A number of astronomical standard stars were observed in each instrument campaign to obtain a valid absolute flux calibration. Stars with a range of fluxes were observed at a number of positions across the array and many times throughout the mission to monitor any changes that may have occurred. Calibration stars with measured spectral types and accurate absolutely calibrated fluxes in the IRAC bands have been determined. These absolute calibration stars were in the continuous viewing zone (CVZ) so that they could be observed at any time necessary and could be monitored throughout the mission.
Four stars were observed in the CVZ at the beginning and end of each instrument campaign. These standards remained the same throughout the cryogenic mission, and provide the absolute flux reference for IRAC. Additionally, a calibrator near the ecliptic plane (which was different for each campaign) was observed every twelve hours. Its placement in the ecliptic plane minimized telescope slews. This calibrator was used to monitor any short-term variation in the photometric stability.
Analysis of the flux calibrator data indicates that absolute flux calibration is accurate to 3% (reflecting mostly the uncertainty in the models). Repeatability of measurements of 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, ). The absolute calibration is derived taking several systematic effects into account. The steps are described in detail by Reach et al. (2005, ). If this methodology is not applied, then point source photometry from the Level 1 products (BCDs) can be in error by up to 10%.
IRAC is calibrated using both so-called primary and secondary calibrator stars. The primary stars are used to monitor long-term variations in the absolute calibration. They number 11 stars, are located in the continuous viewing zone (CVZ), and were thus observable year-round. They were observed once at the beginning, and once at the end of each campaign, i.e., about every 10 days whenever the instrument was switched on in the cryogenic mission. The primary calibration stars are observed at the beginning of every two-week campaign in the warm mission. The primary calibrators (in decreasing brightness) are (J2000; with flux densities in mJy in channels 1-4, respectively):
All of the calibration data taken with these stars are public and are available in the Spitzer Heritage Archive. 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, in a tightly constrained window of about 20 degrees. Because of the motion of the Earth about the Sun this window constantly moved and so any one secondary calibrator was visible for only a campaign or two per year. In practice, the calibration values for IRAC appear to be quite temporally stable.
The data are calibrated 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. Unfortunately, ground-based infrared calibrators were too bright to use as calibrators for IRAC. Therefore, one must use models to predict the actual flux for each channel as a function of spectral type (Cohen et al. 2003, ). Table 4.1 lists the calibration factors that are used in the final processing of all IRAC data. The absolute calibration is described in detail in Reach et al. (2005, ), with further refinements at the 1%–3% level, based on better models for the calibration stars and a better estimate of the corrections to photometry (pixel phase, array-location dependent photometric correction, etc.).
Table 4.1: The photometric calibration and zero magnitude flux for IRAC. Values in parentheses are for warm mission.
The absolute gain calibration is accurate to better than 3%. The stellar photometry is repeatable at the < 1% level. The absolute fluxes of the calibration stars are known to 2% – 3% (Cohen et al. 2003, ). To obtain photometry at this accuracy, photometric corrections for the location of the source within its peak pixel, and the location of the source within the array, must be made.
Note that IRAC is 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 calibration assembly, but it was not operated in flight, in order to minimize mission risk. Therefore, the offset level in IRAC images is referred to laboratory measurements before 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 changes depending on whether the detector was recently annealed. Thus, for diffuse surface brightness measurements, we recommend making differential measurements among at least two sky positions, preferably from the same campaign.