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The high-level instrument capabilities are summarized in Fig. 8. In the following paragraphs, we describe the instruments and their detectors. For more detailed technical information and test results, interested readers should see, for example, Figer et al. [2] for NIRCam, NIRSpec, and FGS/TI. For MIRI's Si:As SCAs, Hoffman, et al. [3] presents some more recent information.

Figure 8. JWST's science instruments are housed in the Integrated Science Instruments Module (ISIM). These instruments are as follows: (1) Near-Infrared Camera, (2) Near-Infrared Spectrograph, (3) Mid-Infrared Instrument, and (4) Fine Guidance Sensors with tunable filters. During an earlier phase of the JWST project, MIRI used a dewar to cool its detectors. Although the dewar has since been replaced by a cryocooler located in the spacecraft, the former dewar location can still be seen in this figure.

Figure 8. JWST's science instruments are housed in the Integrated Science Instruments Module (ISIM). These instruments are as follows: (1) Near-Infrared Camera, (2) Near-Infrared Spectrograph, (3) Mid-Infrared Instrument, and (4) Fine Guidance Sensors with tunable filters. During an earlier phase of the JWST project, MIRI used a dewar to cool its detectors. Although the dewar has since been replaced by a cryocooler located in the spacecraft, the former dewar location can still be seen in this figure.

With the exception of MIRI's three actively cooled A,co=27 ^m Si:As SCAs, which are operated at T < 7 K, all of JWST's near-IR SCAs will be operated a few degrees Kelvin warmer than the SI optical benches, T~37.5-39 K. At these temperatures, we have found the dark current from the most recent Rockwell/JWST HAWAII-2RG SCAs to be ~0.001-0.005 e-/s/pixel.

In the case of a few non-flight NIRCam parts, dark currents <0.001 e"/s/pixel were measured [4].

One interesting phenomena seen in the near-IR JWST detectors is popcorn noise (see Fig. 9), or "popcorn mesa noise" [5,6]. The popcorn mesa noise appears as an almost digital toggling between states as charge integrates up during long, dark exposures. It is thought that popcorn mesa noise is associated with single charge trapping effects in sensitive regions of the multiplexer. Individual popcorn events can happen on timescales shorter than one frame (12 seconds), as well as on scales longer than 1000 seconds. We anticipate doing more detailed statistical analysis on popcorn mesa noise when Engineering Test Unit (ETU) SCAs begin to arrive in late 2005.

Figure 9. Popcorn mesa noise is occasionally seen in individual pixels as charge integrates during exposures. In this particular figure, popcorn events endure for several frames (100s of seconds). In other pixels, popcorn is sometimes detected on timescales shorter than 12 seconds [5,6]. Conversion gain for this figure is about g=1.3 e-/ADU and the frame rate is about 10.7 seconds per frame.

Figure 9. Popcorn mesa noise is occasionally seen in individual pixels as charge integrates during exposures. In this particular figure, popcorn events endure for several frames (100s of seconds). In other pixels, popcorn is sometimes detected on timescales shorter than 12 seconds [5,6]. Conversion gain for this figure is about g=1.3 e-/ADU and the frame rate is about 10.7 seconds per frame.

NIRCam: The NIRCam (see Fig. 10) is a X = 0.6-5 |m imager provided by the University of Arizona with Marcia Rieke as the Principal Investigator. NIRCam also performs an observatory function to support wavefront sensing [1]. NIRCam uses refractive optics and beam splitters to create two wavelength channels and ther are two fully redundant modules. Each channel uses 4 XCo = 2.5 |m Rockwell HgCdTe HAWAII-2RG SCAs for X=0.6-2.5 |m, and 1 Xco = 5 |m SCA for X =2.5-5 |m. In total, NIRCam uses 10 SCAs, with 5 in each channel. Each SCA is controlled using one Rockwell SIDECAR ASIC, which is described in more detail in Section 2.3. SIDECAR stands for System for Image Digitization, Enhancement, Control And Retrieval, and ASIC is an Application Specific Integrated Circuit.

Figure 10. Optical layout of NIRCam. The instrument consists of two of these assemblies bolted together.

