IRAC Instrument Handbook - IRSA - California Institute of Technology

4 Calibration
Calibration 31 Darks
IRAC Instrument Handbook
The Spitzer Science Center (SSC) performed routine calibrations of IRAC using observations of standard
stars and other astronomical objects. The data obtained in these observations were used to construct the
necessary calibration inputs to the pipeline for the IRAC data processing of science observations. The
calibration data files are available to the general user in the Spitzer Heritage Archive maintained by
IRSA.
4.1 Darks
Dark current and bias offsets were calibrated via the standard ground-based technique of dark subtraction.
As part of routine operations, the SSC observed a dark region of the sky (skydark) near the north ecliptic
pole at least twice per campaign (at the beginning and end). These data were reduced and combined in
such a way as to reject stars and other astronomical objects with size-scales smaller than the IRAC array.
The resulting image (Figure 4.1) of the minimal uniform sky background contains both the bias and dark
current. When subtracted from the routine science data, this eliminates both of these instrumental
signatures. Naturally, this also subtracts a component of the true celestial background. The SSC included
a COBE-based model estimate of the true celestial background. Note that the lack of an isolated
measurement of the dark current and bias offset during shutterless operations limits the ability of IRAC to
measure the true celestial background.