch All Release Selected Row 1 Hold Configuration

Figure 2. The shutter selection begins with all shutters closed. The voltage on all rows (vertical electrodes) is set to ~-30 V and on all columns (shutters) to ~+30 V. When the magnet is scanned across the array, the shutters are rotated against the vertical electrode and captured by the electrostatic field between the shutter and vertical electrode. In the third frame, shutters on the top row that are required to be closed are released. The row is grounded. All shutters on columns with +30 V bias remain captured (open), but those which are grounded are released. Also, all shutters on the grounded column whose vertical electrode is set to –30 V are held. Only those where both row and column are grounded are released. In this way, all shutters can be individually released, allowing an arbitrary pattern to be generated. After the pattern has been produced, the rows and columns are set to their original voltages to securely hold the pattern.

3. REQUIREMENTS AND DESIGN The Microshutter Array must meet the scientific and technical requirements set by the NIRSpec instrument (Table 1). Listed here are a subset of key requirements. In this section, we discuss progress in meeting these requirements. • Random Access Addressing – Must allow the opening of any shutter distribution • > 200 objects simultaneously targeted • Must cover NIRSpec FOV (9 sq. arcmin) • Contrast – Must have open to closed transmission ratio of >2000 (10,000 goal) • Lifetime – Must operate for 9.4 x 104 cycles with minimal failures • Must operate in the JWST environment – T ~ 37 K • Must meet power dissipation requirements (40 mW average at 37 K) – Must fit envelope in NIRSpec instrument – Must meet mass requirements (10 kg) – Radiation - 48 kRad life dose Table 1. Performance requirements for the Microshutter Array.

3.1. Optical Performance The basic function of the microshutter array is to block light where it is closed position, and to transmit efficiently where it is open. The NIRSpec instrument requires that this contrast ratio be >2,000 with a goal of 10,000. In order to block light effectively when closed, the shutter vane must be opaque, and leakage around the closed shutter should be limited. The shutter is covered with 2000 Å of aluminum, which is sufficiently opaque to provide adequate blocking. The leakage around the shutter is limited by having a cantilevered light shield that overlaps the metallized shutter vane in the closed position. With proper alignment, any residual leakage is due only to internal reflections and multiple diffraction, allowing the contrast ratio goal of 10,000 to be met. The transmission efficiency of the array is set by two loss factors, the geometrical blocking of the slit, where only part of the image fits within the slit, and the light which is transmitted through the slit, but is diffracted at angles large enough to miss the subsequent pupil. Both of these losses are also suffered using conventional slits. The shutter’s size is selected to be well-matched to the diffraction image of JWST at the long wavelength of the NIRSpec band. The slits have been chosen to be 0.2” in the dispersion direction and 0.4” in cross dispersion. To cover the entire 3.6’ x 3.6’ field of view of the NIRSpec, we use four arrays in a 175 x 384 format ( the 175 shutters are in the cross dispersion direction)( Fig. 4), with permanent fixed slits between the arrays for observations requiring the very highest contrast. During the 10 year operational life of the NIRSpec, the shutters may be configured as many as 93,000 times. The microshutter arrays and their actuating mechanism must be reliable over this lifetime, so that at the end of life, no more than 5% of the rows in the microshutter contain permanently open or leaky shutters.

3.2. Mechanical Requirements and design 3.2.1. Magnet transport The shutters are opened by scanning a tripole magnet across detector array. In the present design, the magnet is 500 µm above the shutter array as it scans across. The scan mechanism must reliably transport the magnet back and forth across the array 93,000 times in the mission lifetime. The average power dissipation must be low, allowing us to meet the requirement of 40 mW time averaged dissipation for the entire cryogenic system. While achieving low power dissipation and long life operation at cryogenic temperatures is challenging, no new techniques are required to produce this mechanism. 3.2.2. Interface and support of microshutter array The microshutter array must be operable over a wide range of temperatures, from room temperature to its operating temperature of ~37 K. One area requiring particular attention is the packaging and mounting of the microshutter array. We have chosen to mount the most challenging component of the system, the microshutter array itself, on a silicon carrier, so this system is as athermal as possible. The support structure and magnet transport stage are made of titanium, so a kinematic mount is required avoid the transmission of internal stresses into the silicon carrier. The required flexures will be machined in the titanium carrier, and the silicon will be bonded to them. The connection between the microshutter carrier and the titanium flexures and any heat straps must perform the thermal function of cooling the device from room temperature and maintaining it at an adequately low operating temperature with the addressing electronic operating.

3.3. Electronic design The addressing of the microshutter array is accomplished through the control of voltages on the shutter vanes and vertical electrodes. The vertical electrodes are connected in rows on the back side of the wafer, the shutter vanes connected in columns from the front side of the array. Electrical connections from the front side of the microshutter to the carrier are made through indium bumps. Connections from the rows of vertical electrodes to the carrier are made through wire bonds. Each row and column is connected to an output of a 128 bit shift register with high voltage outputs, allowing logical control of all rows and columns for full addressing capability. The warm electronics package must drive the scan motor and provide the proper pattern of voltages on the rows and columns to generate the required patterns of open shutters for each observation. . The voltages which are required are ~±30 V, so we require high voltage drivers. Initial work has been done using a Supertex HV583 address driver. It works well at room temperature and at the required operating temperature of 37 K, but does not meet the JWST requirement of nominal operation after exposure to 48 kRad of ionizing radiation. A custom chip is being developed to reduce the sensitivity to radiation exposure.

4. TEST RESULTS
The process for shutter fabrication has been under development for about three years, and devices which are now being built can meet many of the requirements for the NIRSpec flight arrays. We will discuss the areas of success, and identify areas that remain to be demonstrated for flight. 4.1. Actuation, Latching, and Addressing The evaluation of a microshutter must establish; 1) proper electrical function, no shorts or opens. 2) proper mechanical function of shutters. 3) the ability to latch shutters. 4) the ability to deselect shutters and create a desired pattern. The electrical tests are carried out at many steps in the production process, at the completion of processing, and after bonding to the carrier chip. Mechanical function of the shutters is evaluated at the completion of processing prior to mounting by rotating the shutter in a strong magnetic field (the rotisserie facility)(Fig. 3). After mounting on the carrier board, it can be tested with the tripole magnet, both warm and cold in the comprehensive test cryostat. In this facility, we can also test our ability to latch and to address the shutters (Fig. 4). In general, if shutters can be latched, and there are no shorts or opens in the electrodes, they will be addressable.

Figure 3. In this mechanical test, the microshutter array is illuminated from the rear. In the frame at the left of the figure, the shutters are opened with a strong large area solenoidal magnetic field. Only about 25 of the 8192 shutters in this prototype fail to open. In the frame on the right, the field is turned off, and the shutters close. In this sample, there are essentially no shutters which remain permanently open at beginning of life. These pictures show that shutters with high pixel yield can be produced.

Figure 4. A prototype microshutter array is configured to display a grid of ESAs. The addressing is generally good except for the inactive columns to the right of center , and partial addressability on several rows (due to electrical shorts in this sample). This array is >95% operable mechanically , so in this case, most observed imperfections are due to electrical problems rather than mechanical ones. Source Shutters open Shutters closed

Figure 5. Shutter transmission and contrast are measured using a slightly extended f/12 source, to simulate the beam at the position of the microshutter array in NIRSpec. The ratio of the intensity of the unobstructed beam to that with the open shutters in place yields the open transmission. The ratio of the open shutter intensity to the closed shutter intensity is the contrast ratio. For this shutter, the ratio was > 100,000. The NIRSpec requirement is >2,000 with a goal of 10,000.

4.2. Contrast

The contrast of the microshutter array is defined as the ratio of the transmission of a shutter when open to that measured when closed. This is an important parameter for the NIRSpec, because light leaking through closed shutters can cause excess photon noise in the spectra being measured, or systematic errors that will be difficult to remove from the spectra, especially in crowded fields. We have developed an optical bench that can provide a beam of similar focal ratio to that in the NIRSpec at the position of the microshutter array4 . In the tests (Fig. 5), we have focused a slightly extended source on the shutter plane, and measured its total intensity with the shutter removed, with the shutter in place and open, and with the shutter in place and closed. From these measurements, we can determine the contrast ratio, and have found that for modest light shield overlap (~1 µm), extremely high contrast ratios (> 100,000) can be achieved. A remaining challenge is to demonstrate that the high contrast ratio can be maintained over the operational life of the device. 4.3. Reliability One of the most demanding requirements on the microshutter array is the reliability. At the beginning of life, there can be permanently open shutters in at most 1% of the rows, and less than 5% of the shutters can be permanently closed. At the end of life, less than 5% of the rows can have a permanently open shutter, while 20% of the array may be permanently closed. The much more stringent requirement on the open failures is due to the fact that the grating will disperse light from a permanently open shutter over much of an entire row of detectors along the dispersion direction, significantly reducing their utility. These operability requirements are stringent, and impose significant fabrication and testing requirements. Tests of arrays fabricated during the process development have been obtained which suggest that significant infant mortality of shutters may occur5 . The probability of failure decreases proportionally with the number of cycles. Preliminary results from devices from a more mature version of the process show much less of this type of failure, so it is possible that we can produce arrays with little probability of failure in the required lifetime. If not, a burn in period followed by repairs will be required to reduce the probability of failure during the instrument life.

5. SUMMARY AND PLANS We are developing microshutter arrays for use as the programmable field selection on the NIRSpec on JWST. Prototype arrays demonstrate actuatability and addressability, and reach the contrast goals of JWST. Two major tasks remain in this development, scaling the arrays up to the full 175 x 384 format and demonstrating the required reliability. Rapid progress in being made in both these areas, and the JWST schedule requires that these problems be resolved by Aug. 2005.
We are confident that microshutter arrays will meet the requirements of NIRSpec, and will provide an important enhancement of the capability of JWST.

REFERENCES 1. H. Stockman, “The Next Generation Space Telescope: visiting a time when galaxies were young”, The Association of Universities for Research in Astronomy, Inc., 1997. 2. S. H. Moseley et al. ASP conf. 207, NGST Science and Technology Exposition, Sept 13-16, 1999 Woods Hole, MA, p 262, 2000 3. H. I. Teplitz, E. Malumuth, B. E. Woodgate, S. H. Moseley, J. P. Gardner,R. A. Kimble, C.W. Bowers, A. S. Kutyrev, R. K. Fettig, R. P.Wesenberg, and E. E. Mentzell, “Redshift Estimation from Low-Resolution Prism Spectral Energy Distributions with a Next Generation Space Telescope Multiobject Spectrograph,” PASP 112, pp. 1188–1199, Sept. 2000 4. A. S. Kutyrev, R. Arendt, S. H. Moseley, R. A. Boucarut, T. Hadjimichael, M. Jhabvala, T. King, M. Li, J. Loughlin, D. Rapchun, D. S. Schwinger, and R. F. Silverberg, “Programmable Microshutter Array for the JWST NIRSPEC”, accepted for publication in the Journal on Selected Topics in Quantum Electronics on Optical Microsystems, April 2004 5. T. T. King, G. Kleteschka, M. A. Jah, M. J. Li, M. D. Jhabvala, L.L. Wang, M. A. Beamesderfer, A. S. Kutyrev, R. F. Silverberg, D. Rapchun, D. W. Schwinger, G. M. Voellmer, S. H. Moseley, and L. M. Sparr, “Cryogenic Characterization and Testing of Magnetically-actuated Microshutter arrays for the James Webb Space Telescope”, to be published in the Technical Digest for the Hilton Head Solid-State Sensor, Actuator and Microsystem Workshop, 2004.

The NIRSpec enables scientists to obtain simultaneous spectra of more than 100 objects in a 9-square-arcminute field of view.

It provides medium-resolution spectroscopy over a wavelength range of 1 to 5 micrometers and lower-resolution spectroscopy from 0.6 to 5 micrometers. The NIRSpec employs a micro-electromechanical system "microshutter array" for aperture control, and it has two HgCdTe detector arrays.

### Other Resources:

NIRspec-ESA-05.JPG

NIRspec-ESA-04.JPG    MSA = Micro shutter Array ,  FPA = Focal Plane Assembly

NIRspec-ESA-03.JPG

NIRspec-ESA-02.JPG

NIRspec-ESA-01.JPG

( SEE 5 images saved NIRSPEC-ESA above  )

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SCIENCE MISSIONS
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JWST - NIRSpec's Design - JWST NIRSpec
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In a nutshell > https://www.cosmos.esa.int/web/jwst-nirspec/home
NIRSpec's Design
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Documents
- https://www.cosmos.esa.int/web/jwst-nirspec/papers

CONTENT:

NIRSPEC AND ITS DESIGN

NIRSPEC DESIGN OVERVIEW

NIRSpec optical design
(IMAGE) Schematic of the optical design of NIRSpec.   FORE, COLL, and CAM

NIRSpec is an all-reflective system with a total of 14 mirrors along with seven interchangeable dispersive elements and eight interchangeable filters.
The three major optical sub-systems (FORE, COLL, and CAM) are implemented as three mirror anastigmats (TMAs) that were built, aligned, and verified as stand-alone units in order to simplify instrument alignment.

FIGURE 1 >  ...

The Figure ABOVE (FIGURE 1) shows a simplified diagram of the optical path from the optical telescope element (OTE) to the NIRSpec focal plane assembly (FPA).

"jwst" "optical telescope element (OTE)"     SOURCE: https://jwst.nasa.gov/content/observatory/ote/index.html
"... The OTE Consists Of :

The optical train has three image planes (shown in blue : "OTE focus", MSA, FPA) and three pupil planes (shown in red).

MSA = Micro shutter Array ,  FPA = Focal Plane Assembly  [

The pupil planes are located (i) at the OTE primary mirror, which is the limiting pupil stop of the system, (ii) at the position of the optical filters, and (iii) at the position of the dispersive element (prism or grating).

The three image planes of the NIRSpec optical train are located (i) at the OTE focal surface, (ii) the micro-shutter assembly (MSA), and (iii) at the FPA.

THE NIRSPEC OPTICAL PATH
NIRSpec optical pathNIRSpec layout

Schematic of the NIRSpec optical train (top) and CAD rendering of the optical bench with the light path denoted in red (bottom).

The first element in the NIRSpec optical path is the Coupling Optics. It consists of two flat mirrors that "pick off" the NIRSpec portion of the OTE focal plane, and redirect the diverging OTE beam (with an f/20 focal ratio) into the NIRSpec fore optics (FORE). A field mask is positioned just in front of the first coupling mirror. The field mask together with a closed optical base plate structure and an internal pupil mask acts to minimize the amount of out-of-field stray light entering the NIRSpec beam.

The FORE optics telecentrically reimage part of the curved OTE focal surface onto the flat MSA aperture plane, creating an accessible pupil image at the filter wheel assembly (FWA). The field of view (FOV) is rotated 41.5° on the sky with respect to most other instruments, in order to take advantage of the natural symmetry axis of the OTE. At the position of the FWA, the beam is nearly collimated (minimizing longitudinal chromatic aberrations), with a nominal incidence angle (of the chief ray) of 5.3º in order to avoid ghosting. The filter wheel itself contains two broadband filters, four long-pass filters, and one clear aperture, all on CaF2 or BK7 substrates, as well an opaque position that serves as an instrument shutter and mirror for the internal calibration assembly (CAA).

The refocus mechanism assembly (RMA) determines the exact position where the telescope focal surface is reimaged onto the MSA. The RMA compensates for any changes in OTE focal position that may occur during launch and throughout the JWST lifetime. Due to the penta-prism geometry of the RMA, adjusting the focus does not affect wave front error (WFE) and does not translate the optical beam laterally.

After passing through the one or more apertures in the MSA plane, the f/12.5 optical beam enters the spectrograph optical system that starts with the collimator optics (COLL). The purpose of the COLL is to transform the divergent f/12.5 beam from the MSA into a highly collimated beam at the grating wheel position. The grating wheel assembly (GWA) contains seven dispersive elements for spectroscopy (a double-pass CaF2 prism and six reflection gratings), and an imaging mirror for target acquisition, flat fielding, and optical distortion calibrations.

After being dispersed or reflected by an optical element in the GWA, the NIRSpec beam enters the camera (CAM), which is the third and final TMA. The CAM focuses the collimated and dispersed beam onto the focal plane assembly. The nominal image scale at the FPA is 5.56 arcsec/mm, so an 18 μm pixel projects to 100 mas on the sky. The focal ratio at the FPA is f/5.6.
( IMAGE )

THE FOCAL PLANE ASSEMBLY AND SIDECAR ASIC

( IMAGE) SIDECAR ASICNIRSpec FPA
Drawings of the NIRSpec FPA (left) and a SIDECAR ASIC (right).
The NIRSpec FPA contains two closely butted HAWAII-2RG HgCdTe sensor chip arrays (SCAs) manufactured by Teledyne Imaging Systems (TIS). Each SCA has 2048 x 2048 pixels (of which 2040 x 2040 are sensitive to light) that can be addressed individually and read out in a non-destructive way. The light-sensitive portions of the two SCAs are separated by a physical gap of no more than 3.144 mm which – given the mean image scale at the FPA of 5.66"/mm in the dispersion direction – corresponds to 17.8" on the sky.

Because thermal stability is crucial for a good performance of NIR detectors, the FPA is mounted to a thermal strap that connects to a dedicated radiator. In this way, the NIRSpec SCAs can be cold-loaded, and maintained at a stable operating temperature using heaters controlled by a thermal control circuit. The operating temperature will be around 40 K.

The System for Image Digitalization, Enhancement, Control, and Retrieval (SIDECAR) ASIC is used for detector readout and control. It is a special-purpose electronic device individually matched to its corresponding SCA.

The complete detector system is provided by the Goddard Space Flight Center (GSFC) as one of NASA's contributions to the NIRSpec instrument.

FIXED SLITS, MSA, AND INTEGRAL FIELD UNIT

NIRSpec offers three different kinds of apertures for spectroscopy: several fixed slits for high contrast observations of single objects, and an Integral Field Unit (IFU) for 3D-spectroscopy, and the MSA for obtaining spectra of multiple sources (up to ~100) simultaneously. The MSA is mounted on the NIRSpec Optical Assembly (OA) through a mounting bracket that is also used to mount IFU.

NIRSpec MSA and slit layout

FIGURE: Superimposed sketch of the MSA and FPA image planes.

NIRSpec offers three different kinds of apertures for spectroscopy: several fixed slits for high contrast observations of single objects, and an Integral Field Unit (IFU) for 3D-spectroscopy, and the MSA for obtaining spectra of multiple sources (up to ~100) simultaneously. The MSA is mounted on the NIRSpec Optical Assembly (OA) through a mounting bracket that is also used to mount IFU.

The MSA consists of a 2 × 2 arrangement of quadrants. Each quadrant has 365 shutters along the dispersion axis and 171 shutters along the spatial axis. Pairs of quadrants are separated by approximately 9 mm (22.88") along the dispersion axis and 13.95 mm (36.1") along the spatial axis. The latter gap holds the so-called "cruciform" that provides the permanently open fixed slits and the IFU aperture. Within each quadrant, the shutter pitch is ~0.26" along the dispersion axis and ~0.51" along the spatial axis. The open area of an open shutter is ~0.20" x 0.45". The "walls" separating individual shutters are ~0.06" wide. The MSA is provided by NASA's GSFC as the second contribution to the NIRSpec instrument.

The benefit of the IFU is the ability to obtain the spectrum of a contiguous, extended area on the sky. This is achieved by optically re-arranging all spatial resolution elements within the field of view into a virtual long slit by means of "image-slicing" techniques. The resulting virtual slit can then be dispersed without confusion by neighboring spatial elements. In the case of NIRSpec, a 3" x 3" square field of view (within the ~5" circular window) is dissected into 30 slices of 0.1" width and 3" length each.

THE CALIBRATION ASSEMBLY
Calibration Assembly
(Image of the NIRSpec Calibration Assembly. )
The purpose of the calibration assembly (CAA) is to enable on-orbit calibration and monitoring of a number of important instrument parameters such as (i) the geometric distortion between the MSA and the FPA, (ii) the response as a function of both field angle and wavelength, and (iii) the dispersion of the various spectral elements.

Because its light beam does not pass through the NIRSpec filter wheel, the CAA has to provide passband filters that closely mimic the NIRSpec filters. This is achieved by eleven light source assemblies (or "telescopes") feeding light into an integrating sphere. Each telescope contains two lamps for redundancy, dedicated filters for the correct bandpass, and a set of lenses that ensure that the beam from either lamp passes through the filter at the correct angles. Each lamp uses a bulb-enclosed Tungsten filament designed for low power dissipation operation. Due to the integrating sphere design the CAA provides highly uniform illumination.

doc FOOTER:

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SOURCE:  https://sci.esa.int/web/jwst/-/55459-08-new-detectors-and-micro-shutters-for-nirspec ( ABOVE)

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https://jwst.nasa.gov/resources/017457.PDF  < GOT IT >  file:///C:/Users/Susan/Documents/HANSANDCAssady2/REplaceMent-Root-Cause-AND-fiX-4JWST.PDF   < GOT IT

"...   Teledyne Awarded $23 Million Contract to Supply Infrared Detectors to NASA’s WFIRST Astronomy Mission ( https://en.wikipedia.org/wiki/Nancy_Grace_Roman_Space_Telescope ) (Photo: Business Wire) ( http://www.teledyne-si.com/products-and-services/imaging-sensors/hawaii-4rg ) The H4RG-10 focal plane array [FPA] has 4096×4096 pixels, with 16.7 million pixels in each array. The H4RG-10 is the largest infrared array qualified for use in space, a significant step beyond the 1 million pixel (H1R) array used in the Hubble Space Telescope and the 4.2 million pixel (H2RG) arrays used in the James Webb Space Telescope (JWST). THOUSAND OAKS, Calif.--()--Teledyne Technologies Incorporated (NYSE:TDY) announced today that NASA has awarded a contract to Teledyne Scientific & Imaging, LLC, located in Camarillo, California, for the Short Wave Infra-Red (SWIR) Sensor Chip Assembly (SCA) for the Wide Field Infrared Survey Telescope (WFIRST) Project at the NASA/Goddard Space Flight Center in Greenbelt, Maryland. The contract is Cost-Plus-Award-Fee with a value of$23,035,123. The 29 month period of performance commences immediately and concludes October 31, 2020. In this contract, Teledyne will produce 72 SWIR SCA devices for the WFIRST Space Flight Focal Plane Assembly.

