10.1117/2.1201401.005319 Photometric calibration for the InfraRed Array Camera on the Spitzer Space Telescope Sean Carey, James Ingalls, and Joseph Hora A novel instrument design, combined with careful calibration analysis techniques, enables accurate astronomical observations. The InfraRed Array Camera (IRAC) is one of three instruments on the Spitzer Space Telescope, which is part of NASA’s Great Observatories program. 1, 2 Since August 2003, the IRAC instru- ment has been used to make astronomical observations in the 3–8m wavelength range. In the past, it has been difficult to cal- ibrate IR photometers to better than 5% because of uncertainties associated with the primary calibrators and the temperature sta- bility of the detectors. Through a combination of novel design and careful analysis, it has been possible to calibrate the IRAC instrument to an ab- solute photometric accuracy of 2.5%. 3 The exquisite stability of IRAC, which is due to millikelvin thermal control of the detec- tors together with a detailed understanding of the instrument’s systematics, has enabled several Spitzer discoveries. These find- ings include a precise (<3% error) determination of the Hubble constant, 4 mass measurements of the highest redshift galaxies yet discovered, 5 measurements of weather patterns on brown dwarf planets, 6 and the first measurement of an exoplanet’s lon- gitudinal temperature profile. 7 We have been able to achieve the high precision of IRAC by analyzing more than nine years of in-flight data. We iden- tified and corrected instrumental systematics in the photome- try measurements, which enabled precisions of <100ppm over timescales of hours and 0.1% relative photometry over year- long timescales. This has resulted in photometric stability of better than a fraction of a percent over the 5.5-year cryogenic (see Figure 1) and warm (ongoing since 2009) missions. Most of the uncertainties that remain in our final calibration of IRAC are due to limitations in our knowledge of the fundamental IR flux calibrator used (the star Vega). Figure 1. Photometric stability of the InfraRed Array Camera (IRAC). Measurements of a primary calibrator for the instrument were made throughout the cryogenic mission, at a wavelength of 3.6m. The flux from a network of observed calibration objects is measured in units of ’data numbers per second.’ We perform the absolute calibration for IRAC by determining a scaling factor that converts these to physical units—janskys (Jy)—of flux den- sity. There are, however, two main complications with this pro- cedure. First, we have to correct for the instrumental photometry systematic effects that the measured fluxes contain, and second, our knowledge of the flux densities of the calibrator sources is imperfect. So that our calibration is robust to uncertainties in photometric truth, we find average values from two different kinds of calibration sources (stars with A0V and KIII spectral Continued on next page