THE INFRARED ARRAY CAMERA (IRAC) FOR THE SPITZER SPACE TELESCOPE G. G. Fazio, 1 J. L. Hora, 1 L. E. Allen, 1 M. L. N. Ashby, 1 P. Barmby, 1 L. K. Deutsch, 1,2 J.-S. Huang, 1 S. Kleiner, 1 M. Marengo, 1 S. T. Megeath, 1 G. J. Melnick, 1 M. A. Pahre, 1 B. M. Patten, 1 J. Polizotti, 1 H. A. Smith, 1 R. S. Taylor, 1 Z. Wang, 1 S. P. Willner, 1 W. F. Hoffmann, 3 J. L. Pipher, 4 W. J. Forrest, 4 C. W. McMurty, 4 C. R. McCreight, 5 M. E. McKelvey, 5 R. E. McMurray, 5 D. G. Koch, 5 S. H. Moseley, 6 R. G. Arendt, 6 J. E. Mentzell, 6 C. T. Marx, 6 P. Losch, 6 P. Mayman, 6 W. Eichhorn, 6 D. Krebs, 6 M. Jhabvala, 6 D. Y. Gezari, 6 D. J. Fixsen, 6 J. Flores, 6 K. Shakoorzadeh, 6 R. Jungo, 6 C. Hakun, 6 L. Workman, 6 G. Karpati, 6 R. Kichak, 6 R. Whitley, 6 S. Mann, 6 E. V. Tollestrup, 7 P. Eisenhardt, 8 D. Stern, 8 V. Gorjian, 8 B. Bhattacharya, 9 S. Carey, 9 B. O. Nelson, 9 W. J. Glaccum, 9 M. Lacy, 9 P. J. Lowrance, 9 S. Laine, 9 W. T. Reach, 9 J. A. Stauffer, 9 J. A. Surace, 9 G. Wilson, 9 E. L. Wright, 10 A. Hoffman, 11 G. Domingo, 11 and M. Cohen 12 Received 2004 March 26; accepted 2004 May 26 ABSTRACT The Infrared Array Camera (IRAC) is one of three focal plane instruments on the Spitzer Space Telescope. IRAC is a four-channel camera that obtains simultaneous broadband images at 3.6, 4.5, 5.8, and 8.0 m. Two nearly adjacent 5A2 ; 5A2 fields of view in the focal plane are viewed by the four channels in pairs (3.6 and 5.8 m; 4.5 and 8 m). All four detector arrays in the camera are 256 ; 256 pixels in size, with the two shorter wavelength channels using InSb and the two longer wavelength channels using Si:As IBC detectors. IRAC is a powerful survey instrument because of its high sensitivity, large field of view, and four-color imaging. This paper sum- marizes the in-flight scientific, technical, and operational performance of IRAC. Subject headin gg s: infrared: general — instrumentation: detectors — space vehicles: instruments 1. INTRODUCTION The three Spitzer Space Telescope focal plane instruments were designed to investigate four major scientific topics: (1) the early universe, (2) brown dwarfs and superplanets, (3) active galactic nuclei, and (4) protoplanetary and planetary debris disks. Of these topics, the most important in defining the Infrared Array Camera (IRAC) design was the study of the early universe, and in particular the study of the evolution of normal galaxies to z > 3 by means of deep, large-area surveys. The 3–10 m wavelength range was selected because stars have a peak emission at a wavelength of 1.6 m, at the minimum of the H  opacity (John 1988). The emission peak is a ubiquitous feature of stellar atmospheres and can be used to determine a photometric redshift for 1 < z < 5 (Wright et al. 1994). The IRAC sensitivity requirement was set such that IRAC could achieve a 10  detection of an L  galaxy at z ¼ 3. This in turn required the measurement of a flux density at 8 m of 8 Jy (10 ; Simpson & Eisenhardt 1999). Channel 1 (3.6 m) was selected to be at the minimum of the zodiacal back- ground radiation (Wright 1985) and to avoid the water ice absorption band at 3.1 m. The central wavelengths of the remaining IRAC filters and their bandwidths (approximately 25%) were then optimized (considering the detector materials available) to reach the sensitivity requirement and to permit the measurement of a photometric redshift for 1 < z < 5 (Simpson & Eisenhardt 1999). The number of individual channels in IRAC was limited by the number of array cameras that could fit in the specified volume. To reduce costs and maximize reliability, moving parts had to be minimized; hence, no filter wheels, only fixed filters, were used. Transmissive rather than reflective optics were used because of limited space. The pixel size was op- timized to achieve the best point-source sensitivity for weak sources while maximizing the survey efficiency. The shutter at the entrance aperture was the only moving part allowed in IRAC. Although the design was optimized for the study of the early universe, IRAC is a general-purpose, wide-field camera that can be used for a large range of astronomical investi- gations. In-flight observations with IRAC have already dem- onstrated that IRAC’s sensitivity, pixel size, field of view (FOV), and filter selection are excellent for studying galaxy structure and morphology, active galactic nuclei, and the early stages of star formation and evolution and for identifying brown dwarfs. IRAC was built by the NASA Goddard Space Flight Center (GSFC), with the Smithsonian Astrophysical Obser- vatory (SAO) having management and scientific responsibil- ity. Additional information on the operation and performance 1 Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138; gfazio@cfa.harvard.edu. 2 Deceased 2004 April 2. 3 Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721. 4 Department of Physics and Astronomy, University of Rochester, Rochester, NY 14627. 5 NASA Ames Research Center, Moffett Field, CA 94035. 6 NASA Goddard Space Flight Center, Laboratory for High Energy Astrophysics, Code 662, Greenbelt, MD 20771. 7 Institute for Astronomy, 640 North A’ohoku Place, Hilo, HI 96720. 8 Jet Propulsion Laboratory, California Institute of Technology, MS 264- 767, 4800 Oak Grove Drive, Pasadena, CA 91109. 9 Spitzer Science Center, MC 220-6, California Institute of Technology, Pasadena, CA 91125. 10 Department of Physics and Astronomy, UCLA, P.O. Box 951562, Los Angeles, CA 90095. 11 Raytheon Infrared Operations, 75 Coromar Drive, Building 2, MS 8, Goleta, CA 93117. 12 Department of Astronomy, University of California, 601 Campbell Hall, Berkeley, CA 94720. 10 The Astrophysical Journal Supplement Series, 154:10–17, 2004 September # 2004. The American Astronomical Society. All rights reserved. Printed in U.S.A.