“Teledyne is proud to be NASA’s partner for space astronomy with the infrared and visible detectors of WFIRST provided by our Digital Imaging segment”

WFIRST will utilize 18 of Teledyne’s H4RG-10 arrays in the focal plane assembly. With over 300 million pixels, it will be the largest infrared focal plane operating in space when it launches in the mid-2020s. WFIRST will have nearly 300 times as many pixels as there are in the infrared camera of the Hubble Space Telescope, enabling it to take images that have 100 times the field of view of Hubble. The large field of view enables WFIRST to survey large areas of the sky to measure the effects of dark matter and dark energy on the distribution of galaxies in the universe.

The H4RG-10 infrared array is at a mature state, advanced to NASA technology readiness level 6 (TRL-6) during a 45-month development contract that commenced in 2014. TRL-6 is the level at which components are qualified for use in the harsh environment of space. In addition to the infrared arrays, Teledyne has a development contract with NASA for the visible light detectors that will be used in the coronograph instrument of WFIRST.

“Teledyne is proud to be NASA’s partner for space astronomy with the infrared and visible detectors of WFIRST provided by our Digital Imaging segment,” said Robert Mehrabian, Chairman, and Chief Executive Officer of Teledyne. “This mission exemplifies Teledyne’s commitment to NASA, with Teledyne detectors also being used on Hubble, the James Webb Space Telescope, the Wide-field Infrared Survey Explorer, and many Earth science and planetary missions.”

About Teledyne Technologies : Teledyne Technologies is a leading provider of sophisticated instrumentation, digital imaging products and software, aerospace and defense electronics, and engineered systems. Teledyne Technologies’ operations are primarily located in the United States, Canada, the United Kingdom, and Western and Northern Europe. For more information, visit Teledyne Technologies’ website at www.teledyne.com.  ..."

"...

JWST

JWST (James Webb Space Telescope)

Concept    Launch    Observatory   Mission Status   Sensor Complement    Spacecraft Bus and Sunshield
Spinoff Technologies    References

JWST is an orbiting optical observatory and a key element in NASA's Origins Program, optimized for observations in the infrared region of the electromagnetic spectrum. It is considered the successor mission of HST (Hubble Space Telescope) while operating over a different spectral range. At the NIR and MWIR wavelengths, it benefits from operating at intrinsically lower backgrounds than any comparably sized telescope on the ground. JWST, previously known as NGST (Next Generation Space Telescope), will be the premier space facility for astronomers in the decade following its launch. The overall objectives are to study the first stars and galaxies after the big bang. Major science goals (themes) of the mission are to find answers to the following questions: 1) 2)

• What is the shape of the Universe?

• How do galaxies evolve?

• How do stars and planetary systems form and interact?

• How did the Universe built up its present chemical/elemental composition?

• What is the nature of dark matter?

The radiation from the very distant objects to be observed is practically all in the infrared region. Many of the early events happened when the Universe was between 1 million and 1 billion years old, a period that is not known to earthlings (the dark ages of the Universe). To accomplish the goals of the science themes, the main JWST design requirement calls for the detection of objects up to 400 times fainter than those observable by current ground-based or spaceborne observatories.

Historical background: Large next-generation projects with high-performance observation requirements take about two decades (and more) from first studies to launch. Initial planning for the new mission started in 1989 (visions, conceptual studies). The goal was to have a successor mission for HST ready for launch well before 2010.

In the mid-1990s, a telescope design with an 8 m aperture was considered. The challenge was to come up with a lower cost for the large telescope than for previous much smaller space telescopes. This involved conceptual studies by industry. In 1996, a committee report was written, based on these studies: "Next Generation Space Telescope, Visiting a Time When Galaxies Were Young." This report established also a roadmap to NGST activities, defining the new building blocks and to search for enabling technologies and concepts - in particular in the fields of large-aperture lightweight mirrors that are actively controlled, of advanced detector designs, of suitable cooling techniques for all critical components, and of precision metrology to achieve the goal of measuring ultra precise stellar positions.

A broad range of talent on a national and international level and from many institutions, academia and industry was directly involved in the NGST detailed definition phase (Phase A) including simulations and feasibility studies. In 1997, an ad hoc Science Working Group was formed which came up with thematic science goals and developed a so-called "Design Reference Mission" (DRM), representing a hypothetical suite of key science observing programs [stating the expected physical properties (number density and brightness), the desired observation modes (wavelength band, spectral resolution, number of revisits), and a minimum operational life of 2.5 years to complete the mission] for NGST - which provided a yardstick for technology testing. DRM was and is the primary tool against which any JWST architectures are being measured. The shear complexity of the project and the performance requirements demanded a technology development and validation strategy to address and demonstrate a critical path to a workable design of the mission. 3) 4) 5) 6)

In 2000/1, the NGST project experienced a rescoping of the telescope size (from 8 m aperture to 6.5 m) to keep projected costs in bounds. There were also some technology maturity uncertainties.

The project started in 2002 with a Mission Definition Review. NASA began to realize that the critical technologies had reached a level of sufficient maturity to justify a go-ahead with the next phase of the project.

In September 2002, NASA renamed NGST to JWST (James Webb Space Telescope) in honor of James E. Webb (1906-1992), NASA's second administrator during the Apollo Program of the 1960s (1961-1968). At the same time in Sept. 2002, NASA awarded the prime contract of the JWST observatory development (spacecraft, telescope, integration and testing) to Northrop Grumman Space Technology (formerly TRW) of Redondo Beach, CA.

In the fall of 2003 ICR(Initial Confirmation Review) was given, starting the Phase B of the JWST project. The C/D Phase started in 2008.

The CDR (Critical Design Review) of the JWST (James Webb Space Telescope) is planned for December 2013 (Ref. 28).

Project partners: NASA leads an international partnership in the joint JWST mission that includes ESA (European Space Agency) and CSA (Canadian Space Agency). Both agencies (ESA, CSA) collaborated in the JWST project already at an early planning stage (1996). Aside from instrument contributions, ESA will also launch the JWST spacecraft on an Ariane 5 launcher as agreed to with NASA. NASA/GSFC is managing the JWST project, while STScI (Space Telescope Science Institute) of Baltimore, MD, is responsible for JWST science and mission operations, as well as ground station development (STScI is the same organization that is operating the Hubble Space Telescope). A formal JWST and LISA (Laser Interferometer Space Antenna) cooperation agreement between NASA and ESA was signed on June 18, 2007 at the International Paris Air Show at Le Bourget, France. 7) 8) 9) 10) 11)

A most interesting and valuable side effect of the technology development effort for JWST is that these new technologies will also be available to many other space projects (astronomy, space science, Earth observation, etc.) providing potentially a quantum step in observation performance.

Mission concept

The JWST mission concept is an ambitious and most challenging development program, requiring a lot of innovative technology introduction as well as conceptual breakthroughs on various levels to meet the proposed observational performances. The objectives of the science themes can only be met by a combination of a large-aperture telescope in space (6.5 m φ ), a very low detection temperature to eliminate noise, and an ideal observing environment (elimination of stray light).

The observatory will be shielded from the sun and Earth by a large deployable sunshade, the entire telescope assembly will be passively cooled to about 37 K, giving JWST exceptional performance in the near-infrared and mid-infrared wavebands. The baseline wavelength range for the instrumentation is 0.6 - 28 µm, and the telescope will be diffraction-limited above 2 µm. The sensitivity of the telescope will be limited only by the natural zodiacal background, and should exceed that of ground-based and other space-based observatories by factors of 10 to 100,000, depending on the wavelength and type of observation. The JWST observatory will have a 5 year design life (with a goal of 10 years of operations) and will not be serviceable by astronauts (as is Hubble). The total mass of JWST at launch is estimated to be 6,500 kg.

Like Hubble, the JWST will be used by a broad astronomical community to observe targets ranging from objects within our Solar System to the most remote galaxies seen during their formation in the early universe.

Major enabling technologies are:

• Large deployable and lightweight beryllium mirrors (a folding 6.5 meter mirror made up of 18 individual segments, adjustable by cryogenic actuators). To fit inside the launch vehicle, the large space telescope prime mirror must be folded in sections for launch, then unfolded (deployed) precisely into place after launch, making it the first segmented optical system deployed in space.

• Deployment of large structures. Once in space, the multilayer sunshield that was folded over the optics during launch will deploy to its full size and keep the telescope shadowed from the sun.

• Introduction of MEMS technology to the microshutter system of the NIRSpec instrument. The programmable microshutters to allow object selection for the spectrograph.

• NIRCam (Near-Infrared Camera), funded by NASA with the University of Arizona as prime contractor. CSA is participating in the development of the NIRCam instrument.

• NIRSpec (Near-Infrared multi-object Spectrograph), funded by ESA with EADS Astrium GmbH as prime contractor (the detector arrays and a micro-shutter are supplied by NASA/GSFC)

• MIRI (Mid-Infrared Camera-Spectrograph) a joint instrument of JPL and ESA. The instrument (about 50%) is being provided by ESA member states, coordinated but not funded by ESA.

• FGS (Fine Guidance Sensor) with TFI (Tunable Filter Imager), funded by CSA (Canadian Space Agency)

Figure 1: Photometric performance of JWST instruments as compared to those of current observatories (image credit: STScI)

Legend to Figure 1: Plotted is the faintest flux for a point source that can be detected at 10 sigma in a 104 s integration. The fluxes are given in Janskies as well as AB magnitudes. 12)

Figure 2: Comparison of JWST light gathering power vs spectral range with Hubble and Spitzer telescopes (image credit: STScI) 13)

Launch: On Saturday, December 25 (Christmas), 2021 at 9:20 am local time (12:20 UTC), an Ariane 5 rocket lifted off from the Guiana Space Center, Europe's Spaceport in Kourou, French Guiana (South America), injecting the Webb Space Telescope, developed by NASA in partnership with ESA and the Canadian Space Agency (CSA), into its transfer orbit. The telescope was successfully separated from the launcher 27 minutes after liftoff. 14) 15)

The telescope now embarks on a voyage lasting 29 days to reach the second Lagrange point.

• On the third day, the heat shield will begin to deploy. On the eleventh day, the secondary mirror will begin positioning.

• Between the 13th and 14th day, the primary mirror, comprising 18 hexagonal segments and measuring 6.5 meters in diameter, will be assembled.

• The telescope is slated to arrive at its final destination, 1.5 million kilometers from Earth, approximately 29 days after launch.

The space agencies of the United States (NASA), Europe (ESA) and Canada (CSA) teamed up to develop this telescope. Europe played an important role in this mission, with ESA providing the launch onboard Ariane 5, as well as the Nirspec spectrometer built by Airbus. The astrophysics department of the Saclay-based CEA (French Alternative Energies and Atomic Energy Commission) and the Paris Observatory designed the MIRI camera. This is the most ambitious telescope ever sent into space.

"Today's launch is the mission of the decade," said Stephane Israël, Chief Executive Officer of Arianespace, "one that demonstrates the reliability of Arianespace's launch services in the eyes of the international space community. It's a great honor for us to have been chosen for this launch, which will enable humanity to take a giant step forward in its knowledge of the Universe. The mission demanded 20 years of preparation hand in hand with NASA. It's the third launch we have performed for the American space agency, clearly illustrating the advantage of large-scale international collaboration in space. I would like to thank ESA, NASA and CSA for entrusting us with their invaluable payload. To launch on Christmas morning 42 years after the takeoff of the first Ariane from this same Kourou site ... What a great end of year present for the space community gathered today for this launch.

NASA: A joint effort with ESA (European Space Agency) and the Canadian Space Agency, the Webb observatory is NASA's revolutionary flagship mission to seek the light from the first galaxies in the early universe and to explore our own solar system, as well as planets orbiting other stars, called exoplanets. 16)

"The James Webb Space Telescope represents the ambition that NASA and our partners maintain to propel us forward into the future," said NASA Administrator Bill Nelson. "The promise of Webb is not what we know we will discover; it's what we don't yet understand or can't yet fathom about our universe. I can't wait to see what it uncovers!"

Ground teams began receiving telemetry data from Webb about five minutes after launch. The Arianespace Ariane 5 rocket performed as expected, separating from the observatory 27 minutes into the flight. The observatory was released at an altitude of approximately 870 miles (1,400 km). Approximately 30 minutes after launch, Webb unfolded its solar array, and mission managers confirmed that the solar array was providing power to the observatory. After solar array deployment, mission operators will establish a communications link with the observatory via the Malindi ground station in Kenya, and ground control at the Space Telescope Science Institute in Baltimore will send the first commands to the spacecraft.

Engineers and ground controllers will conduct the first of three mid-course correction burns about 12 hours and 30 minutes after launch, firing Webb's thrusters to maneuver the spacecraft on an optimal trajectory toward its destination in orbit about 1 million miles from Earth.

"I want to congratulate the team on this incredible achievement – Webb's launch marks a significant moment not only for NASA, but for thousands of people worldwide who dedicated their time and talent to this mission over the years," said Thomas Zurbuchen, associate administrator for the Science Mission Directorate at NASA Headquarters in Washington. "Webb's scientific promise is now closer than it ever has been. We are poised on the edge of a truly exciting time of discovery, of things we've never before seen or imagined."

The world's largest and most complex space science observatory will now begin six months of commissioning in space. At the end of commissioning, Webb will deliver its first images. Webb carries four state-of-the-art science instruments with highly sensitive infrared detectors of unprecedented resolution. Webb will study infrared light from celestial objects with much greater clarity than ever before. The premier mission is the scientific successor to NASA's iconic Hubble and Spitzer space telescopes, built to complement and further the scientific discoveries of these and other missions.

"The launch of the Webb Space Telescope is a pivotal moment – this is just the beginning for the Webb mission," said Gregory L. Robinson, Webb's program director at NASA Headquarters. "Now we will watch Webb's highly anticipated and critical 29 days on the edge. When the spacecraft unfurls in space, Webb will undergo the most difficult and complex deployment sequence ever attempted in space. Once commissioning is complete, we will see awe-inspiring images that will capture our imagination."

The telescope's revolutionary technology will explore every phase of cosmic history – from within our solar system to the most distant observable galaxies in the early universe, to everything in between. Webb will reveal new and unexpected discoveries and help humanity understand the origins of the universe and our place in it.

NASA Headquarters oversees the mission for the agency's Science Mission Directorate. NASA's Goddard Space Flight Center in Greenbelt, Maryland, manages Webb for the agency and oversees work on the mission performed by the Space Telescope Science Institute, Northrop Grumman, and other mission partners. In addition to Goddard, several NASA centers contributed to the project, including the agency's Johnson Space Center in Houston, Jet Propulsion Laboratory in Southern California, Marshall Space Flight Center in Huntsville, Alabama, Ames Research Center in California's Silicon Valley, and others.

Figure 3: The James Webb Space Telescope lifted off on an Ariane 5 rocket from Europe's Spaceport in French Guiana, at 13:20 CET on 25 December on its exciting mission to unlock the secrets of the Universe (image credit: ESA/CNES/Arianespace)

Figure 4: Highlights of the launch campaign for the James Webb Space Telescope, from its arrival at Europe's Spaceport in Kourou, French Guiana, weeks of launch preparations, to launch on board an Ariane 5, and separation of the spacecraft and solar panel deployment (video credit: ESA/CNES/Arianespace) 17)

Orbit:

The orbit of JWST has been selected to be at L2. The spacecraft will be in a Lissajous (or halo) orbit about the Lagrangian point L2. In the Sun‐Earth system the L2 point is on the rotating Sun-Earth axis about the same distance away as L1 (1.5 million km, representing 1/100 the distance from Earth to the Sun) but at the opposite side of the Earth. The L1 location is inside the Earth orbit while the L2 location is outside the Earth orbit.

The halo orbit of JWST is in a plane slightly out of the ecliptic plane. This orbit avoids Earth and moon eclipses of the sun. The halo orbit period is about 6 months. Nominal station keeping maneuvers will be performed every half orbit (i.e. in intervals of about 3 months).

Figure 5: Locations of the five Lagrangian points in the Sun-Earth system

The L2 location is considered to offer the most advantageous viewing for astronomical targets (looking toward the universe) due to nearly constant lighting conditions (minimum of stray light). Another advantage of the L2 location is that it offers a stable thermal environment. The telescope is kept in perpetual shadow by looking into the deep space direction. The deep space provides a 2.7 K black body radiation. This ideal heat sink is being used to provide the passive cooling for the payload to a temperature range of about 37 K, shielded from sunlight (entering the spacecraft from the opposite direction) by a five-layer sunshield [passive cooling is the most elegant and economical method available to obtain the required operating temperatures for infrared detection].

Figure 6: Overview of JWST trajectory to L2 (image credit: NASA)

Figure 7: Artist's rendering of the JWST observatory (image credit: NASA)

JWST deployment sequence:

During the transfer orbit to L2 different elements of the JWST will be deployed and commissioning will start. The observatory has five deployment stages involving the following elements: 18)

1) Deployment of spacecraft appendages (solar arrays, high gain antenna)

2) Deployment of the sunshield (unfolding 2 days after launch)

3) Extension of the tower

4) Deployment of the secondary mirror (positioned on a tripod structure)

5) Deployment of the primary mirror wings

The deployment of the solar arrays and the high gain antenna is scheduled for the first day to provide the capabilities of onboard power generation and a spacecraft communications link. The unfolding of the sunshield will occur two days after launch, while the timeline for secondary and primary mirror deployment is foreseen after four days. "First light" will occur about 28 days after launch, initiating wavefront sensing and control activities to align the mirror segments. Instrument checkout will start 37 days after launch, well before the final L2 orbit insertion is obtained after 106 days. This is being followed by full commissioning procedures expected to last until about 6 months after launch. 19)

Figure 8: Deployment sequence of the OTE (image credit: NASA, STScI)

Observatory

The Observatory architecture is comprised of three elements: OTE (Optical Telescope Element), ISIM (Integrated Science Instrument Module), and the spacecraft (bus and sunshield). A key aspect of the JWST architecture is the use of semi-rigid primary mirror segments mounted on a very stable and rigid backplane composite structure. The architecture is referred to as "semi-rigid" because it has a modest amount of flexibility that allows for on-orbit compensation of segment-to-segment radius of curvature variations. 20) 21) 22) 23) 24) 25) 26) 27) 28)

Figure 9: The three elements of the JWST flight segment (image credit: NASA) 29)

Figure 10: The JWST spacecraft, reflecting the addition of the trim flap and the new solar panel array (image credit: NASA)

 Item Feature Benefit OTE (Optical Telescope Element) - TMA (Three Mirror Anastigmatic) design, f/20, 25 m2 collecting area - Fine steering mirror (FSM) with line-of-sight (LOS) stabilization < 7.3 marcsec (or mas) - Four separate deployments - Semi-rigid hexagonal mirror segments and graphite composite backplane structure - Superior image quality over the ISIM FOV, provides science resolution and sensitivity - Excellent pointing control and stability in conjunction with the spacecraft attitude control - Simple, reliable and robust deployment - Allows ground verification of the OTE, provides stable optical performance over temperature Primary mirror - Primary mirror deploys in two steps (2-chord fold) - Composed of 18 semi-rigid hexagonal segments, each with set-and-monitor wavefront control actuators - Mirror segment material is Beryllium - Highly reliable deployment - All segments are mechanically near-identical, achieving efficiencies in manufacturing, assembly and testing - Known material properties with demonstrated optical performance over temperature Secondary mirror - Tripod configuration for support structure - Deployment using a single redundant actuator - Semi-rigid optic with 6 degrees of freedom (DoF) alignment - Provides rigidity, minimizes obscuration and scattered light into the field of view - Low risk, high margin (torque margin > 32 times the friction load) - Permits reliable and accurate telescope alignment Aft optics Fixed baffle Reduces stray light and houses the tertiary mirror and the FSM ISIM - Simple semi-kinematic mount; 8 m2 of thermal radiators, and 19.9 m3 volume. - Contains all science instruments (SI) and FGS - Provides a simple interface for the ISIM to decouple ISIM development from the OTE - Allows for parallel development and early testing Tower - Integral 1 Hz passive vibration isolators - Thermally isolates the OTE from the spacecraft - Reduces S/C dynamic noise onto OTE/ISIM - Achieves small mirror temperature gradients Sunshield - 5 layer "V" groove radiator design reduces solar energy to a few 10's of mW - Folded about OTE during launch - Sized (~19.4 m x 11.4 m) and shaped to limit solar radiation induced momentum buildup - Provides a stable thermal environment for passively cooling the OTE and the ISIM - Reliable deployment, protects OTE during launch - Reduces the time and fuel for momentum unloading, increases operational efficiency S/C bus - Chandra-based attitude control subsystem - Two-axis gimbaled high gain Earth-pointing antenna (omni-directional), Ka- and S-band - 471 Gbit solid state recorder - Propellant for >11 years - Flight-proven low noise dynamic environment that minimizes line-of-sight jitter - Contingency operations and link margin - Store > 2 days of science & engineering data - Extended operation capability

Table 1: Overview of key design features and benefits of the Observatory

 Parameter Capability Parameter Capability Wavelength 0.6 - 29 µm. Reflective gold coatings Orbit Lissajous orbit about L2 Sensitivity NIRCam NIRCam FGS tunable NIRSpec NIRSpec MIRI NIRSpec Med MIRI Spec MIRI Spec - SNR=10, integration time = τi, R=λ/Δλ and Zodicial of 1.2 times that at north ecliptic pole - 12 nJy (1.1 μm, τi=10,000 s, and λ/Δλ= 4) - 10.4 nJy (2.0 μm, τi=10,000 s, and λ/Δλ = 4) - 368 nJy (3.5 μm, τi=10,000 s, and λ/Δλ = 100) - 120 nJy (3.0 μm, τi=10,000 s, and λ/Δλ = 100) - 560 nJy (10 μm, τi=10,000 s, and λ/Δλ = 5) - 5000 nJy (21 μm, τi=10,000 s, and λ/Δλ = 4.2) - 5.2 x 10-22 Wm-2 (2 µm, τi=100,000 s, R=1000) - 3.4 x 10-21 Wm-2 (9.2 µm, τi=10,000 s, R=2400) - 3.1 x 10-20 Wm-2 (22.5 µm, τi=10,000 s, R=1200) Celestial sphere coverage Overall observing efficiency - 100% annually - 39.7% at any given time - 100% of sphere has at least 51 contiguous days visibility - 30% for > 197 days - Continuous within 5º of ecliptic poles - Observatory ~80.7% Spatial resolution & stability - Encircled Energy of 75% at 1 µm for 150 mas radius - Strehl ratio of ~ 0.86 at 2 µm. - PSF stability better than 1% Mission life - 5-year minimum lifetime - 11 years for fuel - Commissioning in < 6 months Telescope FOV - 166 arcmin x 166 arcmin FOV -ISIM instruments share FOV with common aperture Launch 2019

Table 2: Overview of the predicted performance of the JWST observatory

OTE (Optical Telescope Element):

The OTE is of course the key element of the observatory with a primary mirror aperture diameter of 6.5 m. A lightweight design is mandatory to keep the launch costs in bounds. Early in the JWST program, an AMSD (Advanced Mirror System Demonstrator) project was launched to address the feasibility and readiness level of the required enabling technologies.

