A number of astronomical standard stars were observed to obtain a valid absolute flux calibration. Stars with a range of fluxes were observed at a number of positions across the array many times throughout the mission, to monitor any changes that may have occurred. Spectral types and accurate absolutely calibrated fluxes in the IRAC channels were determined for the calibration stars. Their flux densities are given in Table 4.1. All of the calibration data taken with these stars are public, and are available in the Spitzer Heritage Archive.
The observational program to measure the flux calibration of IRAC and to ensure its constancy had two components. The primary calibrator stars in the continuous viewing zone (CVZ) were observed in every campaign, and used to derive the overall flux calibration. The secondary calibrator stars were used to monitor short-term variations in the absolute calibration. To avoid slew overheads, they were observed close to downlinks, and, therefore, had to be located near the ecliptic plane, within a tightly constrained window of about 20 degrees in width. Because of the motion of Earth about the Sun, this window constantly moved, and so any one secondary calibrator was visible and near the downlink pointing for only a campaign or two per year. Using the secondary standard stars, the IRAC calibration was measured to be stable over the mission.
The absolute calibration was derived by means of aperture photometry, using a 10 native pixel radius (≈ 12 arcseconds) aperture. The background was measured using a robust average in a 12 - 20 native pixel annulus around the centroid of the star. The steps to remove known systematics are described in detail by Reach et al. (2005), Hora et al. (2008) and Carey et al. (2012). To obtain photometry at the highest possible accuracy, photometric corrections for the location of the peak of the source within a single pixel, and the location of the source within the array, have to be made. If this methodology is not applied, then point source photometry from the Level 1 products (BCDs) can be in error by up to 10%.
Analysis of the flux calibrator data indicated that absolute flux calibration is accurate to 3%. Repeatability of the measurements of the individual stars is better than 1.5% (dispersion), and can be as good as 0.01% with very careful observation design (e.g., Charbonneau et al. 2005). Unfortunately, ground-based infrared calibrators were too bright to use as calibrators for IRAC. Therefore, we had to use models to predict the actual flux for each channel as a function of star spectral type. Uncertainty in those models, and, therefore, in the absolute flux densities of the calibration stars, are 2% – 3% (Cohen et al. 2003). Please note that if you measure aperture photometry on IRAC images, then in our nomenclature you will have measured F*K*(where F* is the flux density of the calibration source at the nominal wavelength, and K*is the color correction factor for the calibration source spectrum, see Section 4.4). If a comparison of values at the nominal IRAC channel wavelengths (listed in
Table 4.3) to measurements of the star at other wavelengths is desired (e.g., the spectral energy distribution), then the F*K* values should be divided by the corresponding K* values in Table 4.1.
Table 4.1: IRAC primary calibrator quoted flux densities and color corrections.
Calibration Source Name(s)
Quoted Flux Densities F* (mJy) and Color Corrections K*
Table 4.2 lists the calibration factors that were used in the final processing of all IRAC data.
Table 4.2: The photometric calibration and zero magnitude flux densities for IRAC. Warm mission corrections are given as the second set of values in channels 1 and 2. Note that at the beginning of the warm mission the calibration was changing frequently. Different FLUXCONV values for these first months can be found on IRSA’s IRAC web pages (under “Warm IRAC Characteristics”).
Table 4.2 lists the calibration factors derived from the primary calibration program that were used in the final processing of all IRAC data. These calibration factors can be used to convert native IRAC data units (DN/second) into units of surface brightness (MJy/sr). Note that the zero magnitude flux densities listed below are evaluated at exactly 3.6, 4.5, 5.8, and 8.0 µm and not at the nominal channel wavelengths given in Table 4.3. To estimate the zero magnitude flux densities at the nominal channel wavelengths, scale the values in Table 4.2 by the square of the ratio of the wavelengths. For example, in channel 1 the zero-magnitude flux density at 3.544 μm would be (3.544/3.6)2 × 280.9 Jy = 272.2 Jy. The difference between the two is a few percent.
Note that IRAC was not an absolute background photometer, so the total brightness in IRAC images should be used with great caution. There was a cold shutter in the cold assembly, but it was not operated in flight, in order to minimize the mission risk. Therefore, in the cryogenic mission the offset level in IRAC images was referenced to laboratory measurements before the launch, where the offset level was observed to change very significantly from one laboratory experiment to another. In laboratory tests, the absolute offset of IRAC images was found to vary at levels that are comparable to the minimum celestial background in channels 1 and 2. Furthermore, the offset level changed depending on whether the detector was recently annealed (annealing means heating the detectors to 23 - 27 K for a short time with a small current flowing through the detectors. It restores the distribution of charges that are trapped to the same distribution each time, but does not restore most hot pixels; annealing was done only during the cryogenic mission). Thus, for diffuse surface brightness measurements, we recommend making differential measurements among at least two sky positions, preferably from the same campaign.