IRAC Instrument Handbook - IRSA - California Institute of Technology

IRAC Instrument Handbook
~1.2” × 1.2”. 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, mapping
capabilities, and simultaneous four-color imaging.
IRAC consists of the Cryogenic Assembly (CA) installed in the Multiple Instrument Chamber (MIC) in
the CTA, and the Warm Electronics Assembly (WEA) mounted in the spacecraft. Harnesses connect the
detectors and calibration subsystem in the CA to the WEA. The WEA communicates with the spacecraft
over three RS-422 serial lines that allow receiving commands from, and sending acknowledgments and
image data to, the spacecraft Command & Data Handling (C&DH) computer.
The IRAC Cryogenic Assembly, depicted in Figure 2.1, consists of the following major subassemblies:
the Pickoff Mirrors; the Shutter; the Optics Housings, which hold the doublet lenses, beamsplitters,
filters, and cold stops; the Focal Plane Assemblies (FPAs) that include the detector arrays and associated
components; the Transmission Calibrator with its Source and Integrating Spheres; and the Housing
Structure, consisting of the Main Housing Assembly and the wedge-shaped MIC Adapter Plate.
2.2 Description of Optics
2.2.1 Field of View (FOV)
The IRAC optical layout is shown in Figure 2.2 and Figure 2.3. Light from the telescope is reflected into
the IRAC structure by the pickoff mirrors for the two fields of view (FOVs). Each pair of channels has a
doublet lens which re-images the Spitzer focal plane onto the detectors. A beamsplitter reflects the short
wavelength light to the InSb detectors (Channels 1 and 2) and transmits the longer wavelength light to the
Si:As detectors (Channels 3 and 4). Channels 1 and 3 view the same telescope field (within a few pixels),
and Channels 2 and 4 view a different field simultaneously. The edges of the two IRAC fields of view are
separated by approximately 1.52’, with no overlap on the sky. The IRAC pixel scale is nearly the same in
all channels (~1.2” per pixel), providing a 5.2’×5.2’ FOV.
Instrument Description 5 Description of Optics