The following requirements were placed on JWST's optics (based on an "optical telescope element" study of 1996:

• The mirror should be sensitive to 1-5 µm (0.6-30 µm extended)

• It should be diffraction limited to 2 µm

• It will have to operate in the temperature range of 30-60 K

• It should have an areal density of < 15 kg/m2.

Figure 11: Isometric drawing of the OTE telescope structure (image credit: NASA, STScI)

The JWST prime contractor, NGAS (Northrop Grumman Aerospace Systems) in consultation with the JWST Telescope Team, selected the beryllium-based mirror technology design made by BATC (Ball Aerospace & Technologies Corporation) as the primary mirror material with the following features: 30) 31)

• 1.318 m point-to-point light-weighted beryllium semi-rigid mirror (element size)

• 13.4 kg/m2 beryllium substrate areal density

• 19.3 kg/m2 areal density for the mirror system - including mirror, reaction structure, flexures, and actuators

• A SBMD (Subscale Beryllium Model Demonstrator) element achieved a 19 nm rms "surface roughness" at 38 K.

Beryllium was chosen over glass as the mirror material because it is lighter and has a low coefficient of thermal expansion at cryogenic temperatures. Since JWST is an infrared telescope, it must operate at cryogenic temperatures (< 40 K) so that the heat of the telescope does not interfere with the radiation it captures. Beryllium mirrors have a heritage in past astronomy missions such as in IRAS (InfraRed Astronomical Satellite, launch Jan. 25, 1983), COBE (Cosmic Background Explorer, launch Nov. 18, 1989) and the Spitzer Space Telescope (launch Aug. 25, 2003). The material properties of beryllium are known to temperatures of 10 K.

Aside from its lightweight features, the primary mirror must be segmented, so that it can be folded up to fit into the nose cone of a rocket. Once on orbit, the telescope will be deployed, using motors to unfold the primary mirror and other important assemblies. Then the telescope will be cooled down from room temperature to about 37 K by the ambient environment on its way to L2 - a temperature change of about 300 K is experienced which obviously causes misalignments and figure errors of the optics system. Note: Passive cooling is attained by placing the observatory at L2 and keeping the telescope and its instrumentation in perpetual shadow by means of a large deployable sunshade.

The primary mirror design consists of 18 hexagonal segments (1.315 m flat-to-flat side), in two rings around the center, resulting in a 6.5 m flat-to-flat diameter with a collecting area of 25 m2. A TMA (Three Mirror Anastigmatic) design is employed with a Strehl ratio of ~0.84 at λ = 2 µm providing a very low background noise. The telescope has an effective f/number of f/16.67, and an effective focal length of 131.4 m.

The segments of the primary mirror act as a single mirror when properly phased relative to each other. The phasing is achieved via a 6 DoF (Degree of Freedom) rigid body motion of the individual segments, and an additional control for the segment mirror radius of curvature. The 18 segments have three separate segment types (A, B, C) with slightly different aspheric prescriptions depending on placement as shown in Figure 12. The numbers 1 to 6 represent the six-fold symmetry of the hexagonal packing of the primary mirror.

Figure 13 shows the rear portion of the mirror segments and the seven actuators. The architecture is "semi-rigid" because it has a modest amount of flexibility that allows for on-orbit compensation of segment-to-segment radius of curvature (ROC) variations. This ROC adjustment is made independent of any attachment to the backplane structure to prevent mirror distortion.

The six actuators providing rigid body motion are arranged in three bipods to form a kinematic attachment to the backplane. Each bipod attaches to a triangular shaped structure which is attached to the isogrid structure of the mirror segment. This structure spreads the loads over the surface of the mirror. The other end of the actuators attaches through a secondary structure and flexure to the backplane. The seventh actuator controls the segment radius of curvature and is independent of the rigid body actuators. The actuators operate at cryogenic and ambient temperatures, and have both coarse and fine positioning capability. This configuration enables simple rigid body motion of the segments without distorting the segment surface. 32)

Figure 12: Arrangement and designation of primary mirror segments and images of the mirrors (image credit: NASA, BATC)

Legend to Figure 12: JWST completes the gold coating of it's telescope mirrors with segment C1. A microscopically thin layer of gold maximizes the reflectivity of these mirrors to infrared light.

Figure 13: Backside of the primary mirror with the three bipod actuators (image credit: NGAS)

WFS&C (Wavefront Sensing and Control) subsystem: A WF&C semi-rigid structure is being used for phasing (to counteract the misalignments). WFS&C consists of actuators mounted on the telescope primary mirror segments and on the secondary mirror, to deform and displace the critical telescope optics in ways that are very effective in compensating the likely on-orbit deformations. The WFS&C software processes images from the cameras to measure the optical aberrations. The software then computes actuator commands to correct the aberrations.

Figure 14: Illustration of the OTE subsystems/assemblies (image credit: NASA, STScI) 33)

Operating temperatures: The large sunshade will protect the telescope from heating by direct sunlight, allowing it to cool down to temperatures of < 45 K. The near-infrared instruments will work at about 30 K through a passive cooling system. The mid-infrared detectors will work at a temperature of 7 K, using stored cryogen (active cooling).

Figure 15: Conceptual layout of the OTE and interfaces to ISIM (image credit: NGAS, STScI)

Figure 16: As at the end of 2013, all 18 of the JWST primary mirror segment assemblies are complete and have arrived at Goddard,
where they are being stored inside separate stainless steel shock-absorbing canisters --  until it is time for mirror assembly (image credit: NASA)

ISIM (Integrated Science Instrument Module)

The ISIM provides structure, environment, control electronics and data handling for three modular science instruments: NIRCamNIRSpec, and MIRI, and the observatory FGS (Fine Guidance Sensor). ISIM is being provided by NASA/GSFC. In addition to designing the ISIM structure, NASA Goddard provides other infrastructure subsystems critical for the operation of the instruments, including the ISIM Thermal Control Subsystem; ISIM Control and Data Handling Subsystem; ISIM Remote Services Unit; ISIM Flight Software; ISIM Electronics Compartment, and ISIM Harness Assemblies. 34) 35) 36) 37) 38)

ISIM is a distributed system consisting of cold and warm modules.

• The cryogenic instrument module is integrated with the OTE and the sensor complement, all of which are passively cooled to the cryogenic temperature of 39 K. This passively cooled cryogenic (39 K) section houses the instruments NIRCam, NIRSpec, MIRI and the FGS (Fine Guidance Sensor). The MIRI instrument is further cooled by a cryocooler to 7 K.

• The second area is the IEC (ISIM Electronics Compartment), which provides the mounting surfaces and a thermally-controlled environment for the instrument control electronics (region 2 maintained at 298 K). The ICE package is mounted onto the exterior of the ISIM structure.

• The third area (warm module) is the ISIM Command and Data Handling (ICDH) subsystem, which includes ISIM flight software, and the MIRI cryocooler compressor and control electronics (region 3 maintained at 298 K). The warm region of ISIM is located in the spacecraft on the warm side of the Observatory. This more benign environment allows for relaxed thermal requirements on major portions of the electronics with higher power dissipation, and it avoids unnecessary heat loads in the cold section.

Figure 17: ISIM is the science instrument payload of JWST (image credit: NASA) 39)

Figure 18: Components of the integrated ISIM with the FGS mounted inside the structure (image credit: NASA)

Each ISIM instrument reimages the OTE focal plane onto its FPA (Focal Plane Array) assembly, allowing for independent selection of detector plate scale for sampling of the optical PSF (Point Spread Function). A fine steering mirror (FSM) is used for accurate optical pointing and image stabilization. The FSM is located at the image of the pupil, after the tertiary mirror but forward of the focal plane interface to the ISIM. The FSM, coupled with the low structural noise spacecraft, suppresses line-of-sight jitter to allow diffraction-limited performance at 2 µm. The V1, V2, and V3 coordinate systems are defined by the vertex of the primary mirror as shown in Figure 15.

The four scientific instruments onboard JWST are contained in the ISIM (Integrated Science Instrument Module) which is mounted to the BSF (Back Plane Support) behind the primary mirror. ISIM contains four instruments: MIRI,FGS/IRISS, NIRCam, and NIRSpec. The IEC (ISIM Electronics Compartment) is also mounted to the BSF and holds a number of high-power boxes, totaling 200 W of dissipation, at room temperature on the cold side of the sunshield. This is an order of magnitude above the summed dissipation of the remainder of the cold side. Its proximity to the cryogenic instruments is driven by the noise-sensitive science data that must be processed by electronics with the IEC. 40)

The IEC has been designed to hold room-temperature electronics boxes in close proximity to the cryogenic telescope and instrument module and to direct the 200 W dissipation so that is does not have a negative affect on the observatory performance. This is made possible through multiple radiative isolators in series, conductive isolation, and directional baffles. Analysis has shown that this design will meet the requirements levied on the IEC by the observatory, allowing the IEC to function as an integral part of the James Webb Space Telescope.

Figure 19: ISIM components within the Observatory (image credit: NASA)

 Instrument Spectral range (µm) Optical elements FPA Plate scale (marcsec/pixel) FOV (Field of View) NIRCam (Short Wave) 0.6 - 2.3 Fixed filters (R~4, R~10, R~100), coronagraphic spots Two 2 x 2 mosaics of 2048 x 2048 arrays 32 2.2' x 4.4' (arcmin) NIRCam (Long Wave) 2.4 - 5.0 Fixed filters (R~4, R~10, R~100), coronagraphic spots Two 2048 x 2048 arrays 65 2.2' x 4.4' NIRSpec (prism, R=100 resolving power) 0.6 - 5.0 Transmissive slit mask: 4 x 384 x 175 micro-shutter array, 250 (spectral) by 500 (spatial) marcsec; fixed slits 200 or 300 marcsec wide by 10 cm long Two 2048 x 2048 arrays 100 3.4' x 3.1' NIRSpec (Grating, R=1000) 1.0 - 5.0 NIRSpec (IFU, R=3000) 1.0 - 5.0 IFU (Integrated Field Unit) 3.0" x 3.0" (arcsec) MIRI (Imaging) 5 - 28 Broad-band filters, coronagraphic spots & phase masks 1024 x 1024 110 1.4' x 1.9' (26" x 26" coronagraphic) MIRI (Prism spectroscopy) 5 -10 R ~ 100 MIRI (Spectroscopy) 5 - 28 Integral field spectrograph (R~3000) in 4 bands Two 1024 x 1024 arrays 200-470 depending on band 3.6" x 3.6" to 7.5" x 7.5" FGS 1.5 – 2.6 3.1 – 4.8 Order-blocking filters+etalon (R~100) 2048 x 2048 68 2.3' x 2.3'

Table 3: Overview of science instrument characteristics

The ISIM instruments are located in an off-axis position, which yield excellent image quality over the 9.4 arcminute field, as shown by the contours of residual wavefront error as a function of field location in Figure 22. The cold portion of the ISIM is integrated with the OTE.

Figure 20: Schematic diagram of the accommodation of the four science instruments in ISIM (image credit: NASA)

Figure 21: NASA engineers check out the unwrapped ISIM structure in a clean room in 2009 (image credit: NASA) 41)

Figure 22: ISIM focal plane allocation layout (image credit: STScI, NASA)

Legend to Figure 22: Placement of the ISM instruments in the telescope field of view. The field of view of each instrument is fully contained within the instrument allocation regions. The numbers indicate the wavefront error contribution by the optical telescope element (in nm) at each location.

Figure 23: The cryogenic portion of the ISIM system (left) is shown in its test configuration (right) for the CV-1RR (image credit: NASA)

Legend to Figure 23: A high fidelity simulation of the JWST telescope beam is fed from below into the ISIM by an Optical SIMulator (OSIM) that is mounted on vibration isolators. The SES vacuum vessel is equipped with nitrogen and helium shrouds to enable testing at the 40 K nominal flight operating temperature. 42)

The ISIM structure and assembly has a total mass of ~ 1400 kg which is about 23% of the JWST mass.

ISIM status:

• Summer 2015: The ISIM enters this final testing sequence in its full flight configuration. After some precursor integration and test activities, which included two very successful cryo-vacuum campaigns (called CV1-RR and CV2, the latter of which was in a nearly-final configuration), the ISIM underwent a series of activities to upgrade its instruments and systems to full flight readiness. These activities included: 43)

- Completion of the upgrade of the near-infrared detector arrays in NIRCam, NIRSpec, and FGS/NIRISS to a newer, more robust design that eliminates a dark current degradation mechanism suffered by the earlier generation arrays.

- Installation of new Microshutter Arrays in the NIRSpec with improved stability against the acoustic loads of launch.

- Installation of new grisms in the NIRISS instrument, including a new grism for exoplanet spectroscopy with 2-3 times higher throughput than the original optic.

- Upgraded electronics boards in several instruments for improved performance or reliability.

- Installation of the flight cold head of the MIRI cryocooler system (the Heat Exchanger Stage Assembly, mounted to the ISIM structure).

The first phase of this final environmental test sequence, vibration testing, was completed in June 2015, with vibration of the "ISIM prime" module. Sinusoidal sweep testing was carried out in each of three axes, with amplitudes up to ~2.5g in some frequency bands, in order to verify workmanship by subjecting the system to the low frequency structural dynamic spectrum of the launch environment.

Figure 24: The ISIM structure and flight instruments, re-integrated and ready for environmental testing (image credit: NASA, Chris Nunn)

Note: As of the launch of the JWST spacecraft on December 25 2021, the JWST file has been split into additional files, to make the file handling manageable for all parties concerned, in particular for the user community.

JWST mission status

• January 24, 2022: Today, at 2 p.m. EST, Webb fired its onboard thrusters for nearly five minutes (297 seconds) to complete the final postlaunch course correction to Webb's trajectory. This mid-course correction burn inserted Webb toward its final orbit around the second Sun-Earth Lagrange point, or L2, nearly 1 million miles away from the Earth. 44)

Figure 25: The final mid-course burn added only about 3.6 miles per hour (1.6 m/s) – a mere walking pace – to Webb's speed, which was all that was needed to send it to its preferred "halo" orbit around the L2 point (image credit: Steve Sabia/NASA Goddard)

- "Webb, welcome home!" said NASA Administrator Bill Nelson. "Congratulations to the team for all of their hard work ensuring Webb's safe arrival at L2 today. We're one step closer to uncovering the mysteries of the universe. And I can't wait to see Webb's first new views of the universe this summer!"

- Webb's orbit will allow it a wide view of the cosmos at any given moment, as well as the opportunity for its telescope optics and scientific instruments to get cold enough to function and perform optimal science. Webb has used as little propellant as possible for course corrections while it travels out to the realm of L2, to leave as much remaining propellant as possible for Webb's ordinary operations over its lifetime: station-keeping (small adjustments to keep Webb in its desired orbit) and momentum unloading (to counteract the effects of solar radiation pressure on the huge sunshield).

- "During the past month, JWST has achieved amazing success and is a tribute to all the folks who spent many years and even decades to ensure mission success," said Bill Ochs, Webb project manager at NASA's Goddard Space Flight Center. "We are now on the verge of aligning the mirrors, instrument activation and commissioning, and the start of wondrous and astonishing discoveries."

- Now that Webb's primary mirror segments and secondary mirror have been deployed from their launch positions, engineers will begin the sophisticated three-month process of aligning the telescope's optics to nearly nanometer precision.

• January 12. 2022: Webb has begun the detailed process of fine-tuning its individual optics into one huge, precise telescope. 45)

- Engineers first commanded actuators – 126 devices that will move and shape the primary mirror segments, and six devices that will position the secondary mirror – to verify that all are working as expected after launch. The team also commanded actuators that guide Webb's fine steering mirror to make minor movements, confirming they are working as expected. The fine steering mirror is critical to the process of image stabilization.

- Ground teams have now begun instructing the primary mirror segments and secondary mirror to move from their stowed-for-launch configuration, off of snubbers that kept them snug and safe from rattling from vibration. These movements will take at least ten days, after which engineers can begin the three-month process of aligning the segments to perform as a single mirror.

• January 8, 2022: NASA's James Webb Space Telescope team fully deployed its 21-foot (6.4 m), gold-coated primary mirror, successfully completing the final stage of all major spacecraft deployments to prepare for science operations. 46)

- A joint effort with the European Space Agency (ESA) and Canadian Space Agency, the Webb mission will explore every phase of cosmic history – from within our solar system to the most distant observable galaxies in the early universe.

- "Today, NASA achieved another engineering milestone decades in the making. While the journey is not complete, I join the Webb team in breathing a little easier and imagining the future breakthroughs bound to inspire the world," said NASA Administrator Bill Nelson. "The James Webb Space Telescope is an unprecedented mission that is on the precipice of seeing the light from the first galaxies and discovering the mysteries of our universe. Each feat already achieved and future accomplishment is a testament to the thousands of innovators who poured their life's passion into this mission."

- The two wings of Webb's primary mirror had been folded to fit inside the nose cone of an Arianespace Ariane 5 rocket prior to launch. After more than a week of other critical spacecraft deployments, the Webb team began remotely unfolding the hexagonal segments of the primary mirror, the largest ever launched into space. This was a multi-day process, with the first side deployed Jan. 7 and the second Jan. 8.

- Mission Operations Center ground control at the Space Telescope Science Institute in Baltimore began deploying the second side panel of the mirror at 8:53 a.m. EST. Once it extended and latched into position at 1:17 p.m. EST, the team declared all major deployments successfully completed.

- The world's largest and most complex space science telescope will now begin moving its 18 primary mirror segments to align the telescope optics. The ground team will command 126 actuators on the backsides of the segments to flex each mirror – an alignment that will take months to complete. Then the team will calibrate the science instruments prior to delivering Webb's first images this summer.

- "I am so proud of the team – spanning continents and decades – that delivered this first-of-its kind achievement," said Thomas Zurbuchen, associate administrator for the Science Mission Directorate in NASA Headquarters in Washington. "Webb's successful deployment exemplifies the best of what NASA has to offer: the willingness to attempt bold and challenging things in the name of discoveries still unknown."

- Soon, Webb will also undergo a third mid-course correction burn – one of three planned to place the telescope precisely in orbit around the second Lagrange point, commonly known as L2, nearly 1 million miles from Earth. This is Webb's final orbital position, where its sunshield will protect it from light from the Sun, Earth, and Moon that could interfere with observations of infrared light. Webb is designed to peer back over 13.5 billion years to capture infrared light from celestial objects, with much higher resolution than ever before, and to study our own solar system as well as distant worlds.

- "The successful completion of all of the Webb Space Telescope's deployments is historic," said Gregory L. Robinson, Webb program director at NASA Headquarters. "This is the first time a NASA-led mission has ever attempted to complete a complex sequence to unfold an observatory in space – a remarkable feat for our team, NASA, and the world."

- NASA's Science Mission Directorate oversees the mission. NASA's Goddard Space Flight Center in Greenbelt, Maryland, manages the project for the agency and oversees the Space Telescope Science Institute, Northrop Grumman, and other mission partners. In addition to Goddard, several NASA centers contributed to the project, including Johnson Space Center in Houston, the Jet Propulsion Laboratory in Pasadena, Marshall Space Flight Center in Huntsville, Alabama, Ames Research Center in Silicon Valley, and others.

Figure 26: This artist's conception of the James Webb Space Telescope in space shows all its major elements fully deployed. The telescope was folded to fit into its launch vehicle, and then was slowly unfolded over the course of two weeks after launch (image credits: NASA GSFC/CIL/Adriana Manrique Gutierrez)

• January 5, 2022: While the Webb team was tensioning the sunshield, other activities were also taking place among the instruments. One milestone: unlocking the Mid-Infrared Instrument (MIRI) Contamination Control Cover. We've asked Gillian Wright, European principal investigator for MIRI, to tell us about it. 47)

- "MIRI has a Contamination Control Cover, because the constraints of its extra-cold operating temperature mean that it is not possible to include other means of dealing with ice contamination, such as heaters for sensitive components. For launch it was safest to have this cover locked, and the timing for operating it is driven by the temperatures of the observatory.

- "To unlock the cover, we first had to power on our Instrument Control Electronics and confirm that they were functioning correctly. Then the commands to the cover could be sent. After successfully completing the tests and unlocking the cover, the instrument control electronics were then powered off before the next steps on the sunshield tensioning activities. This key step for MIRI was monitored remotely by team members in Europe, ready to provide assistance if it were needed.

Figure 27: "The picture here shows tired and happy MIRI team members at the Mission Operations Center in Baltimore, after completing this first of the many MIRI commissioning steps. The MIRI Contamination Control Cover will be closed in the next few days to protect the optics from any possible contaminants as the observatory cools. It will then be reopened much later in the timeline, when MIRI has cooled to its operating temperature of just 7K and is ready to look out at the sky."(Gillian Wright, European principal investigator for the Mid-Infrared Instrument, UK Astronomy Technology Centre)

• January 5, 2022: Today, Webb teams successfully deployed the observatory's secondary mirror support structure. When light from the distant universe hits Webb's iconic 18 gold primary mirrors, it will reflect off and hit the smaller, 2.4-foot (0.74 m) secondary mirror, which will direct the light into its instruments. The secondary mirror is supported by three lightweight deployable struts that are each almost 25 feet long and are designed to withstand the space environment. Specialized heating systems were used to warm up the joints and motors needed for seamless operation. 48)

- "Another banner day for JWST," said Bill Ochs, Webb project manager at NASA's Goddard Space Flight Center, as he congratulated the secondary mirror deployment team at the Mission Operations Center in Baltimore. "This is unbelievable…We're about 600,000 miles from Earth, and we actually have a telescope."

