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IRAC: Instrument Description |
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IRAC is a four-channel camera that provides simultaneous 5.2 x 5.2
arcminutes images at 3.6, 4.5, 5.8, and 8 microns. Two adjacent fields of
view are imaged in pairs (3.6 and 5.8 microns; 4.5 and 8.0 microns) using
dichroic beamsplitters. All four detector arrays in the camera are 256 x
256 pixels in size, with a pixel size of ~1.2 x 1.2 arcsec. The two short
wavelength channels use InSb detector arrays and the two longer wavelength
channels use Si:As detectors.
The IRAC instrument was designed to address the four major scientific objectives defining the Spitzer mission. These are (1) to study the early universe, (2) to search for and study brown dwarfs and superplanets, (3) to study ultraluminous galaxies and active galactic nuclei, and (4) to discover and study protoplanetary and planetary debris disks. The utility of IRAC is in no way limited to these objectives, which we only mention to explain the scientific drivers for the instrument design. IRAC is a powerful survey instrument because of its high sensitivity, large field of view, and simultaneous four-color imaging.
OpticsIRAC provides diffraction-limited imaging internally; image quality is limited primarily by the Spitzer telescope. The IRAC optics specifications limit the wavefront errors to <lambda/20 in each channel.
Detectors & HardwareChannels 1 and 2 use InSb arrays operating at ~15K, and channels 3 and 4 use Si:As detectors operating at ~6K.IRAC has no moving parts (other than the shutter, which is not planned to be operated in flight). The instrument takes data by staring at the sky and sampling the arrays between resets. IRAC is capable of operating each of its four arrays independently and/or simultaneously. All four arrays are used during normal, full-array operation.
Fowler samplingMultiple (Fowler) sampling is used to reduce the effective read noise. This mode of sampling consists of taking N non-destructive reads immediately after the reset, and another N non-destructive reads near the end of the integration. Differencing is performed in the IRAC electronics to generate one integer value per pixel per exposure to store on the spacecraft and transmit to the ground. The Fowler N used for an observation depends on integration time and was selected to maximize the S/N, based on in-flight performance tests.
Subarray modeIn subarray mode, only one corner, 32x32 pixels offset by 8 pixels from the edges, is read out from one array. Pixels (9-40, 9-40) of the array are read out. The subarray pixel size is the same as the full array pixel size (~1.2). Fowler sampling is performed as in full array mode, but a set of 64 subarray images are generated and tiled into a single 256x256 image before data are sent from IRAC. In subarray mode, Fowler sampling is performed at 0.01 sec intervals. Subarray mode is useful for observing very bright sources and for obtaining high temporal resolution.
Calibration lampsIRAC contains two types of internal calibration lamps. The transmission calibrator lamps are designed to illuminate all four arrays and provide an internal responsivity measurement. There are two transmission calibrator spheres, each of which contains two lamp elements. To illuminate the arrays, the shutter is closed, a transmission lamp is turned on, and the light from that lamp bounces off a mirror on the back of the shutter. The flood calibrators individually illuminate each detector. The flood calibrators can be controlled individually, and they can be used whether the shutter is open or closed. The flood calibrators are operated at the end of each IRAC campaign and used as a consistency check. Calibration of IRAC observations is described in section 6.3.1 of the SOM.
CreditsThe InfraRed Array Camera (IRAC) was built by the NASA Goddard Space Flight Center (GSFC) with management and scientific leadership by the Smithsonian Astrophysical Observatory (SAO) under principal investigator Giovanni Fazio.
For more information, see the IRAC chapter of the SOM
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help@spitzer.caltech.edu http://ssc.spitzer.caltech.edu/irac/descrip.html
