JWST part 2

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Figure 57: Schematic layout of a NIRCam imaging module (image credit: NASA)

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Figure 58: Schematic of NIRCam coronagraphic design (image credit: STScI)

Legend to Figure 58: 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 58), which translates the image plane so that the coronagraphic masks are shifted onto the active detector area (Ref. 74).

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Figure 59: 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

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Figure 60: 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. 91) 92)

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.

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Figure 61: 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. 83)

Legend to Figure 61: 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. 93)

- 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.

 

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). 94) 95) 96) 97) 98) 99) 100)

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.

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Figure 62: 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. 101) 102) 103)

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Figure 63: 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. 104)

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 64. 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 65).

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Figure 64: Illustration of the GWA mechanism (image credit: Carl-Zeiss Optronics)

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Figure 65: 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. 104).

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. 105)

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

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Figure 66: 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. 106)

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Figure 67: 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.

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Figure 68: Photo of NASA/GSFC engineers inspecting an MSA with a low light test (image credit: GSFC, Chris Gunn, ESA) 107) 108)

Legend to Figure 68: 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. 109)

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.

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Figure 69: 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.

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Figure 70: 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. 110)

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). 111)

• 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. 112) 113) 114)

- 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. 115)

• 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. 116)

• Sept. 30, 2013: The NIRSpec instrument has arrived at the NASA/GSFC. 117)

• 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. 118)

 

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. 119) 120) 121) 122) 123) 124) 125) 126) 127)

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 71). 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.

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Figure 71: 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 72.

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Figure 72: 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 74). 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.

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Figure 73: 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.

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Figure 74: 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. 128) 129)

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Figure 75: Main optics of the MIRI spectrometer (image credit: MIRI European Consortium)

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Figure 76: 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.

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Figure 77: Schematic configuration of the MIRI imager module (image credit: MIRI European Consortium)

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Figure 78: Illustration of the MIRI imager (image credit: MIRI European Consortium)

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Figure 79: 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.

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Figure 80: Scheme of the spectrograph filter wheel (image credit: MIRI European Consortium)

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Figure 81: 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).

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Figure 82: 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. 130) 131)

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

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Figure 83: Schematic of the silicon detector array (image credit: JPL)

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Figure 84: 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. 132) 133) 134)

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Figure 85: 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)

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Figure 86: Illustration of the MIRI cryocooler elements (image credit: NGAS, UA, Ref. 127)

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Figure 87: Schematic view of the distributed MIRI cryocooler subsystem (image credit: NGAS)

Legend to Figure 87: 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. 135)

• 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. 136)

• 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: 137) 138)

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). 139) 140)

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 88. 141) 142)

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Figure 88: 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. 143) 144)

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 89. 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.

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Figure 89: 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 90 (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 90 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%. 145)

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. 146) 147)

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Figure 90: 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)

- Gain: 1.5 e-/ADU

- 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. 148)

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Figure 91: Schematic view of the NIRISS SCA (image credit: STScI)

 

Observation modes: NIRISS has four observing modes (Ref. 143):

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 92, 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.

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Figure 92: 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 93). 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.

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Figure 93: 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.

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Figure 94: Line-flux sensitivity in the NIRISS WFSS mode for various blocking filters (image credit: CSA, COM DEV Ltd.)

Legend to Figure 94: 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 95) 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.

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Figure 95: 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.

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Figure 96: 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. 139)

 

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! 149)

• 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! 150)

• 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.138).

• 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.

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Figure 97: FGS and NIRISS are two instruments in one package (image credit: CSA)

Legend to Figure 97: 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. 140).

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Figure 98: FGS full instrument level test (image credit: CSA, COM DEV Ltd., Ref. 148)

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Figure 99: Photo of the fully assembled NIRISS (bottom) and FGS-Guider (image credit: CSA, NASA) 151)

 

 


 

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: 152)

• 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. 153) 154)

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Figure 100: Top view of the JWST spacecraft bus (image credit: NASA)

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Figure 101: 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).

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Figure 102: Various FM (Flight Model) and EM (Engineering Model) components of the JWST spacecraft (image credit: NASA, Ref. 80)

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. 155)

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. 156)

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.

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Figure 103: JWST communications system architecture (image credit: NASA) 157)

 

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. 158) 159)

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 104, 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 105). 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.

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Figure 104: Schematic of observatory FOR (image credit: STScI, Ref. 74)

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Figure 105: 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. 160)

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. 161)

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Figure 106: 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.

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Figure 107: Overview of the JWST sunshield analysis process (image credit: NGAS)

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Figure 108: 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. 162) 163)

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. 163).

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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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).

 

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