- The deployment process began at approximately 9:52 a.m. EST, and the secondary mirror finished moving into its extended position at about 11:28 a.m. EST. The secondary mirror support structure was then latched at about 11:51 a.m. EST. At approximately 12:23 p.m. EST, engineers confirmed that the structure was fully secured and locked into place and the deployment was complete.

- "The world's most sophisticated tripod has deployed," said Lee Feinberg, optical telescope element manager for Webb at Goddard. "That's really the way one can think of it. Webb's secondary mirror had to deploy in microgravity, and in extremely cold temperatures, and it ultimately had to work the first time without error. It also had to deploy, position, and lock itself into place to a tolerance of about one and a half millimeters, and then it has to stay extremely stable while the telescope points to different places in the sky – and that's all for a secondary mirror support structure that is over 7 meters in length."

- Next Webb will deploy an important radiator system known as the aft deployable instrument radiator (ADIR), which helps shed heat away from its instruments and mirrors.

• January 4, 2022: The James Webb Space Telescope team has fully deployed the spacecraft's 70-foot sunshield, a key milestone in preparing it for science operations. 49) 50)

- The sunshield – about the size of a tennis court at full size – was folded to fit inside the payload area of an Arianespace Ariane 5 rocket's nose cone prior to launch. The Webb team began remotely deploying the sunshield Dec. 28, 2021, three days after launch.

- "This is the first time anyone has ever attempted to put a telescope this large into space," said Thomas Zurbuchen, associate administrator for NASA's Science Mission Directorate at the agency's headquarters in Washington. "Webb required not only careful assembly but also careful deployments. The success of its most challenging deployment – the sunshield – is an incredible testament to the human ingenuity and engineering skill that will enable Webb to accomplish its science goals."

- The five-layered sunshield will protect the telescope from the light and heat of the Sun, Earth, and Moon. Each plastic sheet is about as thin as a human hair and coated with reflective metal, providing protection on the order of more than SPF 1 million. Together, the five layers reduce exposure from the Sun from over 200 kW of solar energy to a fraction of a watt.

- This protection is crucial to keep Webb's scientific instruments at temperatures of 40 kelvins, or under minus 380 degrees Fahrenheit – cold enough to see the faint infrared light that Webb seeks to observe.

- "Unfolding Webb's sunshield in space is an incredible milestone, crucial to the success of the mission," said Gregory L. Robinson, Webb's program director at NASA Headquarters. "Thousands of parts had to work with precision for this marvel of engineering to fully unfurl. The team has accomplished an audacious feat with the complexity of this deployment – one of the boldest undertakings yet for Webb."

- The unfolding occurred in the following order, over the course of eight days:

a) Two pallet structures – forward and aft – unfolded to bring the observatory to its full 70-foot length

b) The Deployable Tower Assembly deployed to separate the telescope and instruments from the sunshield and the main body of the spacecraft, allowing room for the sunshield to fully deploy

c) The aft momentum flap and membrane covers were released and deployed

d) The mid-booms deployed, expanding perpendicular to the pallet structures and allowing the sunshield to extend to its full width of 47 feet

e) Finally, at approximately 11:59 a.m. EST Tuesday (Jan. 4), the sunshield was fully tensioned and secured into position, marking the completion of the sunshield deployment.

- The unfolding and tensioning of the sunshield involved 139 of Webb's 178 release mechanisms, 70 hinge assemblies, eight deployment motors, roughly 400 pulleys, and 90 individual cables totaling roughly one quarter of a mile in length. The team also paused deployment operations for a day to work on optimizing Webb's power systems and tensioning motors, to ensure Webb was in prime condition before beginning the major work of sunshield tensioning.

- "The sunshield is remarkable as it will protect the telescope on this historic mission," said Jim Flynn, sunshield manager at Northrop Grumman, NASA's primary contractor for Webb. "This milestone represents the pioneering spirit of thousands of engineers, scientists, and technicians who spent significant portions of their careers developing, designing, manufacturing, and testing this first-of-its-kind space technology."

- The world's largest and most complex space science observatory has another 5 1/2 months of setup still to come, including deployment of the secondary mirror and primary mirror wings, alignment of the telescope optics, and calibration of the science instruments. After that, Webb will deliver its first images.

Figure 28: On Jan. 4, 2022, engineers successfully completed the deployment of the James Webb Space Telescope's sunshield, seen here during its final deployment test on Earth in December 2020 at Northrop Grumman in Redondo Beach, California. The five-layer, tennis court-sized sunshield is essential for protecting the telescope from heat, allowing Webb's instruments to cool down to the extremely low temperatures necessary to carry out its science goals (image credits: NASA/Chris Gunn)

- The telescope's revolutionary technology will explore every phase of cosmic history – from within our solar system to the most distant observable galaxies in the early universe, to everything in between. Webb will reveal new and unexpected discoveries and help humanity understand the origins of the universe and our place in it.

- The James Webb Space Telescope is an international partnership with the ESA (European Space Agency) and the Canadian Space Agency. NASA Headquarters oversees the mission. NASA's Goddard Space Flight Center in Greenbelt, Maryland, manages Webb for the agency and oversees work on the mission performed by the Space Telescope Science Institute, Northrop Grumman, and other mission partners. In addition to Goddard, several NASA centers contributed to the project, including the agency's Johnson Space Center in Houston, Jet Propulsion Laboratory in Southern California, Marshall Space Flight Center in Huntsville, Alabama, Ames Research Center in California's Silicon Valley, and others.

• January 4, 2022: Spacecraft controllers started the final steps in the deployment of the sunshield of the James Webb Space Telescope Jan. 3 after fixing two minor issues with the spacecraft. 51)

Figure 29: After launch, JWST requires two weeks of major deployments of its sunshield and telescope, a process with many potential failure modes (image credit: NASA GSFC/CIL/Adriana Manrique Gutierrez)

- NASA announced late Jan. 3 that it had completed the tensioning of three of the five layers of aluminum-coated Kapton that comprise the sunshield, which blocks sunlight from reaching the telescope and its instruments to cool them. That tensioning process, involving a series of motors, pulleys and cables, stretches the layers into the final shape and ensures proper separation between the layers.

- That work appeared to be going faster than expected. During a briefing with reporters around midday Jan. 3, Bill Ochs, JWST project manager at NASA's Goddard Space Flight Center, said tensioning of the outermost of the five layers had started, with plans to work inward layer by layer.

- "Today we are focused on tensioning layer 1. Layer 1 will take most of the day," he said. "Tomorrow through Wednesday we will deploy layers 2 through 5."

- With three of the five layers already deployed, though, NASA now expects to complete the final two layers, and thus the overall sunshield deployment process, Jan. 4. "This was the hardest part to test on the ground, so it feels awesome to have everything go so well today," said James Cooper, JWST sunshield manager at Goddard, in a NASA statement late Jan. 3. "The Northrop and NASA team is doing great work, and we look forward to tensioning the remaining layers."

- The tensioning was originally scheduled to start the day after the deployment of two booms on either side of the spacecraft that extended the sunshield to its full area. However, that deployment took longer than expected Dec. 31, and managers decided to give controllers the day off Jan. 1 to rest.

- Engineers then spent Jan. 2 resolving two minor issues with the spacecraft. One involved what Ochs described as a "preset max duty cycle" setting in the solar arrays that kept them from producing enough power to meet all the spacecraft needs. As a result, the spacecraft was using its battery in addition to power from the arrays.

- Amy Lo, vehicle engineering lead for JWST and prime contractor Northrop Grumman, said at the media briefing that controllers "rebalanced" the array by adjusting settings on each of its five individual panels. "We got the arrays to where they ought to be in order to provide the power that we need on Webb. We're power-positive, the arrays look good," she said.

- The other issue involved motors used for the tensioning process whose temperatures were higher than expected although still within conservative engineering margins. Controllers adjusted the orientation of the spacecraft temporarily to reduce the amount of sunlight hitting, and heating up, the motors. "They're nice and cool," Lo said of the motors. "We've got a lot of margin now on our temperature."

- Once the sunshield is fully tensioned, spacecraft controllers will turn their attention to JWST's mirrors. Ochs said the tripod holding the secondary mirror could be deployed late in the week, followed over the weekend by two wings holding segments of the primary mirror as well as an aft radiator.

- Getting the sunshield in place, though, will be a major step toward completing the full deployment of JWST. Ochs estimated that, once the sunshield is fully tensioned, the mission will have retired 70–75% of the 344 single-point failures in the spacecraft.

- "When I get asked, ‘What keeps you up the most at night?' obviously it's the sunshield deployment," he said. "I don't expect any drama. I always tell folks, the best thing for operations is boring, and that's what we anticipate over the next three days, to be boring. I think we'll all breathe a sigh of relief once we get to the final layer, layer 5, tensioning. But I don't expect drama."

• December 29, 2021: After a successful launch of NASA's James Webb Space Telescope Dec. 25, and completion of two mid-course correction maneuvers, the Webb team has analyzed its initial trajectory and determined the observatory should have enough propellant to allow support of science operations in orbit for significantly more than a 10-year science lifetime. (The minimum baseline for the mission is five years.) 52)

- The analysis shows that less propellant than originally planned for is needed to correct Webb's trajectory toward its final orbit around L2, a point of gravitational balance on the far side of Earth away from the Sun. Consequently, Webb will have much more than the baseline estimate of propellant – though many factors could ultimately affect Webb's duration of operation.

- Webb has rocket propellant onboard not only for midcourse correction and insertion into orbit around L2, but also for necessary functions during the life of the mission, including "station keeping" maneuvers – small thruster burns to adjust Webb's orbit — as well as what's known as momentum management, which maintains Webb's orientation in space.

- The extra propellant is largely due to the precision of the Arianespace Ariane 5 launch, which exceeded the requirements needed to put Webb on the right path, as well as the precision of the first mid-course correction maneuver – a relatively small, 65-minute burn after launch that added approximately 45 mph (20 m/s) to the observatory's speed. A second correction maneuver occurred on Dec. 27, adding around 6.3 mph (2.8 m/s) to the speed.

- The accuracy of the launch trajectory had another result: the timing of the solar array deployment. That deployment was executed automatically after separation from the Ariane 5 based on a stored command to deploy either when Webb reached a certain attitude toward the Sun ideal for capturing sunlight to power the observatory – or automatically at 33 minutes after launch. Because Webb was already in the correct attitude after separation from the Ariane 5 second stage, the solar array was able to deploy about a minute and a half after separation, approximately 29 minutes after launch.

- From here on, all deployments are human-controlled so deployment timing – or even their order — may change. Explore what's planned here.

• December 25, 2021: At 7:50 p.m. EST (corresponding to 00:50 UTC on Dec. 26), Webb's first mid-course correction burn began. It lasted 65 minutes and is now complete. This burn is one of two milestones that are time critical — the first was the solar array deployment, which happened shortly after launch. 53)

- This burn adjusts Webb's trajectory toward the second Lagrange point, commonly known as L2. After launch, Webb needs to make its own mid-course thrust correction maneuvers to get to its orbit. This is by design: Webb received an intentional slight under-burn from the Ariane-5 that launched it into space, because it's not possible to correct for overthrust. If Webb gets too much thrust, it can't turn around to move back toward Earth because that would directly expose its telescope optics and structure to the Sun, overheating them and aborting the science mission before it can even begin.

- Therefore, we ease up to the correct velocity in three stages, being careful never to deliver too much thrust — there will be three mid-course correction maneuvers in total.

- After this burn, no key milestones are time critical, so the order, location, timing, and duration of deployments may change.

• December 25, 2021: From one space flier to another, ESA astronaut Matthias Maurer shares a message of support for the James Webb Space Telescope (Webb) launch, from ESA's Columbus science laboratory on the International Space Station. 54)

- Matthias is currently living and working in space for his first mission known as Cosmic Kiss. He describes the mission of Webb as part of humankind's biggest adventure, as we explore the cosmos to understand our place within it.

Figure 30: Best wishes to the JWST mission from Matthias Maurer on the ISS (video credit: ESA)

- Webb is the next great space science observatory following Hubble, designed to answer outstanding questions about the Universe and to make breakthrough discoveries in all fields of astronomy. It is an international partnership between the European Space Agency ESA, US space agency NASA, and the Canadian Space Agency CSA, and has been launched on an Ariane 5 rocket from Europe's Spaceport in French Guiana.

- Webb is designed to see farther into our origins: from the formation of stars and planets to the birth of the first galaxies in the early Universe, just as the International Space Station enables us to learn more about our home planet.

JWST continued

Sensor complement: (NIRCam, NIRSpec, MIRI, FGS/NIRISS)

NIRCam (Near-Infrared Camera)

NIRCam funded by NASA with the University of Arizona as prime contractor (PI: Marcia J. Rieke). CSA is participating in the development of the NIRCam instrument. The industrial partner is Lockheed-Martin Advanced Technology Center, Palo Alto, CA. The NIRCam objectives are: 55) 56) 57) 58) 59) 60)

• To find "first light" sources. NIRCam surveys will become the backbone of the first light searches and for galaxy evolution studies.

• To assist the space telescope in initial (after deployment) and periodic alignment tests throughout the mission. This requires wavefront sensing to assure perfect alignment and shape of the different primary mirror segments.

• The camera also includes features for studying star formation in the Milky Way and for discovering and characterizing planets around other stars.

The various roles place additional constraints on the camera design. First, the camera must accommodate extra optics and pupil analyzers to enable the wavefront sensing. Secondly, the modules incorporating the wavefront sensing must be fully redundant as the mission depends critically on this functionality. Hence, the NIRCam design includes two identical imaging modules each of which includes dual filter wheels. The dual filter wheels are configured so that one wheel holds bandpass filters while the other wheel holds pupil analyzers thus permitting wavefront analysis as a function of wavelength.

NIRCam employs a compact refractive optics design using dichroics (to split the incoming radiation in 2 wavelengths) to enable simultaneous observation of a field at λ < 2.5 µm and at λ > 2.5 µm. The short wavelength module is Nyquist-sampled at 2 µm while the long wavelength module is Nyquist-sampled at 4 µm.

 Wavelength range 0.6 - 5.0 µm Spectral resolution Selection of R~4 and R~10 discrete filters, R~100 using 2 tunable filters FOV (Field of View) Imaging: 2.16 x 2.16 arcmin at two wavelengths simultaneously R=100 imaging: Two 2.16 x 2.16 arcmin fields (one λ<2.5µm, one λ>2.5µm) Spatial resolution Imaging: 0.0317 arcsec/pixel, λ<2.5µm; 0.068 arcsec/pixel, λ > 2.5µm R=100: 0.0648 arcsec/pixel Coronagraphy Choice of coronagraphic spots and pupils in all instrument sections

Table 4: Overview of the NIRCam capabilities

 Parameter Imaging module 1 Imaging module 2 Tunable filter module short λ Tunable filter module long λ Wavelength range (µm) 0.6 to 2.3 2.4 to 5 0.6 to 2.3 2.4 to 5 1.2 to 2.5 1 to 2.5 goal 2.5 to 4.5 2.3 to 5 goal Nyquist sampling (µm) 2 / 4 2 / 4 4 4 Pixel format 4096 x 4096 (short λ) 2048 x 2048 (long λ) 4096 x 4096 (short λ) 2048 x 2048 (long λ) 2048 x 2048 2048 x 2048 FOV (arcmin) 2.3 x 2.3 2.3 x 2.3 2.3 x 2.3 2.3 x 2.3 Spectral resolution 4, 10 4, 10 100 100

Table 5: NIRCam module characteristics

Figure 31: Schematic layout of a NIRCam imaging module (image credit: NASA)

Figure 32: Schematic of NIRCam coronagraphic design (image credit: STScI)

Legend to Figure 32: An optical wedge in the pupil wheel brings the coronagraphic spots into the field of view. The spots are matched with Lyot stops.

Coronagraphy: To enable the coronagraphic imaging of nearby stars, each of the two identical optical trains in the instrument also contains a traditional focal plane coronagraphic mask plate held at a fixed distance from the FPAs (Focal Plane Assemblies), so that the coronagraph spots are always in focus at the detector plane. Each coronagraphic plate is transmissive, and contains a series of spots of different sizes to block the light from a bright object. Each coronagraphic plate also includes a neutral density spot to enable centroiding on bright stars, as well as point sources at each end that can send light through the optical train of the imager to enable internal alignment checks. Normally these coronagraphic plates are not in the optical path for the instrument, but they are selected by rotating into the beam a mild optical wedge that is mounted in the pupil wheel (Figure 32), which translates the image plane so that the coronagraphic masks are shifted onto the active detector area (Ref. 33).

Figure 33: Layout of a NIRCam imaging module (image credit: University of Arizona)

The NIRCam filters and pupil selections are given in Table 6. All of the camera's filter wheels are identical, 12 position dual wheels. NIRCam also includes a set of broadband filters whose wavelengths and widths have been carefully chosen to support accurate photometric redshift estimation.

 Position Shortwave imaging Longwave imaging Tunable filter Filter wheel (µm) Pupil wheel Filter wheel (µm) Pupil wheel Filter wheel Pupil wheel 1 B1: 0.7 Imaging pupil B5: 2.7 Imaging pupil Blocker-1 Imaging pupil 2 B2: 1.1 Flat field source B6: 3.6 Flat field source Blocker-2 Flat field source 3 B3: 1.5 Outward pinholes B7: 4.4 Outward pinholes Blocker-3 Outward pinholes 4 B4: 2.0 Coron pupil 1 I4: 2.4-2.6 Coron pupil 1 Blocker-4 Coron pupil 1 5 I1: 1.55-1.7 Coron pupil 2 I5: 2.8-3.2 Coron pupil 2 Blocker-5 Coron pupil 2 6 I2: 1.7-1.94 HeI 1.083 µm I6: 3.2-3.5 TBD Blocker-6 Cal pattern 1 7 I3: 2.0-2.22 WFS-1 (Wavefront Sensing) I7-CO2: 4.3 TBD Blocker-7 Cal pattern 2 8 B8: 0.8-1.0 WFS-2 I8-CO: 4.6 TBD Blocker-8 Cal pattern 3 9 Hα: 0.656 WFS-3 Brα: 4.05 TBD Blocker-9 Cal pattern 4 10 [Fell] 1.64 WFS-4 H2: 2.41 TBD 1%-1 TBD 11 Pα: 1.875 WFS-5 H2: 2.56 TBD 1%-2 TBD 12 H2: 2.12 WFS-6 H2: 4.69 TBD 1%-3 TBD

Table 6: Specification of NIRCam filters and pupils

Figure 34: Illustration of the NIRCam instrument (image credit: NASA)

The expected point-source sensitivity is ~3.5 nJy for wavelengths from 0.7 - 5 µm in a 100,000 second exposure at a SNR (Signal-to-Noise Ratio) of 10. All ten detectors arrays needed for NIRCam are using Teledyne Technologies (former Rockwell Scientific )HgCdTe 2k x 2k devices (HAWAII-2RG detector technology, also referred to as H2RG). The short wavelength bands will be sampled at 4096 x 4096 pixels (0.0317 arcsec/pixel), while the long wavelength bands are being sampled by 2048 x 2048 pixels (0.0648 arcsec/pixel). The focal plane array includes detector and cryogenic electronics. 61) 62)

Note: The term "Jy" refers to the "Jansky," the unit of radio‐wave emission strength, in honor of Karl G. Jansky (1905‐1950) an American engineer whose discovery of radio waves (1931) from an extraterrestrial source inaugurated the development of radio astronomy. Jansky published his findings in 1932 while working at Bell Telephone Laboratories in Murray Hill, NJ.
The "Jy" is a unit of radiative flux density (or radio‐wave emission strength) which is commonly used in radio and infrared astronomy. 1 Jy = 10‐26 W/(m2 Hz). The units of Jy (Hz)‐1/2 then refer to the noise power.

Figure 35: This new 2Kx2K pixel NIRCam sensor chip assembly incorporates improved barrier layers to increase the ground storage lifetime (image credit,NASA, Bernie Rauscher, "JWST Detector Update," Ref. 42)

Legend to Figure 35: The Teledyne H2RG detectors are being used in 3 instruments of JWST, namely in NIRCam, NIRSpec, and in FGS/NIRISS.

The NIRCam coronagraph: Each NIRCam module will be equipped with a simple Lyot coronagraph consisting of a selection of focal plane occulters and pupil masks (Lyot stops). The requirements are:

1) Provide imaging to within 0.6 arcsec (4λ/D) of the star at λ = 4.6 μm and to within 0.3 arcsec at λ = 2.1 μm for the detection of extrasolar planets seen in emission.

2) Provide imaging to within 0.8 arcsec (6λ/D) of the star at λ = 4.3 μm, 0.64 arcsec at λ = 3.35 μm, and 0.4 arcsec at λ = 2.1 μm for observations of circumstellar disks seen in reflected light.

3) The occulters must be rigidly mounted and must not interfere with imaging during non-coronagraphic observations, requiring placement outside the normal field of view.

4) Ideally, suppress the diffraction pattern produced by the JWST obscurations to a level equal to or below the scattered light created by the uncorrectable optical surface errors, given the budgeted ~131 nm rms of wavefront error prior to the coronagraphic occulters.

5) Provide sufficient throughput to image 1 Gyr-old Jupiter-mass planets around the nearest late-type stars with 1-2 hours of exposure time.

6) Tolerate 2% pupil misalignments due to pupil wheel positioning errors and telescope-to-instrument rotational offsets.

7) Tolerate 10-40 marcsec (milliarcsecond) of pointing error at λ = 4.6 μm without a significant decrease in performance.

NIRCam status:

• Jan. 6, 2015: The MIRCam instrument surpassed expectations during tests in late 2014. NIRCam performed significantly better than requirements during the first integrated, cryogenic testing program at GSFC (Goddard Space Flight Center), Maryland. 63)

- In April 2014, NASA installed the instrument alongside others in the ISIM (Integrated Science Instrument Module), which finished cryogenic and vacuum testing late last year.

• Flight NIRCam ready for integration into ISIM (Ref. NO TAG#.

