Pixel-to-pixel gain variations are calibrated in IRAC by flatfielding using images of the diffuse zodiacal background, which is the brightest source in the sky that fills the focal plane. At least one region near the ecliptic plane is observed in every IRAC campaign.
These data are processed by a calibration pipeline similar in many respects to the BCD pipeline. Eventually, the data are stacked using a sophisticated outlier rejection scheme to remove the stars and galaxies from the data. The result is an image of the zodiacal background free of foreground and background objects. From this is subtracted a contemporaneous skydark, and the result normalized to one to create a skyflat.
Analysis of the time history of the skyflats indicated no detectable change in the pixel-to-pixel response as a function of time. At the end of the cryogenic mission, all of the skyflat data were combined into a set of "super skyflats." These skyflats have extremely high photometric S/N (0.14%, 0.09%, 0.07%, and 0.01% in channels 1-4, respectively), and are free from residuals from stars and stray light. However, users are cautioned to read Array-Location-Dependent Photometric Corrections for important information regarding the accuracy of these flats for certain types of observations.
These flats are divided into all the BCD data by the BCD pipeline via the software module FLATAP.
Additionally, users should note that the IRAC calibrations form a set; in order for the flux calibration to be internally consistent, the flux conversion must have been measured from data using this flatfield. The flatfields shown here will be used for the final cryogenic processing (along with new flux conversions), and observers who use the BCD or CBCD products will not be affected. However, attempting to use these flatfields with the current FLUXCONV values will result in systematic errors as large as 0.5%.