The IRAC Instrument Handbook is one in a series of documents that explain the operations of the Spitzer Space Telescope and its three instruments, the data received from the instruments and the processing carried out on the data. The Spitzer Space Telescope Handbook gives an overview of the entire Spitzer mission and it explains the operations of the observatory, while the other three handbooks document the operation of, and the data produced by, the individual instruments (IRAC, IRS, and MIPS). The IRAC Instrument Handbook is intended to provide all the information necessary to understand the IRAC standard data products, as processed by the online pipeline system, and which are retrievable from the Spitzer Heritage Archive (SHA). Besides the detailed pipeline processing steps and data product details, background information is provided about the IRAC instrument itself, its observational modes, and all aspects of IRAC data calibration. It should be stressed that this Handbook is not intended to support interactive data analysis. For data analysis advice and suggested data processing procedures, please refer to the separate documentation available at the InfraRed Science Archive (IRSA) documentation website
including the Spitzer Data Analysis Cookbook. This Handbook serves as the reference for both the processing as well as the correct interpretation of IRAC data as available from the Spitzer Heritage Archive. Note that all the earlier versions of the IRAC Instrument Handbook are now obsolete, and IRAC data users should use the current version of the Handbook.
Several appendices are attached to give more detailed information on the topics briefly covered in the chapters mentioned above.
The Digital Object Identifier or DOI for this document is https://doi.org/10.26131/irsa486, and it should be used when quoting this document in articles or journal papers.
1.2 Basic Definitions
This section contains a description of the most commonly used terms in this Handbook. A complete list of acronyms can be found in Appendix F.
An Astronomical Observation Template (AOT) is the generic list of parameters for a distinct Spitzer observing mode. There was one possible IRAC AOT for the cryogenic mission, and one for the warm mission. A fundamental unit of Spitzer observing was the Astronomical Observation Request (AOR), also referred to sometimes as an “observation.” It was an AOT with all of the relevant parameters fully specified. Each AOR is identified in the Spitzer Heritage Archive by a unique observation identification number known as AORKEY. An AOR consists of several Data Collection Events (DCEs), which can be thought of as single frame exposures. The data products consist of Level 0 products (“raw data”) and Level 1 data products that are also called Basic Calibrated Data (BCD), which were derived from the DCEs after pipeline processing. See Chapter 5 for more information about pipeline processing. BCDs (or in the case of IRAC, corrected BCDs or CBCDs; see Section 5.2 for more information about BCDs vs. CBCDs) were designed to be the most reliable data product achievable by automated pipeline processing, and should be the starting point for further data processing. The pipeline also produced Level 2 data products or Post-BCD products, which were derived from data from the whole AOR (i.e., combination of several CBCDs). See Chapter 6 for more information about the data products.
1.3 IRAC Essentials
The most relevant software for IRAC data reduction is MOPEX (mosaicking and point source extraction; Makovoz & Khan 2005 and Makovoz & Marleau 2005). Documentation for it can be found in the data analysis section of Spitzer’s documentation website at IRSA:
See Chapter 8 for a brief introduction into IRAC data analysis. The separate Data Analysis section of the documentation website provides access to tools, user guides and data analysis recipes.
Before you start using IRAC data, we recommend that you familiarize yourself very carefully with this document, and specifically Chapter 7, which discusses the various artifacts in IRAC data. Several of these artifacts are at least partially corrected in the pipeline, but you should still be aware of them. The CBCD frames contain the artifact-corrected observations, and should usually be a starting point for your further data reduction and analysis. However, you always have the option of going back to the BCD frames if you are not happy with how artifacts were corrected in the CBCDs, and perform your own corrections.
Without additional corrections to the photometry, point source flux density measurements from IRAC images are only accurate within about 10%. The additional corrections are discussed in Chapter 4. It is possible to achieve down to a few percent absolute flux density accuracy by carefully performing all the corrections to the data (ultimately limited by the uncertainties in the spectra of calibration stars). Specifically, we urge the reader to read Chapter 4 carefully to understand why point source photometry should be performed with aperture photometry, unless the targets lie in an area of sky that has an extremely high surface density and/or a strongly spatially varying background. Chapter 4 also emphasizes that the pixel size (the area of sky subtended by a pixel) varies across the arrays and, therefore, flux density measurements off the (C)BCDs require corrections. Chapter 4 and Appendix B give much more information on how to best perform point source photometry on IRAC images. An IRAC data user should note that IRAC images are calibrated in surface brightness units. However, extended emission flux densities, and especially surface brightness, are not nearly as well calibrated as point sources. The user should read the caveats about measuring them in Chapter 8 and think carefully before publishing any results about extended emission flux density and surface brightness.
1.4 Standard Acknowledgments for IRAC Publications
Any paper published based on Spitzer data should contain the following text: “This work is based [in part] on observations made with the Spitzer Space Telescope, which was operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA.” If you received NASA data analysis funding for your research, you should use one of the templates listed under https://irsa.ipac.caltech.edu/data/SPITZER/docs/spitzermission/publications/ackn/. We also ask that you cite at least the fundamental IRAC paper (Fazio et al. 2004) in your research paper, and other IRAC-related papers, as appropriate.
1.5 How to Contact Us
A broad collection of information about IRAC and IRAC Data Analysis is available at IRSA’s Spitzer Documentation website, accessible via