NIRSpec (Near-Infrared multi-object Spectrograph)

NIRSpec is funded by ESA (Project Scientist: Peter Jakobsen of ESA/ESTEC) with Airbus Defince and Space (formerly EADS Astrium GmbH) as prime instrument contractor (the detector arrays and a microshutter are supplied by NASA/GSFC). The key objectives are the study of galaxy formation, clustering, chemical abundances, star formation, and kinematics, as well as active galactic nuclei, young stellar clusters, and measurements of the initial mass function of stars (IMF). 64) 65) 66) 67) 68) 69) 70)

The region of sky to be observed is transferred from the JWST optical telescope element (OTE) to the spectrograph aperture focal plane (AFP) by a pick-off mirror (POM) and a system of foreoptics which includes a filter wheel for selecting band passes and introducing internal calibration sources. The nominal scale at the AFP is 2.516 arcsec/mm.

Figure 36: CAD layout of the NIRSpec instrument with outer shroud removed (image credit: ESA)

The NIRSpec baseline design uses a micro-electromechanical system (MEMS), consisting of an array of about 1000 x 500 microshutters, to select hundreds of different objects in a single field of view.

The NIRSpec instrument will be the first slit-based astronomical MOS (Multi-Object Spectrograph) in space providing spectra of faint objects over the near-infrared 1.0-5.0 µm wavelength range at spectral resolutions of R=100, R=1000 and R=2700. The instrument's all-reflective wide-field optics, together with its novel MEMS-based programmable microshutter array slit selection device and its large format low-noise HgCdTe detector arrays (2 detectors of 2 k x 2 k pixels), combine to allow simultaneous observations of > 100 objects within a FOV of 3.4 arcmin x 3.6 arcmin with unprecedented sensitivity. 71) 72) 73)

Figure 37: Schematic layout of the NIRSpec optics (image credit: ESA)

NIRSpec is required to select various spectral band widths and split these up into its comprised wavelengths. These functions are achieved by the FWA (Filter Wheel Assembly) and the GWA (Grating Wheel Assembly). The filters of the FWA select a different bandwidth of the spectrum each while the gratings on the GWA yield specific diffractive characteristic for spectral segmentation. A high spectral sensitivity as well as the ability to detect the spectra of various objects at the same time result in high requirements regarding the positioning accuracy of the optics of both mechanisms in order to link the detected spectra to the 2-dimensional images of the observed objects. 74)

The spectrometer uses diffractive gratings to spatially separate the incoming light and analyze several objects simultaneously. The NIRSpec mechanism yields 6 different gratings and one prism to work with various spectral resolutions and in different ranges of the infrared spectrum. A TAM (Target Acquisition Mirror) allows allocation of the spectra and the corresponding stellar objects. These 8 optical elements are integrated on a GWA (Grating Wheel Assembly) as shown in Figure 38. It exchanges the diffractive optic within the instrument's beam path with high precision to allow correlation of different spectra taken from the same object.

To avoid the overlap of various orders of diffraction on the detector, a set of spectral filters was designed to select the desired wavelength range. These filters are mounted on a mechanism quite similar to the GWA. It moves one filter into the beam path to build a fitting combination of grating in use and preselected range of wavelength. This FWA (Filter Wheel Assembly) holds four edge filters and two band filters for various wavelengths, one clear filter for target acquisition and a mirror assembly for in-orbit calibration and pupil alignment during integration of the mechanism (Figure 39).

Figure 38: Illustration of the GWA mechanism (image credit: Carl-Zeiss Optronics)

Figure 39: Illustration of the FWA mechanism (image credit: Carl-Zeiss Optronics)

Mechanical alignment: Since both FWA and GWA are mechanisms actively influencing the beam path of the instrument, precise and repeatable alignment of the currently used optic, it is essential to ensure a stable image on the detector. Especially the GWA alignment is crucial since its optic works in reflection where every tilt of the optic is carried over directly into the alignment of the instrument. The FWA on the other hand uses planar elements working in transmission inducing but a fraction of their misalignment into an aberration of the beam (Ref. 74).

NIRSpec includes also an IFU (Integral Field Unit) device with the objective to study of the dynamics of high redshift galaxies. This device provides in addition a NIRSpec backup acquisition mode for spectroscopy. The IFU permits a 2-D spectral characterization of astronomical objects with unprecedented depths, especially in the 2-5 µm wavelength range. The IFU covers a FOV of 3 arcsec x 3 arcsec and provides five fixed slits for detailed spectroscopic studies of single objects. The NIRSpec-IFU is expected to be capable of reaching a continuum flux of 20 nJy (AB>28) in R=100 mode, and a line flux of 6 x 10-19 erg s-1 cm-2 in R=1000 mode at an SNR> 3 in an exposure period of 104 s.

The FPA (Focal Plane Array) consists of sub-units, each 2 k x 2 k, forming an array of 2 k x 4 k sampled at 100 marcsec (milliarcsecond) pixels. The detectors are thinned HgCdTe arrays (ASICs) built by the Rockwell Science Center and referred to as SIDECAR (System for Image Digitization, Enhancement, Control and Retrieval). Each of the two ASICs has 2048 x 2048 pixels, pixel size of 18 µm, pixel scale = 100 mas (micro arcseconds), the data are locally digitized. 75)

The NIRSpec also contains a calibration unit with a number of continuum and line sources.

Figure 40: Illustration of a MSA (Microshutter Array) assembly at left and the FPA SIDECAR ASIC at right, (image credit: NASA)

Multiobject spectroscopy: A special MEMS device, referred to as MSA (MicroShutter Array), is being developed at NASA/GSFC to be used as a programmable field selector for NIRSpec. The objective is to provide a means to observe numerous objects simultaneously and to eliminate the confusion caused by all other sources. MSA consists of microshutter arrays arranged in a 2 x 2 quadrant mosaic. Each quadrant represents a closely packed array of 175 x 384 of shutters each of which may be addressed independently - allowing only the light from objects of interest into the instrument. The MSA covers a FOV of 3.6 arcmin x 3.6 arcmin (each microshutter has a FOV of 0.2 x 0.4 arcsec) - allowing the simultaneous observation of about 100 objects.

The microshutters themselves are MEMS devices produced on a thin silicon nitride membrane on 100 µm x 200 µm pitch (spectral x spatial direction). They are actuated magnetically and latched and addressed electrostatically. The MSA object selection feature represents an enabling technology development with a first introduction in spaceborne astronomy. 76)

Figure 41: Schematic layout of the microshutter assembly (image credit: NASA, ESA)

The MSA microshutter array consists of ust under a quarter of a million individually controlled microshutters. By programming the array to only open those shutters coinciding with pre-selected objects of interest, light from these objects is isolated and directed to the spectroscopic stage of NIRSpec to produce the spectra.

Figure 42: Photo of NASA/GSFC engineers inspecting an MSA with a low light test (image credit: GSFC, Chris Gunn, ESA) 77) 78)

Legend to Figure 42: The inspection light source is held by the technician at the front of the picture. Four array quadrants are located within the octagonal frame in the center of a titanium mosaic base plate.

The team, led by Principal Investigator Harvey Moseley of GSFC has demonstrated that electrostatically actuated microshutter arrays — that is, those activated by applying an specific voltage — are as functional as the current technology's magnetically activated arrays. This advance makes them a highly attractive capability for potential Explorer-class missions designed to perform multi-object observations. 79)

Considered among the most innovative technologies to fly on the Webb telescope, the microshutter assembly is created from MEMS technologies and comprises thousands of tiny shutters, each about the width of a human hair. Assembled on four postage-size grids or arrays, the 250,000 shutters open or close individually to allow only the light from targeted objects to enter Webb's NIRSpec, which will help identify types of stars and gases and measure their distances and motions. Because Webb will observe faint, far-away objects, it will take as long as a week for NIRSpec to gather enough light to obtain good spectra.

Figure 43: Alternate view of the NIRSpec instrument (ESA, NASA)

The NIRSpec instrument has a size of about 1.90 m x 1.3 m x 0.7 m and an estimated mass of about 196 kg.

Figure 44: Photo of the NIRSpec engineering test unit in Oct. 2009 (image credit: ESA)

The spectrograph structure is built from silicon carbide (SiC) - a monolithic ceramic providing the properties to meet the extremely high demands for dimensional stability and geometrical accuracy for the optical assembly. Geometrical distortions between NIRSpec and the ISIM, generated by very high temperature differences between cryogenic operational and ambient on ground environment are balanced by so called Kinematic Mounts made from titanium alloy. The need to exchange these parts without losing optical performance of the already aligned instrument led to the development of a highly sophisticated exchange procedure. 80)

The existing Kinematic Mounts already integrated on the Flight Model of NIRSpec were declared non-flight-worthy due to a detection of a manufacturing issue within the tapered areas, dedicated for flexural bending. Consequently a remanufacturing of the three OBKs (Optical Bench Kinematic Mounts) was decided and the development of an exchange philosophy considering all aspects of safety and technical requirements was developed in a joint team of ESA and Airbus Defence and Space.

Due to the detailed planning, preparation and practice, the actual exchange on the NIRSpec flight hardware was performed in five days without any procedure variation. The exchange was successfully performed as trained before.

The dye penetrant investigations performed in between the individual OBK exchange activities confirmed no damage of the SiC interfaces of the OBBP (Optical Bench Base Plate). These results were backed by acoustic monitoring which showed that no shock was introduced and no crack was initiated inside the SiC structure.

The results of the online optical measurements showed that the relative position and the PAR (Pupil Alignment Reference) remained stable within the measurement accuracy better than 3 acrsec angular and 10 µm relative PAR center displacement.

Status of NIRSpec:

• July 20, 2015: Engineers from Airbus and ESA (European Space Agency) work inside NASA Goddard Space Flight Center's large clean room to remove the cover on Webb Telescope's NIRSpec (Near InfraRed Spectrometer) instrument in preparation for the replacement of the MSA (Micro Shutter Array) and the FPA (Focal Plane Assembly). 81)

• Feb. 2015: The past two months have seen a team of engineers engaged in the intricate activity of replacing key components of the NIRSpec (Near InfraRed Spectrograph) on the James Webb Space Telescope. The instrument is now ready for the next series of extensive environmental tests devised to ensure that JWST's instruments can withstand the stresses and strains of launch and operation in space. 82) 83) 84)

- In the summer of 2014, the JWST Integrated Science Instrument Module (ISIM), fitted with all four instruments (NIRSpec, MIRI, NIRCam, and FGS/NIRISS), successfully completed cryogenic testing in a '24/7' campaign that lasted 116 days.

- However, the positive outcome of this important test campaign did not mean that ISIM and the instruments were ready for integration onto JWST's telescope. It has been known for over a year that additional work would be necessary to get some of the instruments into their final flight configuration. As a consequence, a period of a few months was allocated for these activities, immediately after the completion of the cryogenic test campaign.

- In particular, NIRSpec needed to have its detectors, microshutter assembly and optical assembly cover replaced. Also, the NIRCAM and FGS/NIRISS teams had to exchange some components in their instruments. MIRI was the only instrument that remained integrated with the ISIM. However, MIRI's configuration was also updated by installing the flight model cooler Cold Head Assembly (CHA) and exchanging some of the cooler lines and their supports.

- The first generation of JWST's highly sensitive near-infrared detectors were found to suffer from a design flaw that resulted in a progressive degradation of their performance. New detectors have now been installed in all three near-infrared instruments.

- Another crucial component of NIRSpec are its MSA (Microshutter Assembly), a new technology developed for JWST by NASA. - One of the defining and pioneering features of NIRSpec is its ability to analyze the light from more than 100 astronomical objects at the same time. This is made possible by an assembly of four microshutter arrays, totalling almost a quarter of a million individual shutters.

- One of the defining and pioneering features of NIRSpec is its ability to analyze the light from more than 100 astronomical objects at the same time. This is made possible by an assembly of four MSAs, totalling almost a quarter of a million individual shutters.

- The cryogenic test revealed that several thousand of the individual microshutters had become inoperable and could not be opened. This susceptibility to acoustic noise was not expected and had gone undetected because of the difficulty of reproducing the environment to which the microshutters are actually subjected in this instrument. As a result of this problem, the performance of the microshutters in NIRSpec was strongly degraded. The NIRSpec Engineering Test Unit (ETU) provided the most realistic test environment for the MSA. These various tests provided a wealth of information that helped NASA to identify the cause of the 'failed closed' shutters issue.

- The new MSA contains three 'original design' arrays and one 'new design' array. In addition to most arrays being pre-screened at array level, the complete new MSA flight model was acoustically tested in the NIRSpec ETU before it was installed in the flight version of NIRSpec.

• April 4, 2014: An important milestone for JWST was passed on 25 March with the installation of the NIRSpec instrument on the ISIM (Integrated Science Instrument Module) at NASA/GSFC. All four science instruments are now in place on the ISIM, ready for the next series of tests. 85)

• Feb. 2014: The NIRSpec instrument is being installed on the ISIM (Integrated Science Instrument Module) at Goddard in preparation for an extensive series of tests with the full instrument complement. In addition, new detectors have been selected for NIRSpec, to be installed later this year. 86)

• Sept. 30, 2013: The NIRSpec instrument has arrived at the NASA/GSFC. 87)

• In early September 2013, the NIRSpec instrument, built by Astrium GmbH, was formally handed over to ESA. This marks an important milestone in Europe's contribution to the JWST mission. Having undergone rigorous testing in Europe, NIRSpec will be shipped to NASA later this month for integration into JWST's instrument module, followed by further testing and calibration as the whole observatory is built up. 88)

MIRI (Mid-Infrared Camera-Spectrograph)

MIRI is a joint instrument development of NASA and ESA. The instrument optics module and optical bench will be provided by the European MIRI Consortium funded by the ESA member states. NASA/JPL will provide the remainder of the instrument, notably the detector and cryostat subsystems. Within the joint instrument science team, Gillian S. Wright of the UKATC (UK Astronomy Technology Center), Edinburgh, is the PI of the European MIRI Consortium while George H. Rieke at the Steward Observatory of the University of Arizona (UA) is the MIRI PI for NASA. ESA coordinates the activities of the European MIRI Consortium (21 institutes from 10 countries) while EADS Astrium Ltd. functions as the main instrument contractor. The MIRI instrument has a mass of ~ 103 kg. 89) 90) 91) 92) 93) 94) 95) 96) 97)

Note: The ROE (Royal Observatory Edinburgh) comprises the UKTAC (UK Astronomy Technology Center) of the Science and Technology Facility Council (STSC), the Institute of Astronomy of the University of Edinburgh and the ROE Visitor Center.

Further participating European organizations in the MIRI project are: Astron, The Netherlands; CCLRC, Rutherford Appleton Laboratory (RAL), UK; CEA Service d'Astrophysique, Saclay, France; Centre Spatial De Liège, Belgium; CSIC (Consejo Superior de Investigaciones Científicas), Spain; DSRI (Danish Space Research Institute), Denmark; Dublin Institute for Advanced Studies, Ireland; IAS (Institut d'Astrophysique Spatiale), Orsay, France; INTA (Instituto Nacional de Técnica Aeroespacial), Spain; LAM (Laboratoire d'Astrophysique de Marseille), France; MPIA (Max-Planck-Institut fur Astronomie), Heidelberg, Germany; Observatoire de Paris, France; PSI (Paul Scherrer Institut), Switzerland; University of Amsterdam, The Netherlands; University of Cologne, Germany; University of Leicester, UK; University of Leiden, The Netherlands; University of Leuven, Belgium; University of Stockholm, Sweden.

As part of the European cooperation with NASA on the JWST program, MIRI was set up as a 50 : 50 partnership between ESA and NASA, with the European Consortium (EC) in charge of the optical bench assembly and the JPL (Jet Propulsion Laboratory) in charge of the detector system, the cooling system, and the flight software (Figure 45). In addition to the responsibilities shown, GSFC (Goddard Space Flight Center) provides the harness between the optical module and the ICE (Instrument Control Electronics). The formal delivery of the MIRI Optical System, including the detectors chain provided by JPL, to NASA is the responsibility of ESA.

Figure 45: Overview of MIRI instrument concept, contributions, interfaces and responsibilities (image credit: ESA, NASA)

In contrast to other science missions, where each scientific instrument has its own dedicated computer, on JWST there is one unit for all instruments where the flight software for each instrument resides – the ICDH (Instrument Control and Data Handling) electronics. Failure modes and event upsets are handled in this unit. The ICDH interfaces via an IEEE-1553B (MIL-STD-1553B) bus to the dedicated control electronics for the instrument mechanisms (ICE) and, via a remote services unit, for the FPE (Focal Plane Electronics) unit as shown in Figure 46.

Figure 46: Functional block diagram of MIRI optical and cooler subsystem interfaces (image credit: MIRI consortium)

MIRI's principal science objectives relate to the origin and evolution of all cosmic constituents, in particular to galaxy formation, star formation, and planet formation on a wide range of spatial and temporal scales. MIRI is to provide imaging, coronagraphy and low- and medium-resolution spectroscopy in the mid-infrared band (the 5-28 µm), representing a broad wavelength response in the thermal infrared. To achieve an optimized detection sensitivity, MIRI requires a high photon conversion efficiency as well as spectral and spatial passbands matched to the observation targets.

The MIRI design features an imager and a dual spectrometer (Figure 48). Light enters from the telescope through the IOC (Input Optics and Calibration) module. The IOC is part of the MIRI Optical Bench Assembly. It is designed to pick-off the MIRI field of view from the JWST Fine Steering Mirror and to relay the relevant parts of this FOV into the spectrometer and into the imager subsystems. The IOC additionally provides in-flight calibration fluxes to the imager and is mounted onto the MIRI primary structure (deck) and is operated at about 6 K. The IOC is being provided by CSL (Centre Spatial de Liege) of Liege University, Belgium.

The imager and the two spectrometer modules are based on all reflecting designs. The optical configuration of MIRI supports four science modes:

1) Photometric imaging in a number of bands from 5.6-25.5 µm within a FOV of 1.9 arcmin x 1.4 arcmin

2) Coronagraphy with a spectral range 10-27 µm in 4 bands (10.65, 11.4, 15.5, and 23 µm)

3) Low-resolution (R = 100) resolving power slit spectroscopy of single objects in the spectral range 5-11 µm

4) Medium-resolution (~100 km/s velocity resolution) integral field spectroscopy in the spectral range 5-28.5 µm over FOVs growing with wavelength from 3.5 x 3.5 to 7 arcsec x 7 arcsec.

Figure 47: The MIRI optics module (image credit: MIRI consortium)

The optical concept splits the instrument into two separate channels operating over the 5 to 28 µm wavelength range, one for imaging (over a 1.9 x 1.4 arcmin FOV) and one for medium resolution spectroscopy (up to 8 x 8 arcmin FOV). The functional split into two parts was chosen because it was found that it simplified the internal optical interfaces, and the complexity of the layout and of the mechanisms. Both the imager and spectrometer channels are fed by common optics from a single pick-off mirror placed close to the telescope focal plane, and fed also by a common calibration subsystem. - The pick-off mirror in front of the JWST OTE focal plane directs the MIRI FOV towards the imager. A small fold mirror adjacent to the imager light path picks off the small (up to 8 x 8 arcsec) FOV of the spectrometer. A second tilting fold in the spectrometer optical path is used to select either light from the telescope or from the MIRI calibration system.

Figure 48: MIRI instrument optical bench assembly and key subsystem layout (image credit: MIRI consortium)

The MIRI spectrometer is comprised of two parts, the SPO (Spectrometer Pre-Optics), built by UKATC, and the SMO (Spectrometer Main Optics), built by Astron, The Netherlands. The two parts of the spectrometer combine together using a spectrograph filter wheel which is made by MPIA (Max Planck Institute of Astronomy). The SPO houses the image slicers and the dichroic/grating wheels. Light enters the SPO directly from the IOC. Light passes from the image slicer, through a series of mirrors, to the FPM. The FPM in turn is located in the SMO. 98) 99)

Figure 49: Main optics of the MIRI spectrometer (image credit: MIRI European Consortium)

Figure 50: Illustration of the SPO (image credit: MIRI European Consortium)

The light is divided into four spectral ranges by the dichroics, and two of these ranges are imaged onto each of the two detector arrays. Along the way to the appropriate array, the light is dispersed by a diffraction grating. The gratings are mounted on mechanical turrets with three for each spectral range. A full spectrum is obtained by taking exposures at the three settings of each mechanical turret - the turrets are ganged together and operated with a single mechanism, and the dichroics allow the same spot on the sky to be distributed to all four spectrometer arms. Thus, only three exposures are required to obtain a complete spectrum.

 Channel 1 2 3 4 Nr of slices (N) 21 17 16 12 Wavelength range (µm) 5.5-7.7 7.7-11.9 11.9-18.3 18.3-28.3 Slice width (arcsec) Pixel size (arcsec) 0.176 0.196 0.277 0.196 0.387 0.245 0.645 0.273 FOV (arcsec) 3 x 3.87 3.5 x 4.42 5.2 x 6.19 6.7 x 7.73 Resolving power 2400-3700 2400-3600 2400-3600 2000-2400

Table 7: Summary of imager channels

The imager module has a combined FOV for the imager and coronagraph/low-resolution spectrometer modes. The coronagraph masks are placed at a fixed location on one edge of the imager field.

Figure 51: Schematic configuration of the MIRI imager module (image credit: MIRI European Consortium)

Figure 52: Illustration of the MIRI imager (image credit: MIRI European Consortium)

Figure 53: Illustration of the coronagraph (image credit: MIRI European Consortium)

The instrument uses phase mask coronagraphs. They reject the light of a central source by introducing phase shifts using a quadrant-design plate at the instrument input focal plane. These shifts cause the light from the source to interfere destructively at the detector array. Unlike conventional occulting Lyot coronagraphs, phase plates allow measurements to be obtained very close to the central object. Further from the central object, they provide performance similar to that of a conventional occulting coronagraph. The 4-quadrant phase mask is dividing an Airy disk (image of a point source) in the center of the field into 4 domains; and it applies a phase difference of p to two of them, so that the image is eliminated by destructive interference.

The dichroic filter wheel comprises three working positions to move gratings and dichroics simultaneously. Each is located on separate wheel discs. The two wheels feed light in to the four spectrometer channels inside MIRI.

