IRAC Instrument Handbook - IRSA - California Institute of Technology

IRAC Instrument Handbook
Analysis of data from the first two years of operations has shown that the flatfield response of IRAC is
unchanging at the limit of our ability to measure. As a result, so-called “super skyflats" were generated
from the first two years of data. The super skyflats are shown in Figure 7.1.
These flats are extremely low-noise, with stochastic pixel-to-pixel uncertainties of 0.14%, 0.09%, 0.07%,
and 0.01% in channels 1 through 4, respectively. This is smaller in amplitude than the intrinsic pixel-topixel
scatter in the gain. Furthermore, because the super skyflats are derived from data over many parts of
the sky, with many dithers and rotations of the telescope, they are substantially free of errors arising from
gradients in the zodiacal background, or from residual contamination by stars and galaxies. Currently all
IRAC data are reduced with the same set of super skyflats.
Large-scale gradients corrected by the flats are on the order of 10%−15%. Systematic errors in the flats
are due to the gradient in the zodiacal background and straylight removal errors. The former is expected
to be very small based on results from other missions (Abraham et al. 1997 [1], ISOPHOT 25 µm).
Diffuse stray light is a significant contaminant in the raw images at the ~ 5%−10% level. This diffuse
light looks like a “butterfly” across the top of the InSb detectors in channels 1 and 2, or a “tic-tac-toe”
pattern in channels 3 and 4. It is always present, resulting from scattering of the zodiacal background onto
the detectors. In both the skyflats and the science data, a model of the straylight has been subtracted, but
this leaves a residual pattern on the order of 1% which contaminates the flats. These errors are
substantially ameliorated by dithering (errors will decrease as N , where N is the number of dithers, and
will quickly become very small relative to other uncertainties).
Data Features and Artifacts 107 Darks, Flats and Bad Pixels