The 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. The information in this Handbook is based on the design requirements and on the characterization of the flight instrument in pre-flight ground tests and on in-flight performance, including the In-Orbit Checkout (IOC)/Science Validation (SV) in 2003 August - November and the IRAC Warm Instrument Characterization (IWIC) in 2009 May - July.
Figure 2.1: IRAC Cryogenic Assembly model, with the top cover removed to show the inner components.
A brief, high-level summary of IRAC for astronomers appeared in the ApJS Spitzer Special Issue, specifically the paper by Fazio et al. (2004) entitled “The Infrared Array Camera (IRAC) for the Spitzer Space Telescope” and in the paper by Hora et al. (2004) entitled “In-flight performance and calibration of the Infrared Array Camera (IRAC) for the Spitzer Space Telescope.” Links to these papers are available at IRSA’s Spitzer/IRAC documentation website, accessible at
IRAC was a four-channel camera that provided simultaneous 5.2 arcminute × 5.2 arcminute images at 3.6, 4.5, 5.8, and 8.0 μm. Two adjacent fields of view were imaged in pairs (3.6 and 5.8 μm; 4.5 and 8.0 μm) using dichroic beamsplitters. All four detector arrays in the camera were 256 × 256 pixels in size, with a pixel size of ≈ 1.2 arcseconds × 1.2 arcseconds. The two short wavelength channels used InSb detector arrays and the two longer wavelength channels used Si:As detectors. The IRAC instrument was designed to address the four major scientific objectives defining the Spitzer mission. These were (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 was in no way limited to these objectives, which we only mention to explain the scientific drivers for the instrument design. IRAC was a powerful survey instrument because of its high sensitivity, stability, large field of view, mapping capabilities, and near simultaneous four-channel imaging.
IRAC consisted of the Cryogenic Assembly (CA) installed in the Multiple Instrument Chamber (MIC) in the Cryogenic Telescope Assembly (CTA), and the Warm Electronics Assembly (WEA) mounted in the spacecraft. Harnesses connected the detectors and the calibration subsystem in the CA to the WEA. The WEA communicated with the spacecraft over three RS-422 serial lines that allowed 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, consisted of the following major subassemblies: the Pickoff Mirrors; the Shutter; the Optics Housings, which held the doublet lenses, beamsplitters, filters, and cold stops; the Focal Plane Assemblies (FPAs) that included 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.
IRAC operated both during the cryogenic mission and during the warm mission. During the cryogenic mission (or “cryo mission”), the channel 1 and 2 InSb arrays were operating at about 15 K and the channel 3 and 4 Si:As arrays were cooled down to about 6 K. On May 15, 2009, the cryogenic coolant ran out and Spitzer warmed up to 27.5 K. Consequently, the operation of the channel 3 and 4 arrays became impossible because of greatly enhanced thermal noise, but channels 1 and 2 could still be operated with about the same sensitivity as in the cryogenic mission. The temperatures and biases were adjusted. The bias of the channel 1 array was adjusted from 750 mV to 500 mV and the temperatures for the two operating arrays were set to 28.7 K. After the IRAC Warm Instrument Characterization (IWIC) period in 2009 May − July, the warm mission (or “warm IRAC”) operations started on 2009 July 25. The last temperature changes were made on September 18, 2009. The data taken in campaigns PC(post-cryo)1 until the early part of PC4 were taken while the temperatures were still settling and changing, and require the use of specific flux calibration. In general, we caution the user to be extremely careful when using data from PC1 − early PC4 campaigns. The calibration of data from the latter part of the PC4 campaign (which ran 2009 September 10 - 23) onwards is stable.