Figure 54: Scheme of the spectrograph filter wheel (image credit: MIRI European Consortium)

Figure 55: Illustration of the dichroic wheel (image credit: MIRI European Consortium)

The filter wheel has 18 positions: 10 imaging filters, 4 coronagraphic diaphragms/filters, 1 neutral density filter, 1 double prism, 1 lens and 1 clear/blind position (counterweight of prism). The system has to operate in the cryo-vacuum of 7 K up to 10 years. The design is of ISOPHOT wheel mechanisms heritage flown on ESA's ISO (Infrared Space Observatory) mission. The filter wheel assembly houses a wheel disc carrying all the optical elements. Rotation is realized by a central two-phase torque motor (allows for bi-directional movement).

Figure 56: Illustration of the filter wheel (image credit: MIRI European Consortium)

The FPS (Focal Plane System) consists of three FPM (Focal Plane Module) units (two in the spectrometer and one in the imager), a single FPE (Focal Plane Electronics) unit, and a set of low noise FPE/FPM cryogenic harnesses that connect the FPMs to the FPE. Each FPM houses a single SCA (Sensor Chip Assembly) containing a 1024 x 1024 Si:As IBC detector array and readout electronics. The IBC (Impurity Band Conduction) technology of Raytheon Vision Systems has been selected for very sensitive, cryogenically cooled infrared detectors. These arrays are manufactured as a hybrid structure, referred to as SCA (Sensor Chip Assembly), consisting of a detector array connected with indium bumps to a ROIC (Readout Integrated Circuit). The Si:As IBC detector material offers the highest performance for longwave detection in low-background systems. 100) 101)

 Wavelength band (µm) Support mode Sensitivity (10σ, 10,000 s) 5.6 Imaging 0.19 mJy 12.8 Imaging 1.4 mJy 25.5 Imaging 29 Jy 6.4 Line spectroscopy 1.2 x 10-20 W m-2 22.5 Line spectroscopy 5.6 x 10-20 W m-2

Table 8: Overview of expected MIRI sensitivities

Figure 57: Schematic of the silicon detector array (image credit: JPL)

Figure 58: The FPM of MIRI (image credit: JPL)

MIRI cryocooler: The MIRI instrument (optical bench, all focal planes) is cooled to ~7 K by a super-frigid mechanical helium cryocooler system of NASA/JPL built by NGAS (Northrop Grumman Aerospace Systems), Redondo Beach, CA. The cryocooling is achieved by means of a cryostat. Two hydrogen vessels are being used, the larger one for the optical bench, and the smaller one for the detectors. The vessels are designed to hold 1000 liter of solid hydrogen at 7 K.

Active cooling is provided by a dedicated three stage Stirling-cycle PT (Pulse-Tube) to precool a circulating helium flow loop, with a Joule-Thomson (JT) expansion stage to provide continuous cooling to 6.2 K to a single point on the MIRI optical bench. Significant development of the cryocooler occurred as part of the ACTDP (Advanced Cryocooler Technology Development Program) prior to selection as the flight cryocooler for MIRI. 102) 103) 104)

Figure 59: Block diagram of the ACTDP design applied to the MIRI cooler subsystem; the dark lines show the He gas flow in the JT cooler loop (image credit: NGAS)

Figure 60: Illustration of the MIRI cryocooler elements (image credit: NGAS, UA, Ref. 97)

Figure 61: Schematic view of the distributed MIRI cryocooler subsystem (image credit: NGAS)

Legend to Figure 61: The drawing on the left side shows the spacecraft bus (bottom) and the OTE. The CCA (Cooler Compressor Assembly) and the CHA (Cold Head Assembly) are shown as expanded CAD renderings on the right hand side. The CCA is shown in context of the spacecraft bus and tower structures in the immediate vicinity. The CCE (Cryocooler Control Electronics) and the CTA (Cooler Tower Assembly) are not shown.

Status of MIRI:

• Feb. 2014: MIRI has performed beautifully during its first cryo-vacuum test campaign carried out at NASA's Goddard Space Flight Center towards the end of 2013. An examination of data recorded during those tests confirms that the instrument is in good health and performing well. 105)

• July 2013: The ISIM, with the two instruments (MIRI and FGS/NIRISS), is now being prepared for the first series of cryogenic tests, planned for later this summer. These will include optical, electrical and electromagnetic interference tests, all under cold vacuum conditions. The tests will be conducted in the SES (Space Environment Simulator) vacuum chamber at GSFC. 106)

• On April 29, 2013, MIRI was the second instrument to be installed into the ISIM (after FGS/NIRISS).

• MIRI arrived at GSFC on 28 May 2012, having been despatched from the Rutherford Appleton Laboratory in the United Kingdom, where it had been assembled. Engineers from ESA, the MIRI European Consortium and NASA were on hand to take delivery of this, the first of JWST's four instruments to arrive at GSFC.

FGS (Fine Guidance Sensor):

The FGS is a sensitive camera that provides dedicated, mission-critical support for the observatory's ACS (Attitude Control System). The camera can image two adjacent fields of view, each approximately 2.4 arcmin x 2.4 arcmin in size, and can also be configured to read out small subarrays (8 x 8 pixels) at a rate of 16 times/s. Even with these short integration times, the FGS is sensitive enough to reach 58 µJy at 1.25 µm (~Jab = 19.5). This combination of sky coverage and sensitivity ensures that an appropriate guide star can be found with 95% probability at any point in the sky, including high galactic latitudes.

The objectives of FGS are to provide constant directional data for the telescope, enabling it to maintain stability for improved image acquisition. Specific requirements are: 107) 108)

1) To obtain images for target acquisition. Full-frame images are used to identify star fields by correlating the observed brightness and position of sources with the properties of cataloged objects selected by the observation planning software.

2) To acquire preselected guide stars. During acquisition, a guide star is first centered in an 8 x 8 pixel window. Small angle maneuvers are then executed to translate this window to a pre-specified location within the FOV, so that an observation with one of the science instruments will be oriented correctly.

3) To provide the ACS with centroid measurements of the guide stars at an update rate of 16 Hz. These measurements will be used to enable stable pointing at the milli-arcsecond level.

Note: In the course of building and testing of the TFI (Tunable Filter Imager) flight model, numerous technical issues arose with unforeseeable length of required mitigation effort. In addition to that, emerging new science priorities caused that in summer of 2011 a decision was taken to replace TFI with a new instrument, called NIRISS (Near Infrared Imager and Slitless Spectrograph). 109) 110)

FGS/NIRISS (Near-Infrared Imager and Slitless Spectrograph):

FGS is one of the four science instruments on board the JWST, a contribution of CSA (Canadian Space Agency). The FGS-NIRISS science team is jointly led by John Hutchings of NRC (National Research Council) of Canada, Victoria, British Columbia, Canada and René Doyon, University of Montréal. - The FGS consists of two Guider channels and one Near-Infrared Slitless Spectrometer (NIRISS) channel. COM DEV Space Systems of Ottawa Canada is CSA's prime contractor for the FGS instrument. The NIRISS channel makes use of grisms and filters optimized for first-light science and exo-planet observations. This is a recent change in the configuration of the instrument which until the summer of 2011 made use of a tunable filter. The block diagram of the updated instrument configuration is shown in Figure 62111) 112)

Figure 62: Block diagram of the FGS (image credit: CSA, ComDev Ltd.)

The FGS prime function is to work with the ACS (Attitude Control Subsystem) of the Observatory to provide fine guiding. The guiding side of FGS (FGS-Guider) is a near-infrared (IR) camera operating in broadband light over the full 0.6-5 µm bandpass of its two Hawaii-2RG detectors. The FGS-Guider features an all reflective optical design with two redundant 2.3 arcmin x 2.3 arcmin FOV each capable of reading a small (8 x 8) subarray window to select any star in the FOV and to report its centroid every 64 ms (16 Hz) to the ACS, which in turn sends an error signal to the fine steering mirror of the telescope. At this sampling rate, the FGS-Guider is required to have a NEA (Noise Equivalent Angle) less than 4 marcsec (one axis) on a star with an integrated signal of 800 electrons, equivalent to approximately a JAB = 19:5 star. This limiting magnitude guarantees more than 95% of the sky coverage with at least three stars within the FGS-Guider FOV. 113) 114)

FGS features two modules: an infrared camera dedicated to fine guiding of the observatory and a science camera module, the NIRISS (Near-Infrared Imager and Slitless Spectrograph) covering the wavelength range between 0.7 and 5.0 µm with a FOV of 2.2 arcmin x2.2 arcmin.

A schematic optical layout of NIRISS is shown in Figure 63. The optical design is an all reflective design with gold-coated diamond-turned aluminum mirrors. The average WFE ( Wavefront Error) over the FOV of the instrument (telescope excluded) is less than 79 nm RMS.

Figure 63: NIRISS optical layout. The NIRISS optical configuration is identical to the old TFI one except that the etalon is no longer present and that the dual wheel has been repopulated with new filters and grisms as shown in Figure 64 (image credit: CSA, ComDev Ltd.)

NIRISS has a dual pupil and filter wheel assembly. Collimated light first passes through a selected position in the pupil wheel and then through the selected position in the filter wheel. Figure 64 shows the elements of the pupil and filter wheels. The PAR (Pupil Alignment Reference) shown in Figure 1 is used during ground testing to verify the positioning of NIRISS in the ISIM (Integrated Science Instrument Module). Its presence decreases the throughput of the "CLEARP" element by about 10%. 115)

The Dual Wheel is comprised of pupil and filter wheels, bearings, gears, static hub, rear motor/resolver plate and the support bracket. The equipment includes drive motors, resolvers and variable reluctance sensors. Each wheel (~280 mm diameter) is capable of rotating the optical elements to one of 9 desired positions, supported by a preloaded duplex pair of angular contact bearings. All moving parts use MoS2 dry lubricants compatible with the cryogenic environment. A stepper motor with a single-stage planetary gearhead is used to drive each wheel independently, through a reduction gear train. The optical parts are held in place by a metallic spring gasket with a precision holder machined from Ti 6Al-4V ELI annealed, stress-relieved prior to final machining and cryo-cycled prior to installing optical elements. A black tiodize coating is used for stray light control. 116) 117)

Figure 64: NIRISS dual wheel optical elements (image credit: CSA, COM DEV Ltd.)

Detector: The NIRISS detector consists of a single SCA (Sensor Chip Assembly) with the following characteristics:

- 2048 x 2048 pixel HgCdTe array. Each pixel is 18 microns on a side.

- Dark rate: < 0.02 e-/s

- Noise: 23 e- (correlated double sample)

- 2.2 arcmin x 2.2 arcmin FOV

- Plate scale in x: 0.0654 arcsec/pixel; plate scale in y: 0.0658 arcsec/pixel

• The 2048 x 2048 pixels of the SCA are divided into 2040 x 2040 photosensitive pixels and a 4-pixel wide border of non-photosensitive reference pixels around the outside perimeter. The reference pixels do not respond to light, but are sampled and digitized in exactly the same way as the light sensitive pixels. The reference pixels can be used to monitor and remove various low-frequency bias drifts.

• The composition of the detector is tuned to provide a long-wavelength cutoff at approximately 5.3 microns.

• The SCA is fabricated and packaged into a FPA (Focal Plane Assembly ) that includes a HAWAII-2RG readout integrated circuit (ROIC), which is controlled by a SIDECAR ASIC (Application Specific Integrated Circuit). The ASIC is a custom-built chip that clocks the array, sets the bias voltages, and performs the analog-to-digital conversion of the pixel voltages.

• The SCA is fabricated and packaged into a focal-plane assembly (FPA) that includes a HAWAII-2RG readout integrated circuit (ROIC), which is controlled by a SIDECAR Application Specific Integrated Circuit (ASIC). The ASIC is a custom-built chip that clocks the array, sets the bias voltages, and performs the analog-to-digital conversion of the pixel voltages.

• A full-frame read of the SCA is digitized through four readout amplifiers. Each amplifier reads a strip that is 512 x 2048 pixels. 118)

Figure 65: Schematic view of the NIRISS SCA (image credit: STScI)

Observation modes: NIRISS has four observing modes (Ref. 113):

1) BBI (Broadband Imaging) featuring seven of the eight NIRCam broadband filters

2) Low resolution WFSS (Wide-Field Slitless Spectroscopy) at a resolving power of ~150 between 1 and 2.5 µm

3) Medium-resolution SOSS (Single-Object Spectroscopy). The single-object cross-dispersed slitless spectroscopy enabling simultaneous wavelength coverage between 0.7 and 2.5 µm at R~660, a mode optimized for transit spectroscopy of relatively bright (J > 7) stars

4) sparse AMI (Aperture Interferometric Imaging) between 3.8 and 4.8 µm enabling high-contrast (~ 10-4) imaging of M < 8 point sources at angular separations between 70 and 500 marcsec.

Broadband imaging: NIRISS offers the same broadband imaging capability as NIRCam except that NIRISS does not carry the NIRCam F070W filter. The new blocking filters procured for NIRISS, used in combination with NIRCam short wavelength fitters, have measured inband transmission of 95% typically. As shown in Figure 66, NIRISS and NIRCam are predicted to have similar sensitivities within 10%. This sensitivity calculation takes into account the coarser pixel sampling (65 marcsec) of NIRISS at short wavelengths compared to NIRCam (32 marcsec). NIRISS is not expected to be used for broadband imaging unless parallel observing is eventually offered by the Observatory. If so, NIRISS could be easily used in parallel with NIRCam for a wide variety of programs including deep extragalactic surveys aiming at probing the galaxy population of the early universe.

Figure 66: Predicted NIRISS broadband imaging sensitivity (10σ, 104s) compared to NIRCam (image credit: CSA, COM DEV Ltd.)

WFSS (Wide-Field Slitless Spectroscopy): The WFSS mode of NIRISS operation is optimized for Ly α emitters (1-2.5 µm) and makes use of a pair of grisms GR150V and GR150H. In order to break wavelength-position degeneracy two prisms are at 90º angle to each other and are used in two separate imaging sessions. In this scheme, the intersection of the two perpendicular dispersion lines indicates undeviated wavelength and true sky position of the source.

It is implemented through the two GR150R & GR150C grisms operated in slitless mode at R = 150 (2 pixels), enabling low-resolution multi-object spectroscopy between 1 and 2.5 µm in first order. The grisms are resin-replicated on a low refractive index material (Infrasil 301) to minimize Fresnel loss. They were manufactured by Bach Research. The peak efficiency of a flight-like GR150 grism, i.e. manufactured with the same replication process (same substrate prism, same master), was measured to be ~80% (see Figure 67). Wavefront error measured at 90 K on both grism surfaces showed some distortion due to stress induced by CTE (Coefficient of Thermal Expansion) mismatch between the resin and the glass substrate. However, within uncertainties, the distortion was measured to be identical on both sides at 90 K. This distortion effectively turns the grism into a weak meniscus lens which, to first order, has no defocus. Cryogenic (90 K) monochromatic PSF measurements were also secured to estimate the TWFE (Transmitted Wavefront Error) of the GR150 grisms; the results indicate that they should have less than 30 nm (RMS) of TWFE. The image quality in the WFSS mode is therefore expected to be as good as in broadband imaging i.e. with a typical Strehl ratio of ~0.5 at 1.3 µm.

Figure 67: Blaze function of the GR150 grism measured on a flight-like grism. The flight prisms are expected to have very similar performance (image credit: CSA, COM DEV Ltd.)

SOSS (Single-Object Slitless Spectroscopy): This mode of NIRISS operation is optimized for relatively bright stars (e.g. exoplanet transiting systems) in 0.6-2.5 µm spectral range in the first order of dispersion. It is based on a GR700XD grism made of the directly ruled ZnSe. A ZnSe cross-dispersion prism is placed in front of the grism for an optimal separation of the first and second order spectra.

To optimize this mode for very high signal-to-noise ratio observations of bright objects, the entrance face of the ZnSe prism has a built-in cylindrical weak lens that defocusses the spectrum over ~25 pixels along the spatial direction, keeping the point spread function nearly diffraction-limited in the spectral direction. As a result, the spectrum is undersampled at most wavelengths along the spectral direction which, given the non-uniform detector pixel response in the presence of pointing jitter noise, constitutes a potential source of systematic effect for achieving high-precision differential spectrophotometry. To mitigate/minimize this problem, the GR700XD grism is slightly rotated by ~ 2º with respect to the detector. Given that the PSF (Point Spread Function) is spread over 25 pixels in the spatial direction, this rotation effectively provides Nyquist sampling at all wavelengths. Furthermore, since the GR700XD grism is operated in slitless mode, there are no flux variations induced by a slit. All these features, designed for achieving high-precision differential spectrophotometry, combined with the very stable thermal environment expected at L2, will make the NIRISS SOSS mode a powerful capability for atmospheric characterization of transiting exoplanets.

Figure 68: Line-flux sensitivity in the NIRISS WFSS mode for various blocking filters (image credit: CSA, COM DEV Ltd.)

Legend to Figure 68: The dashed line is the predicted NIRSpec sensitivity for the multi-slit low-resolution (R ~100) mode; the solid circles superimposed on the dashed line is the spectral resolution of NIRSpec at that wavelength. The green triangle is the sensitivity that TFI would have had at its shortest wavelength (1.45 µm; zLyα = 10:9); TFI would have been typically a factor ~3 more sensitive than NIRISS at the expense of sampling a very narrow redshift range at a given wavelength and limited to probe zLyα > 10:9.

AMI (Aperture Masking Interferometry): The NIRISS PW includes a seven-aperture non-redundant mask (NRM; Figure 69) used for aperture masking interferometry (AMI). The AMI technique enables high-contrast imaging at inner working angle theoretically as small as 1 λ/2D. This mode is particularly appealing for faint companion detection (brown dwarfs & exoplanets) around relatively bright stars. AMI has been successfully used on the ground for a variety of applications, for example to unveil the spiral structure of the stellar wind of the Wolf-Rayet star WR98A (Monnier et al. 1999), detect brown dwarfs (Lloyd et al., 2006) and to put mass limits on the presence of brown dwarfs and exoplanets within the inner 10 AU of the multi-planetary system HR8799.

Figure 69: NIRISS non-redundant mask design (image credit: CSA, COM DEV Ltd.)

The main scientific application of AMI with NIRISS is for high-contrast imaging of point sources but it can also be used for aperture synthesis applications like probing the inner structure of nearby active galactic nuclei. For the former, simulations suggest that contrast of ~ 2 x 10-4 within one λ/D at 4.3 µm should be achieved on a M = 8 star in 104 seconds. This level of contrast is sufficient to detect 5-10 MJup gas-giant exoplanets around bright nearby young (10-100 Myrs) stars. For comparison, contrast at the level of ~ 10-3 within one λ/D at L0 has been achieved on Keck. Since AMI is particularly sensitive to amplitude errors, a space-based environment is ideal for AMI. The NIRISS simulations take into account the instrumental effect of bad pixels, intra-pixel response and flat field errors and assume one calibrator/reference star; using more than one calibrator should improved the performance. As seen in Figure 12, AMI is probing a unique discovery space between 70 and 500 marcsec which is very complementary to NIRCam and MIRI, both virtually "blind" to companions at separations less than ~0.5 arcsec.

Figure 70: Five sigma contrast curve predicted for the NIRCam/MIRI coronagraphs and the NIRISS/AMI mode. AMI is probing relatively small inner working angles (image credit: CSA, COM DEV Ltd.)

 NIRISS mode Filters Grism Mask BBI (Broadband Imaging) F090W, F115M, F150W, F200W, F277W, F444W, F356W WSS (Wide-Field Slitless Spectroscopy) F115W, F150W, F200W, F140M, F158M GR150H or GR150V SOSS (Single Object Slitless Spectroscopy) Open GR700XD AMI (Aperture Interferometric Imaging) F380M, F430M, F444W NMR (Non Redundant Mask) Pupil Alignment (used only during on- ground testing) Open PAR (Pupil Alignment Reference)

Table 9: Summary of NIRISS filter, grism and mask configurations for different modes of operation (Ref. 109)

FGS/NIRISS integration and status:

• August 27, 2015: Preparations for the third cryo-vacuum test (CV3) of the ISIM (Integrated Science Instrument Module) at NASA's Goddard Space Flight Center continued throughout the summer. For the first time, the flight configuration of the ISIM was vigorously shaken – not stirred! – and bombarded by intense acoustical waves to simulate the harsh conditions of launch. Both NIRISS and FGS sailed through their "system functional tests" before and after these perturbations with no issues. Additional tests to confirm the electromagnetic compatibility of the subsystems of ISIM under conditions that simulate normal operations were also completed successfully. Now that the robustness of the ISIM has been demonstrated, it's "full speed ahead" for the beginning of CV3 in late October! 119)

• Feb. 12, 2015: FGS/NIRISS became the first instrument to be reinstalled in the ISIM (Integrated Science Instrument Module) following the "Half-Time Show." All the planned hardware changes were successfully completed and both instruments passed their electronic check-outs at room temperature with flying colors. FGS/NIRISS is ready for the final series of tests at NASA's Goddard Space Flight Center! 120)

• Oct. 29, 2013: NIRISS completed its first suite of tests under cryogenic conditions in the large vacuum chamber at NASA's Goddard Space Flight Center. The tests featured "first light" observations for all the observing modes of NIRISS. Although a few glitches occurred, initial analysis of the test data show that NIRISS is performing marvelously.

• March 1, 2013: NIRISS and the FGS became the first flight instruments to be attached to the ISIM (Integrated Science Instrument Module), which is currently located in the large clean room at NASA/GSFC (Ref.108).

• Dec. 21, 2012: NIRISS and the FGS successfully completed room-temperature functional tests at NASA/GSFC.

• Nov. 15, 2012: NIRISS and the FGS became the first JWST instruments to be accepted formally by NASA during the Delivery Review Board meeting at the Goddard Space Flight Center.

• The Canadian Space Agency delivered NIRISS and the Fine Guidance Sensor to NASA's Goddard Space Flight Center on July 30, 2012.

• The end-to-end functional and performance cryogenic vacuum testing of NIRISS was successfully completed at the beginning of 2012. The new, compared to TFI, components of the Dual Wheel went through separate qualification process afterwards.