NIRSpec: The NIRSpec is a ^ = 0.6 - 5 |m multi-object spectrograph provided by ESA. NIRSpec incorporates two NASA-provided components. These are: (1) the detector subsystem and (2) a Micro-Machined Silicon (MEMS) Micro-Shutter Array (MSA). NIRSpec's modes are broadly summarized in Fig. 8. Figure 11 shows the instrument layout that has been conceived by EADS Astrium, ESA's Prime Contractor for the instrument. The FPA is being designed by ITT Industries, under sub-contract to the NIRSpec FPA provider, Rockwell Scientific.

NIRSpec's FPA consists of two, ^ = 0.6 -5 |m Rockwell HgCdTe HAWAII-2RG SCAs. Each SCA will be controlled using one SIDECAR ASIC. ITT has used an elliptical footprint for the FPA structure (see Fig. 11). According to their analysis, this geometry helps them to meet cryogenic FPA flatness requirements.

FGS/TI: The Fine Guidance Sensor/Tunable Imager (FGS/TI; see Fig. 12) is provided by the Canadian Space Agency. Because JWST is a large and relatively flexible telescope structure, tracking targets to the required accuracy cannot be accomplished by steering the bulk telescope structure. The telescope is therefore configured as a three-mirror anistigmat, with a fine steering mirror located at a pupil. Tracking is achieved by imaging a guide star with the FGS on an array identical to the ones used in NIRCam and NIRSpec. A centroid is measured, and an error signal is generated and fed to the fine steering mirror. The spacecraft's body pointing is updated as needed to keep the mirror within its range of travel.

The FGS/TF incorporates a tunable filter module. This portion of the instrument has the same field of view as one half of NIRCam. A dichroic is used to view this field by using short wavelength and long wavelength tunable filters simultaneously. These filters have R~100 and also include a coronagraphic capability. In total, the FGS/TF incorporates 4 Rockwell HAWAII-2RG SCAs. One SIDECAR ASIC will be used to control each SCA.

Grating Mechanism Nasa

Figure 11. JWST NIRSpec. Major instrument structure is entirely silicon carbine (SiC).

Notable exceptions include the FPA, micro-shutter array, and filter wheels. Credit: Overall instrument design is by EADS Astrium and FPA assembly design is by ITT Industries.

Figure 11. JWST NIRSpec. Major instrument structure is entirely silicon carbine (SiC).

Notable exceptions include the FPA, micro-shutter array, and filter wheels. Credit: Overall instrument design is by EADS Astrium and FPA assembly design is by ITT Industries.

MIRI: Mid-infrared imaging and spectroscopy are provided for JWST by MIRI (see Fig. 13). MIRI is being developed jointly by a consortium of European astronomical institutes, led by the United Kingdom Astronomy Technology Center, which has responsibility for the optical bench, and by the Jet Propulsion Laboratory, which has responsibility for the focal plane system, cooler, and flight software. MIRI's detectors need to operate near 7K so more cooling is needed than can be achieved with a passive radiator system, and the extra cooling will be supplied by a cryocooler.

Housing

Figure 12. JWST Fine Guidance Sensor / Tunable Imager.

Figure 13 shows the layout of MIRI with its imager on one side of its deck and the spectrometer modules on the other. The imager has a 1.9^1.4 arcminute field-of-view and 9 filters to cover 5.6 to 25.5 ^m. MIRI's spectrometer is configured as four integral field units to cover the spectral range from 5 to 28 ^m with a spectral resolution of order 3000. The field of view scales from 3.5"*3.5" at the short wavelength end to 7"x7" at the long wavelength end.

Planet detection is an important science program for MIRI and it will be equipped with two types of coronagraph, a traditional focal plane mask and a phase mask. The coronagraphs share a detector with the imager. The phase mask has the advantage of being able to observe closer to the central source in the region where the traditional mask is opaque. It has the drawback of working over only a limited spectral range so MIRI will have three phase masks to provide a wavelength choice.

Figure 13. (left) MIRI with mounting struts to ISIM and (right) detail of a MIRI FPM.

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