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Pixel-to-pixel gain variations are calibrated in IRAC by flatfielding
using images of the diffuse zodiacal background. 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.
After the first year of operations, analysis of the time history of
the skyflats indicated no detectable change in the pixel-to-pixel
response. The first year of skyflat data were combined into a set of
"super skyflats." These skyflats have extremely high photometric S/N (~
0.5, 0.3, 0.2, and 0.06% in channels 1-4, respectively), and are
substantially free from residuals from stars and stray light. However,
users are cautioned to read this
page 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. At this time (S13), all BCD images are
processed with the same set of flats linked below.
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