Figure 71: FGS and NIRISS are two instruments in one package (image credit: CSA)

Legend to Figure 71: The left image shows the components of FGS. Light from the telescope is redirected by the POM (Pick-Off Mirror), and refocused by the TMA (Three-Mirror Assembly) onto the Fine Focus Mechanism before entering the detector assembly. The FGS has two detectors, called FPAs (Focal Plane Assemblies), which record the light . — The right image shows the components of NIRISS. Light from the telescope is redirected into NIRISS by its Pick-Off Mirror. The collimator makes the light rays parallel to each other so they pass correctly through various combinations of filters or light-splitting grisms in the Pupil and Filter Wheel. Finally, the light is focused by the camera onto the detector (Ref. 110).

Figure 72: FGS full instrument level test (image credit: CSA, COM DEV Ltd., Ref. 118)

Figure 73: Photo of the fully assembled NIRISS (bottom) and FGS-Guider (image credit: CSA, NASA) 121)

Spacecraft bus and sunshield

The JWST spacecraft bus provides the necessary support functions for the operation of the JWST observatory. The bus is the home for six major subsystems: 122)

• ACS (Attitude Control Subsystem)

• EPS (Electrical Power Subsystem)

• C&DHS (Command and Data Handling Subsystem)

• RF communications subsystem

• Propulsion subsystem

• TCS (Thermal Control Subsystem)

The spacecraft is 3-axis stabilized. Two star trackers (+ 1 for redundancy) point the observatory toward the science target prior to guide star acquisition, and they provide roll stability about the telescope line of sight (V1 axis.) Six reaction wheels (two are redundant) are mounted on isolators near the center of gravity of the bus to reduce disturbances to the observatory. These reaction wheels offload the fine steering control (operation from a 16 Hz update from the FGS) to maintain the fine steering mirror near its central position to limit differential distortion-induced blurring onto the target star. 123) 124)

Figure 74: Top view of the JWST spacecraft bus (image credit: NASA)

Figure 75: Observatory schematic block diagram (image credit: NASA)

A propulsion subsystem, containing the fuel tanks and thrusters, is used to support trajectory maneuvers to L2 and to maintain the halo orbit at L2.

The avionics design of JWST employs the FPE (Focal Plane Electronics) onboard network which uses the SpaceWire specification and a transport layer (not part of SpaceWire). SpaceWire is used to provide point‐to‐point links to ISIM (Integrated Science Instrument Module). A MIL‐STD‐1553 data bus is being used to communicate with the ICEs (Instrument Control Electronics) of each instrument, and FGS (Fine Guidance Sensor).

Figure 76: Various FM (Flight Model) and EM (Engineering Model) components of the JWST spacecraft (image credit: NASA, Ref. 39)

RF communications: JWST will be using CCTS (Common Command and Telemetry System), a modified multimission COTS system of Northrop Grumman which is based on Raytheon's ECLIPSE product line (Raytheon was responsible for developing this system for Northrop Grumman. ECLIPSE is a commercial off-the-shelf command and telemetry product that is configured to support both satellite flight operations and integration and test for JWST. 125)

Onboard storage is provided by a solid-state recorder with a capacity of 58.9 GB (manufacturer: SEAKR Engineering, Inc.). Operating like a digital video recorder, the spacecraft flight unit records all science data together with continuous engineering "state of health" telemetry for the entire observatory 24 hours a day, seven days a week. The data is downloaded to the ground station when the telescope communicates with Earth during a four-hour window every 12 hours. 126)

A high gain antenna provides Ka-band and S-band communications. The Ka-band downlink from L2 is used for science data at the selectable rates of 7, 14, or 28 Mbit/s. A pair of omni-directional antennas (S-band) provide near hemispherical coverage for emergency communications. The S-band nominal downlink is 40 kbit/s and the uplink is 16 kbit/s.

Note: Unlike Hubble, JWST was never meant to be repaired. But in May 2007, NASA announced that it is considering installing a grapple attachment anyway, just to be safe.

Figure 77: JWST communications system architecture (image credit: NASA) 127)

JWST Sunshield:

The sunshield provides a very stable passively cooled cryogenic environment to the OTE and ISIM instrumentation - taking full advantage of the steady thermal conditions of the JWST halo orbit at L2. Thermal stability is further enhanced by the two-chord fold architecture of the primary mirror. The folding architecture allows simple thermal straps across the hinge lines and results in a uniform temperature distribution on the primary mirror structure. With these features, the observatory can maintain its optical performance and optical stability for any pointing within its FOR (Field of Regard) without relying on active thermal control or active wavefront control. The sunshield deployment concept is based on Northrop Grumman's precision antenna mesh system. 128) 129)

The FOR (Field of Regard) is the region of the sky in which observations can be conducted safely at a given time. For JWST, the FOR is a large annulus that moves with the position of the Sun and covers about 40% of the sky at any time. This coverage is lower than the ~80% that is accessible by Hubble. The FOR, as is shown in Figure 78, allows one to observe targets from 85º to 135º of the Sun. Most astronomical targets are observable for two periods separated by 6 months during each year. The length of the observing window varies with ecliptic latitude, and targets within 5º of the ecliptic poles are visible continuously, and provides 100% accessibility of the sky during a year period. The sunshield permits the observatory to pitch toward and away from the sun by approximately 68º, while still keeping the telescope in the shade (Figure 79). The continuous viewing zone is important for some science programs that involve monitoring throughout the year and will also be useful for calibration purposes. Outside the continuous viewing zone every area in the sky is observable for at least 100 days per year. The maximum time on target at a given orientation is 10 days.

Figure 78: Schematic of observatory FOR (image credit: STScI, Ref. 33)

Figure 79: FOR directions of the OTE in relation to the Sun, Earth and Moon (red arrow), image credit: STScI

The sunshield has dimensions of about 20 m x 14 m providing ample shielding from light of the sun and the Earth. The sunshield provides a 5 layer, "V" groove radiator design of lightweight reflecting material. It reduces the 300 kW of radiation it receives from the sun on its sunward side, to a mere 23 mW (milliwatt) at the back, sufficient to sustain a 300 K temperature drop from front to back. With a back sunshield temperature of ~ 90 K, the primary mirror, the optical truss, and the instrument payload can radiate their heat to space (at 2.7 K) and reach cryogenic temperatures of 30-50 K. These low temperatures and the total blocking of direct or reflected sunlight are crucial to the scientific success of JWST. 130)

The five sunshield layers of ultra-thin membrane are constructed from DuPont Kapton® E. The first layer, at the hot side, is 50.8 µm thick. The remaining four layers are each 25.4 µm thick, similar in thickness to a human hair. The membranes use a vapor-deposited aluminum coating to produce a highly reflective surface and can sustain a 300 K temperature drop. Z-folded at launch, the sunshield will be signaled to begin deploying two days into launch, as the spacecraft heads toward its orbit. 131)

Figure 80: The five-layer finite element model of the JWST sunshield (image credit: NGAS)

Historically, membranes have been designed to induce a biaxial-tension stress state, thus guaranteeing that wrinkles do not form. The large-scale geometry of the JWST sunshield, along with its complex design features, may hinder such a biaxial stress state. Therefore, the ability to accurately predict the response of the membrane becomes critical to mission success. This article addresses the analytical problems involved in meeting those objectives and looks ahead to the challenges remaining in manufacturing the sunshield.

Figure 81: Overview of the JWST sunshield analysis process (image credit: NGAS)

Figure 82: Deployed observatory, back view: Spacecraft bus, solar arrays, communications antenna, and ISIM (image credit: NGAS)

 Total mass of spacecraft ~ 6200 kg, including observatory, on-orbit consumables and launch vehicle adaptor Mission duration 5 years (10 year goal) Diameter of primary mirror 6.5 m Clear aperture of primary mirror 25 m2 Primary mirror material Beryllium Mass of primary mirror 705 kg Mass of a single primary mirror segment 20.1 kg for a single beryllium mirror, 39.48 kg for one entire PMSA (Primary Mirror Segment Assembly) Focal length 131.4 m Number of primary mirror segments 18 Optical resolution ~0.1 arcsecond Wavelength coverage 0.6 - 28 µm Size of sunshield 21.2 m x 14.2 m Telescope operating temperature ~45 K Launch vehicle Ariane 5 ECA (an ESA sponsored flight from Kourou) Launch 2021

Table 10: Overview of JWST mission parameters

Introduction of JWST spinoff technologies:

In the timeframe 2010/12, new technologies developed for NASA's JWST (James Webb Space Telescope) have already been adapted and applied to commercial applications in various industries including optics, aerospace, astronomy, medical and materials. Some of these technologies can be explored for use and licensed through NASA's Office of the Chief Technologist at NASA's Goddard Space Flight Center, Greenbelt, MD. - Note: NASA's JWST is also simply referred to as the Webb. 132) 133)

1) Optics Industry: Telescopes, Cameras and More

The optics industry has been the beneficiary of a new stitching technique that is an improved method for measuring large aspheres. An asphere is a lens whose surface profiles are not portions of a sphere or cylinder. In photography, a lens assembly that includes an aspheric element is often called an aspherical lens.

Stitching is a method of combining several measurements of a surface into a single measurement by digitally combining the data as though it has been "stitched" together.

Because NASA depends on the fabrication and testing of large, high-quality aspheric (nonspherical) optics for applications like the JWST, it sought an improved method for measuring large aspheres. Through SBIR (Small Business Innovation Research) awards from NASA/GSFC, QED Technologies, of Rochester, New York, upgraded and enhanced its stitching technology for aspheres.

QED developed the SSI-A® (Subaperture Stitching Interferometer for Aspheres) metrology technology, which earned the company an "R and D 100" award, and also developed a breakthrough machine tool called the aspheric stitching interferometer. The equipment is applied to advanced optics in telescopes, microscopes, cameras, medical scopes, binoculars, and photolithography.

2) Aerospace and Astronomy

In the aerospace and astronomy industries, the JWST program gave 4D Technology its first commercial contract to develop the PhaseCam interferometer system, which measures the quality of the JWST telescope's mirror segments in a cryogenic vacuum environment. This is a new way of using interferometers in the aerospace sector.

• The PhaseCam interferometer verified that the surfaces of the JWST telescope's mirror segments were as close to perfect as possible, and that they will remain that way in the cold vacuum of space. To test the Webb mirror segments, they were placed in a "cryovac" environment, where air is removed by a vacuum pump and temperatures are dropped to the extreme cold of deep space that the space craft will experience. A new dynamic interferometric technique with very short exposures that are not smeared by vibration was necessary to perform these measurements to the accuracy required, particularly in the high-vibration environment caused by the vacuum chamber's pumps. - The interferometer resulting from this NASA partnership can be used to evaluate future mirrors that need to be tested in vacuum chambers where vibration is a problem.

• Restoring Hubble: Integrated circuits used in camera repair. Webb investments in cryogenic ASICs (Application-Specific Integrated Circuits) led to the development of the ASICs that are now flying on the Hubble Space Telescope. This is a unique example of "future heritage": a program in development (Webb) invented a technology for a program well into the operations phase (Hubble). Webb's investments into this technology allowed the ASICs to be programmable, which was important in the repair of Hubble's Advanced Camera for Surveys that has produced stunning views of our universe.

• Astronomical Detectors: The benefits of the near-infrared detectors developed for Webb's instruments have already spread far and wide in the world of science. "Infrared sensors based on the technology developed for Webb are now the universal choice for astronomical observations, both from space and the ground," said Dr. James Beletic, Senior Director at Teledyne. This technology is also being used for Earth science and national security missions. An early pathfinder version of Webb's HAWAII-2RG 4 Megapixel array has been used in several NASA missions including Hubble, Deep Impact/EPOXI, WISE, and the OCO-2 (Orbiting Carbon Observatory-2), and the HAWAII-2RG is already in use at dozens of ground-based observatories around the world. The availability of these high-performance detectors developed for Webb has been critical to a breathtaking collection of missions, both present and future (Ref. 133).

3) Medical Industry: Eye Health

New "wavefront" optical measurement devices and techniques were created for making the JWST telescope mirrors. Those have led to spinoffs in the medical industry where precise measurements are critical in eye health, for example.

• The technology came about to accurately measure the JWST primary mirror segments during manufacturing. Scientists at AMO WaveFront Sciences, LLC of Albuquerque, N.M. developed a new "wavefront" measurement device called a Scanning Shack Hartmann Sensor.

• The optical measuring technology developed for the JWST, called "wavefront sensing" has been applied to the measurement of the human eye and allowed for significant improvements.

• "The Webb telescope program has enabled a number of improvements in measurement of human eyes, diagnosis of ocular diseases and potentially improved surgery," said Dan Neal, Director of Research and Development AMO (Abbott Medical Optics Inc.) in Albuquerque, N.M. The Webb improvements have enabled eye doctors to get much more detailed information about the shape and "topography" of the eye in seconds rather than hours.

4) Materials Industry: Measuring Strength

The JWST technologies have opened the door to better measurement in testing the strength of composite materials. Measuring strain in composite materials is the same as measuring how much they change in certain environments. Measuring step heights allows one to understand very small changes in a surface profile and doing all of this at high speed allows the device to work even in the presence of vibration that would normally blur the results.

"Technology developed for the Webb telescope has also helped 4D Technologies, Inc. to develop unique technology to measure strain in composite materials, to measure step heights in precision machined surfaces, and for high speed wavefront detection," said James Millerd, President, 4D Technology Corporation, Tucson, AZ.

The Webb telescope technologies have also been beneficial to the economy. The technologies have enabled private sector companies such as 4D to generate significant revenue and create high-skill jobs. Much of 4D's growth from a two man start-up to over 35 people can be traced to projects originally developed for the telescope. 4D has also been able to adapt these technologies for a wide range of applications within the astronomy, aerospace, semiconductor and medical industries.

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109) Michael Maszkiewicz, "Near- Infrared Imager and Slitless Spectrograph (NIRISS): a new instrument on James Webb Space Telescope (JWST)," Proceedings of the ICSO (International Conference on Space Optics), Ajaccio, Corsica, France, Oct. 9-12, 2012, paper: ICSO-037, URL: http://congrex.nl/icso/2012/papers/FP_ICSO-037.pdf

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111) Neil Rowlands, Sandra Delamer, Craig Haley, Eric Harpell, Maria B. Vila, Gerry Warner, Julia Zhou, "Cryogenic performance test results for the flight model JWST fine guidance sensor," Proceedings of. SPIE, Vol. 8442, 8442{130 (2012), doi: 10.1117/12.926672

112) Neil Rowlands, "The JWST Fine Guidance Sensor (FGS) Overview and Current Status," Proceedings of the 65th International Astronautical Congress (IAC 2014), Toronto, Canada, Sept. 29-Oct. 3, 2014, paper: IAC-14-A7.2.5

113) René Doyon, John Hutchings, Mathilde Beaulieu, Loic Albert, David Lafrenière, Chris Willott, Driss Touahri, Neil Rowlands, Micheal Maszkiewicz, Alex Fullerton, Kevin Volk, André Martel, Pierre Chayer, Anand Sivaramakrishnan, Roberto Abraham, Laura Ferrarese, Ray Jayawardhana, Doug Johnstone, Micheal Meyer, Judith Pipher, Marcin Sawicki, "The JWST Fine Guidance Sensor (FGS) and Near-Infrared Imager and Slitless Spectrograph (NIRISS)," Proceedings of SPIE, 'Space Telescopes and Instrumentation 2012: Optical, Infrared, and Millimeter Wave,' Vol. 8442, 84422R © 2012 SPIE, doi:10.1117/12.926578

114) "Fine Guidance Sensor/Near InfraRed Imager and Slitless Spectrograph (FGS/NIRISS)," NASA, URL: http://jwst.nasa.gov/fgs.html

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First_Spacecraft_Flight_Recorder_for_NASA_JWST_Delivered_to_Northrop_Grumman_999.html

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The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (herb.kramer@gmx.net).

Concept    Launch    Observatory   Mission Status   Sensor Complement    Spacecraft Bus and Sunshield

#### Directory

Satellite Missions | Airborne Sensors
Observation of the Earth and its Environment

...

SOURCE: https://www.spiedigitallibrary.org/conference-proceedings-of-spie/10563/105634I/The-NIRspec-assembly-integration-and-test-status/10.1117/12.2304097.full?SSO=1

bY: Thomas Wettemann, Ralf Ehrenwinkler, Thomas E. Johnson, Marc Maschmann, Peter Mosner, Maurice te Plate, Andreas Rödel

Author Affiliations +

Proceedings Volume 10563, International Conference on Space Optics — ICSO 2014; 105634I (2017) https://doi.org/10.1117/12.2304097
Event: International Conference on Space Optics — ICSO 2014, 2014, Tenerife, Canary Islands, Spain

Abstract

The Near-Infrared Spectrograph (NIRSpec) is one of the four instruments on the James Webb Space Telescope (JWST) scheduled for launch in 2018. NIRSpec has been manufactured and tested by an European industrial consortium led by Airbus Defence and Space and delivered to the European Space Agency (ESA) and NASA in September 2013. Since then it has successfully been integrated into the JWST Integrated Science Instrument Module (ISIM) and is currently in ISIM Cryo-Vacuum Test#2. Since however two of its most important assemblies, the Focal Plane Assembly (FPA) and the Micro-Shutter Assembly (MSA) need to be replaced by new units we will present the status of the instrument, the status of its new flight assemblies in manufacturing and testing and give an outlook on the planned exchange activities and the following instrument re-verification.

## I. INTRODUCTION

NIRSpec, the Near-Infrared Spectrograph, is one of the four Science Instruments (SI) which will be flown on the James Webb Space Telescope (JWST) schedule for launch in 2018. The JWST is the follow-on mission to the Hubble Space Telescope (HST) and is developed to receive more information about the origins of the universe by observing infrared light from the first stars and galaxies.

A comprehensive description of the observatory and its main science goals is given by Gardner et al [1] and its current status is summarized at the SPIE Astronomical Telescopes and Instrumentation conference 2014 [2].

Fig. 1 shows the Optical Assembly (OA) of NIRSpec without its optical assembly cover mounted. Three electronic boxes, the Instrument Control Electronics (ICE), the Micro-Shutter Control Electronics (MCE) and the Focal Plane Control Electronics (FPE) complete the instrument. These electronic boxes will be mounted in the warm ISIM Electronics Compartment (IEC).

The NIRSpec Instrument in the Airbus DS cleanroom prior to Instrument Cover integration

NIRSpec is a multi-object spectrograph that is capable of simultaneously measuring the near infrared spectrum of up to 100 objects like stars or galaxies with low, medium and high spectral resolutions. The observations are performed in a 3 arcmin x 3 arcmin field of view over the wavelength range from 0.6 micrometer to 5.0 micrometer. It also features a set of slits and an aperture for high contrast spectroscopy of individual sources, as well as an integral-field unit (IFU) for 3D spectroscopy.

The instrument is one of the ESA contributions to JWST and is built by Airbus DS GmbH (former Astrium GmbH), a group of European Subcontractors and NASA which contributed the Micro-Shutter Subsystem (MSS) and the Detector Subsystem (DS).

## II. OVERVIEW OF THE NIRSPEC INSTRUMENT

The architecture of the instrument is constituted by a baseplate on which the optical components are attached such, that after the light from the JWST telescope mirrors is coupled-in by the Coupling Mirrors (COM), the optical path is routed parallel to the baseplate. The functional assemblies of NIRSpec are shown in Fig. 2 and were explained in detail by Bagnasco et al. [3].

The NIRSpec Optical Assembly with its main functional Assemblies (Instrument Cover removed)

The optical path (cf. Fig. 3) is mainly defined by 3 three-mirror anastigmats (TMAs), the fore optics TMA (FOR) - which provides a telecentric intermediate focal plane where the multi-object selection mechanism called Micro-Shutter Assembly (MSA) is located, the collimator TMA (COL) - which collimates the light onto the disperser carrying Grating Wheel Assembly (GWA) and the camera TMA (CAM) - which images the light onto the two 2k x 2k MCT detectors.

NIRSpec Optical Path and Design Features

Further assemblies are the RMA which carries two plane mirrors for refocussing, a Filter Wheel Assembly (FWA) which carries filters for wavelength selection and an on-board calibration assembly (CAA) which provides several spectral line and flat-field illumination sources for in-orbit instrument calibrations.

In order to enable very long exposures of up to 10,000 seconds in the near infrared wavelength regime the instrument operates at -235°C (38K) and utilizes a highly a-thermal concept with all mirrors, the mirror mounts and the optical bench base plate all manufactured out of the silicon carbide ceramic BOOSTEC® SiC.

The instrument size is approximately 1900x1400x700 mm and it weighs about 196 kg with about 100 kg of SiC. The optical assembly is for straylight and thermal reasons protected by an externally aluminized and internally black coated Kapton Cover. The operation of the instrument is performed with three electronic boxes.

## A. Status of NIRSpec at Delivery to NASA

NIRSpec has been shipped to NASA in September 2013 after having passed successfully the verification and test campaign. The campaign included a cryo-vacuum verification and calibration test at qualification temperature which was followed by an acoustic noise test at acceptance level and a sine vibration test at protoflight level. After the sine vibration test a further cryo-vacuum verification test was performed in order to verify that the instrument performance did not degrade due to the mechanical tests.

During NIRSpec and other JWST Science Instrument tests it was found that the NIR detectors suffer from a gradually increasing number of hot pixels. This effect was found to be accelerated by higher temperatures. An extrapolation of the degradation lead to the conclusion that the performance of the detectors would, by the time of JWST launch, become unacceptable.

In order to determine the root cause of the degradation a NASA failure review board was established which traced the degradation back to a detector design flaw that allowed indium to interdiffuse with the gold contacts and migrate into the HgCdTe detector layer [4]. As a consequence of this, design improvements have been derived and new batches of detectors which do not suffer from this problem have been manufactured and the design improvements validated [4]. Based on the new design it was decided to manufacture new NIR detectors for all SIs which would then be integrated into a new FPA and after testing into the respective instruments.

Another finding of the NIRSpec environmental test campaign was that some of the 4 Micro-Shutter Arrays, that form the multi-object selection apertures of the Micro-Shutter Assembly (MSA) [5], were sensitive to Acoustic Noise. The impact of this was that, due to the acoustic noise loads, the amount of shutters that were stuck closed, i.e. shutters that cannot be opened but are permanently ‘stuck closed’, significantly increased. An effect of this was additionally a strong reduction in shutter-array closure contrast. The Flight Model (FM) MSA is shown in Fig. 4.

FM Micro-Shutter Assembly (Credit: NASA)

Due to the significant degradation and unclear expectable performance after higher level acoustic noise tests a NASA Failure Review Board was initiated. The performed investigations revealed that due to manufacturing tolerance add-up some quadrants had arrays which were susceptible for acoustic noise and others not. Since the amount of stuck shutter became critical for the NIRSpec Target Acquisition Mode it was decided to build a new Flight Spare (FS) MSA with slight design modifications implemented. In case the FS MSA performance would be better than the currently integrated FM then it would become the new FM MSA (FM2).

## B. Integration of NIRSpec into ISIM and current Status

After its delivery NIRSpec has been prepared for the integration into the JWST Integrated Science Instrument Module (ISIM) which is a large CFRP structure carrying all 4 SIs. In March 2014 NIRSpec, which was the last instrument to be integrated, was finally mounted to ISIM (Fig. 5, left).

During the integration great care had to be taken to avoid contact of the ceramic instrument with any of the ISIM structures finally enabling all of the 3 NIRSpec Titanium Kinematic Mounts to engage with their pre-aligned counterparts on ISIM.

With all instruments integrated a successful ISIM System Functional Test has been performed in April 2014. Afterwards ISIM was moved to the Space Environment Simulation Chamber (Fig. 5, right) and prepared for its Cryo-Vacuum Test #2 which was started in mid of June 2014. At the time of writing this article the cryo test was running since about 1 month and thus the majority of the 3.5 months of test was still to be done.

NIRSpec integration to ISIM (left) and ISIM integration into the SES Chamber for CV#2 (right) (Credit: NASA)

## A. Status of the new FPA

At the end of July 2014 the new Focal Plane Assembly (S/N 106) was completely assembled (Fig. 6, left) vibration tested and had passed most of its characterization tests at its nominal cryo operational temperature ranging from 36.5K – 42.8K. The new detectors have been shown to meet or exceed the required performance specifications, and have a small and stable hot pixel population [6]. The expected delivery of the completely verified and characterized FPA is scheduled for September this year.

The new NIRSpec FPA (S/N 106) completely assembled (left) and the FM2 MSA prior to integration of the MSA magnet arm mechanism and cover (Credit: NASA)

## B. Status of the MSA Flight Spare

The Flight Spare MSA was completely assembled in May 2014 (Fig. 6, right, shows the MSA at intermediate assembly stage). After a first cold performance test at cryogenic temperatures the MSA went into the acoustic and vibration test campaign which it successfully passed. Afterwards it went into cryo testing which was still ongoing at the time of writing the article.

The data obtained so far shows that compared to the FM1 MSA the number of failed closed shutters is reduced by 2/3 and generally a far better average and median shutter contrast is achieved (Fig. 7). Therefore the FS MSA will become the new FM MSA (FM2).

FM1 (left) and FM2 (right) MSA shutter-quadrant contrast histograms. The new MSA shows generally much better contrast. (Credit: NASA)

## V. OUTLOCK

Once the ISIM CV#2 is finished the SIs will be de-integrated from ISIM in order to swap out their degrading detectors and in case of NIRSpec additionally the MSA.

The instrument de-integrations are scheduled for mid of December 2014. Directly thereafter the exchanges are planned to be conducted.

## A. Detector Subsystem Exchange

For the Detector Subsystem the exchange means that the complete set of FPA, ASICs and instrument internal harness will be replaced (cf. Fig. 8). The Focal Plane Electronics (FPE) will be reused. The exchange of the ASIC assembly and the harness is a mechanical removal and integration only. The integration of the FPA requires however alignment activities. Since the infrared detectors cannot be operated at ambient temperature the alignment cannot be performed via optical stimuli. Therefore the alignment is purely based on reference coordinate metrology data.

Detector Subsystem hardware that is to be exchanged (Grey box represents SiC Camera housing)

The new FPA (S/N 106) will be integrated with a dedicated shim set which will be calculated from the NASA provided FPA metrology data and the metrology data of the Best Fit Plane (BFP) of the Camera detector focal plane.

For this the FPA 106 will first be mounted to an alignment jig and laterally pre-aligned (in the X/Y plane). Using this jig the FPA will then be installed on the Camera and the detector fine alignment (clocking) will be performed. In the case that the pre-alignment to the jig was not completely correct new metrology data will be measured to refine the pre-alignment. Fig. 9 shows the planned FPA exchange and alignment approach.

FPA exchange and alignment approach

## B. MSA Exchange

In the case of the MSS (Micro-Shutter Subsystem) only the MSA will be exchanged. The corresponding harness and the Micro-shutter Control Electronics (MCE) will be reused.

The integration of the FM2 MSA requires great care as it weighs close to 10kg and needs to be lowered by a crane to a central position of the instrument which is very close to several other sensitive assemblies. The MSA is mounted to the MSA/IFU bracket (see Fig. 10). This bracket carries also the Integral Field Unit (IFU) which is aligned to the residual NIRSpec optical train and whose position cannot easily be changed anymore. For illumination of the IFU the MSA part which separates the four shutter quadrants, the so called cruciform, has a 1.4mm x 1.4mm square opening (see Fig. 10, right). As part of the MSA alignment this aperture needs to be co-aligned in X,Y and rotation to the slightly smaller IFU entrance aperture leaving ~ 100-150μm for alignment accuracy in X and Y direction.

CAD view of MSA and IFU mounted on the bracket (left) and location of the IFU aperture on the MSA Cruciform (right)

The MSA alignment to the bracket is ensured by two pins which have a tolerance of about +/-40μm but do not allow for larger lateral alignments. In order to avoid any additional delta alignment effort, NASA had to replicate the previous FM MSA metrology data to the FM2 MSA. This was performed very successfully and with the achieved residual pin offset of ~10μm in X and ~17μm in Y the new MSA can be integrated to the MSA/IFU bracket merely relying on the already implemented alignment pins. Thereafter the alignment will be checked with Laser Tracker and Theodolites.Fig. 11 shows the MSA exchange and alignment approach.

MSA exchange and alignment approach

As a fallback solution, in case a delta alignment becomes necessary, a special alignment tool is manufactured and alignment pins with smaller diameter will be used to ensure that the necessary alignment range is available. The final check of the correct alignment will be performed using the correlation of the IFU aperture and the IFU window on the MSA cruciform. The IFU will be back illuminated such that the MSA window shift versus the IFU aperture can be observed though the instrument front optics (FOR and COM optics).

## C. NIRSpec re-integration to ISIM and succeeding environmental ISIM Tests

Once the FPA, ASICs, FPA harness and the MSA are mechanically exchanged, harness safe-to-mates will be performed and after the electrical integration MSS and DS Short Functional Tests to re-verify the functionality. Thereafter the NIRSpec cover will be reinstalled. The total exchange activity is scheduled to last 28 working days.

As soon as all Science Instrument assembly exchange activities are done, the instruments will be re-integrated in reverse order into ISIM. This, according to the current planning, is scheduled to occur in the Feb. 2015 timeframe. Once that is completed ISIM will be vibration and acoustic noise tested which will serve also as SI workmanship acceptance test. In May/June 2015 ISIM will undergo EMI / EMC Testing which will be followed by the ISIM Cryo-Vacuum Test#3 (CV#3). During the cryo test the final performance verification and calibration of NIRSpec will be performed. This will primarily concern verification of the DS and MSS. After the CV#3 ISIM will be delivered to OTIS (Dec. 2015).

## VI. SUMMARY

Following the delivery to ISIM, NIRSpec has been integrated to ISIM and is currently (Aug. 2014) in cryo-vacuum test #2. After the test it will be de-integrated from ISIM to exchange its non-flight worthy Focal Plane Assembly and its degraded Micro-Shutter Assembly with new improved FM builds. The exchange is planned such that a minimum of alignment steps are necessary. The instrument re-verification will occur on ISIM level tests which are scheduled to be finished by end of 2015.

## REFERENCES

[1] Gardner, J. P., Mather, J. C., Clampin, M., Doyon, R., Flanagan, K. A., Franx, M., Greenhouse, M. A., Hammel, H. B., Hutchings, J. B., Jakobsen, P., Lilly, S. J., Lunine, J. I., McCaughrean, M. J., Mountain, M., Rieke, G. H., Rieke, M. J., Sonneborn, G., Stiavelli, M., Windhorst, R., and Wright, G. S., “The James Webb Space Telescope,” 1 –29 (2009). Google Scholar

[2] Clampin, M., “Recent progress with the JWST Observatorys,” in Proc. SPIE, 9143-1 (2014). Google Scholar

[3] Bagnasco, G., Kolm, M., Ferruit, P., Honnen, K., Koehler, J., Lemke, R., Maschmann, M., Melf, M., Noyer, G., Rumler, P., Salvignol, J., Strada, P., and Te Plate, M., “Overview of the Near Infrared Spectrograph (NIRSpec) Instrument on-board the James Webb Space Telescope (JWST),” in Proc. SPIE, (2007). Google Scholar

[4] Rauscher, B. J., Stahle, C., Hill, R. J., Greenhouse, M., Beletic, J., Babu, S., Blake, P., Cleveland, K., Cofie, E., Eegholm, B., Engelbracht, C. W., Hall, D. N. B., Hoffman, A.,Jeffers, B., Jhabvala, C.,Kimble, R. A., Kohn, S., Kopp, R., Lee, D., Leidecker, H., Lindler, D., McMurray, R. E., Misselt, K., Mott, D. B., Ohl, R., Pipher, J. L., Piquette, E., Polis, D., Pontius, J., Rieke, M., Smith, R., Tennant, W. E., Wang, L., Wen, Y., Willmer, C. N. A., and Zandian, M., “Commentary: JWST near-infrared detector degradation-finding the problem, fixing the problem, and moving forward,” AIP Advances 2, (2), 021901 (2012). Google Scholar

[5] Kutyrev, A. S., Collins, N., Chambers, J., Moseley, S. H., Rapchun, D., “Microshutter arrays: high contrast programmable field masks for JWST NIRSpec,” in Proc. SPIE 7010, Space Telescopes and Instrumentation 2008: Optical, Infrared, and Millimeter, 70103D (2008). Google Scholar

[6] Hill, J. H., J., R. M., Rauscher, B. J., Greenhouse, M. A., Wen, Y., Lindler, D. J., and Mott, D. B., “New detectors for the JWST near-IR instruments,” in Proc. SPIE, 9154-13 (2014). Google Scholar

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"...

# NIRSpec Multi-Object Spectroscopy

One of the showcase observing modes of JWST is the NIRSpec multi-object spectroscopy (MOS) mode using the micro-shutter assembly (MSA).  The MSA can obtain simultaneous spectra of many science sources within a 3.6' × 3.4' field of view.

See also:  Using NIRSpec MOS mode: Summary and Helpful Hints Video Tutorial,   NIRSpec Multi-object Spectroscopy Overview and Tool Demo Video Tutorial

The JWST NIRSpec multi-object spectroscopy (MOS) mode provides multiplexing 0.6–5.3 μm spectroscopy capabilities over a 3.6' × 3.4' field of view. This mode uses tiny configurable shutters in the micro-shutter assembly (MSA) to acquire dozens to hundreds of spectra of astronomical sources within a single exposure. This is a very powerful feature for spectroscopic surveys. For example, potential use cases for the NIRSpec MOS mode include, but are not limited to: spectral characterization of the faintest objects in our universe, surveys to investigate galaxy formation and evolution, stellar population studies, star cluster formation, and the evolution and properties of extended solar system bodies.

The NIRSpec MSA consists of four quadrants of 365 × 171 shutters that can be individually opened and closed to create the spectral slit configurations for this multi-object spectroscopy mode. In Figure 1, the four NIRSpec MSA quadrants are plotted on a Hubble Space Telescope WFC3 F555W image of the Tarantula Nebula in the Large Magellanic Cloud.

The NIRSpec MSA can be opened in contiguous columns of several shutters to create what are called “slitlets” to acquire MOS spectra of science sources of interest. Figure 2 shows a zoomed-in view of several three-shutter slitlets, configured to open on (faint) sources, over-plotted on the Hubble Ultra Deep Field WFC3 UVIS image.

# Properties of the MSA

The MSA consists of four quadrants, each with 171 rows of 365 shutters, totaling ~250,000 shutters. The four quadrants are labeled Q1, Q2, Q3, and Q4, as shown in Figure 3.  The open area of each MSA shutter is 0.20" × 0.46" (width in the dispersion direction × spatial direction).

MSA shutters are mounted on a fixed support grid. Support bars between all shutters are 0.07" wide, so that the shutter pitch (center-to-center distance) is 0.27" in the dispersion direction and 0.53" in the spatial (cross-dispersion) direction.

The area in each MSA quadrant is about 95" in the dispersion (X) direction by 87" in cross-dispersion (Y), and the total extent of the MSA sky field of view is 3.6' × 3.4'. The MSA quadrants are separated from one another by ~23" along the dispersion (X) axis (that is, between the inner edges of the tan-shaded shutter arrays shown in Figure 3 from the right edge of Q3 to the left edge of Q1, and similarly between Q4 and Q2). Note that this gap not the same as the gap between detectors in the dispersion (X) direction, which is ~ 18". It is the detector gap that is responsible for missing wavelength information in spectral observations with the MSA. The MSA mounting plate also separates Q1 from Q2, and Q3 from Q4, by about 37" in the spatial (Y) direction. The fixed slits, located in this 37" gap between quadrants, are always open.

To create an MSA configuration, the ~250,000 shutters can be individually selected to open or close. The NIRSpec MSA is configured to open sets of science shutters using a two-step process: (1) the MSA magnet arm sweeps across the MSA to open all shutters in the quadrants, then (2) the magnet moves back across the quadrants to close unused shutters and leave open those configured to observe specific science sources. This configuration process takes about 90 seconds.

Some of the shutters in the NIRSpec MSAs are defective and either permanently "failed open" or "failed closed." There are presently a small number (less than twenty) permanently failed open shutters; these are detrimental to science because spectra from spurious sources can overlap and contaminate science spectra. Additionally, approximately 15% of the quadrant shutters are permanently failed closed or impacted by electrical shorts and therefore inoperable (see Figure 4).

Of those shutters that are failed closed, there are three types:

1. Some failed closed shutters are physically stuck closed;
2. Some shutters used to be failed open but were deliberately blocked closed during the manufacturing process to limit detrimental contaminating spectra;
3. Some entire rows and columns of shutters in the MSA are masked closed due to electrical shorts in the MSA that prevent them from being addressed properly by the configuration magnet.

The NIRSpec observation planning software will search for optimal MSA shutter science configurations and automatically plan around all failed shutters.

A few considerations for MOS observing:

# Words in bold italics are buttons  or parameters in GUI tools. Bold style represents GUI menus/panels & data software packages.

All of the available disperser and filter combinations can be used in NIRSpec MOS mode.
Table 1 below outlines usable instrument configurations, spectral resolutions and wavelength ranges.

Table 1. Spectral configurations available in NIRSpec MOS mode

Disperser-filter combination Nominal resolving power Wavelength range
(μm)
G140M/F070LP ~1,000

0.70–1.27
G140M/F100LP 0.97–1.84
G235M/F170LP 1.66–3.07
G395M/F290LP 2.87–5.10
G140H/F070LP ~2,700

0.81–1.27
G140H/F100LP 0.97–1.82
G235H/F170LP 1.66–3.05
G395H/F290LP 2.87–5.14
PRISM/CLEAR ~100 0.60–5.30

`

Wavelength range values presented here are approximate. Note that the nominal spectral ranges for medium and high-resolution dispersers may be shortened due to red-end detector cutoffs. The cutoff wavelengths depend on the target aperture location (slit or shutter). Detailed limits are found on the wavelength ranges and gaps pages for the IFUFS, and BOTS, and in the ETC. Information on wavelength ranges for MOS, which depend on the position of the shutter in the MSA, can be determined using the MSAViz Tool.

MSA spectra are projected onto the two NIRSpec detectors (NRS1 and NRS2). In the NIRSpec MOS mode, some shutters will not capture the full spectral range at the high resolution, R ~ 2,700, configurations. This is because the right-most MSA shutters in MSA quadrants 1 and 2 (Figure 3) project the longest wavelengths beyond the right edge of detector NRS2 in the R = 2,700 configurations. Alternatively, R~1000 spectra do not extend past the right hand side of NRS2 (see Figure 5).

# Detector wavelength gaps

In the MOS mode, there are gaps in spectral coverage caused by the physical distance between the two detectors, referred to as detector wavelength gaps. The range of wavelengths lost in the gap are different for different shutters in the MSA since the spectra from different shutters maps to different locations on the detectors. Unlike fixed slits (FS) and integral field unit (IFU) observations, which suffer wavelength gap losses only in the R ~ 2,700 high spectral resolution mode, all grating and filter combinations in the MOS mode have shutters that lose wavelengths to the gap.

The NIRSpec MOS mode has a specialized MSA Planning Tool (MPT) within the JWST Astronomers Proposal Tool (APT) software. Using this planning tool, it is possible to create dither options to move targets by ~18" (or more) in the dispersion direction. This 18" is the approximate minimum dither distance necessary to span the detector wavelength gap and acquire complete spectra of science sources. Also, it is possible to inspect MSA configurations designed for MOS observations using the MPT to ensure that wavelengths of interest fall into operable regions on the detectors (in areas unaffected by the detector gap or the long wavelength cutoffs, for instance). The tool is called the NIRSpec MSA Spectral Visualization Tool (MSAViz).

The detector wavelength gap discussed here is different from the gap between quadrants of the MSA shown in Figure 3.

# Subarrays

NIRSpec MOS mode exposures are only acquired in FULL frame 2048 × 2048 detector pixel readout; no subarrays are used.

# Exposure specification

NIRSpec MOS exposure times are tied to the timing of the detector readout patterns. There are four readout patterns available for NIRSpec MOS observations:

• NRSRAPID
• NRS
• NRSIRS2RAPID
• NRSIRS2

The readout patterns are split over two readout modes: (1) traditional and (2) improved reference sampling and subtraction (IRS2). The traditional mode, which is used for the NRSRAPID and NRS readout patterns, is similar to the detector readout for NIRCam and NIRISS. In FULL detector readout, NRSRAPID has a single frame (10.7 s), and NRS averages four frames (42.8 s).

The IRS2 mode, which is used for the NRSIRS2RAPID and NRSIRS2 readout patterns, intersperses reference pixels within the science pixel reads to improve noise characteristics achievable during data processing, resulting in longer frame times and higher data volumes. Like the traditional readout, the NRSIRS2RAPID is a single frame, but unlike the traditional readout equivalent, NRSIRS2 has five frames averaged into a single group. These IRS2 readout patterns improve performance and sensitivity in long exposure MOS observations of faint objects.

Additional information on NIRSpec MOS exposure specification and how this translates to exposure time and sensitivity can be found using the JWST Exposure Time Calculator (ETC). Users interested in determining which readout pattern is best for their science should refer to the NIRSpec Detector Recommended Strategies article.

# Options for dithering

See also: NIRSpec MOS Dither and Nod Patterns,  NIRSpec Dithering Recommended Strategies,  NIRSpec MOS Recommended Strategies

Most observations with JWST will require dithering. This is especially true for NIRSpec since the PSF is under-sampled at most wavelengths. The NIRSpec MOS mode provides two options for creating offsets or dithers:

• NoddingThe telescope can be repositioned slightly between exposures to place the sources in different shutters within the slitlets of an MSA configuration.
• Fixed Dithers: Larger telescope dithers which position the sources onto different shutters of the MSA, and their spectra onto different areas of the detectors.The NIRSpec MOS dithering page and the NIRSpec MOS Recommended Strategies page provide an in-depth review of the available options mentioned here. Users interested in learning which dithering strategies are recommended for their science should read the NIRSpec Dithering Recommended Strategies article.

# What do NIRSpec MOS data look like?

Figure 5 shows NIRSpec MOS mode data acquired with a ground calibration test lamp using the R = 1,000 G140M/F100LP short wavelength spectral configuration. The four MSA quadrant spectra are shown. This observation was acquired with a special five-shutter calibration-only slitlet pattern that had three shutters open with two closed in-between them. Two of the slitlet configurations are highlighted. Failed open shutters cause contaminating single shutter spectra. The MSA planning software is designed to automatically optimize an MSA configuration around failed open shutters so that science spectra of observed sources are not contaminated by dispersed light from these shutters.

# References

Böker, T. 2016 ESAC JWST "On Your Mark" Workshop (ppt) (pdf)
The NIRSpec Multi-Object Spectroscopy (MOS) mode

Dorner, B., Giardino, G., Ferruit, P. et al. 2016, A&A, 592, A113
A model-based approach to the spatial and spectra calibration of NIRSpec onboard JWST

Kutyrev, A.S., Collins, N., Chambers, J. et al. 2008 SPIE, 7010, 70103d
Microshutter arrays: High contrast programmable field masks for JWST NIRSpec

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"...  The JWST NIRSpec multi-object spectroscopy (MOS) mode provides multiplexing 0.6–5.3 μm spectroscopy capabilities over a 3.6' × 3.4' field of view.

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::  3.6' × 3.4'  "3.6 feet x 3.4 feet" :   '  feet ft  ::  https://en.wikipedia.org/wiki/Foot_(unit)

This mode uses tiny configurable shutters in the micro-shutter assembly (MSA) to acquire dozens to hundreds of spectra of astronomical sources within a single exposure. This is a very powerful feature for spectroscopic surveys. For example, potential use cases for the NIRSpec MOS mode include, but are not limited to: spectral characterization of the faintest objects in our universe, surveys to investigate galaxy formation and evolution, stellar population studies, star cluster formation, and the evolution and properties of extended solar system bodies.

The NIRSpec MSA consists of four quadrants of 365 × 171 shutters that can be individually opened and closed to create the spectral slit configurations for this multi-object spectroscopy mode. In Figure 1, the four NIRSpec MSA quadrants are plotted on a Hubble Space Telescope WFC3 F555W image of the Tarantula Nebula in the Large Magellanic Cloud.

The NIRSpec MSA can be opened in contiguous columns of several shutters to create what are called “slitlets” to acquire MOS spectra of science sources of interest. Figure 2 shows a zoomed-in view of several three-shutter slitlets, configured to open on (faint) sources, over-plotted on the Hubble Ultra Deep Field WFC3 UVIS image.

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