The superbias subtraction step removes the fixed detector bias from a science data set by subtracting a superbias reference image.
The 2-D superbias reference image is subtracted from every group in every integration of the input science ramp data. Any NaN’s that might be present in the superbias image are set to a value of zero before being subtracted from the science data, such that those pixels effectively receive no correction.
The DQ array from the superbias reference file is combined with the science exposure “PIXELDQ” array using a bitwise OR operation.
The ERR and GROUPDQ arrays in the science ramp data are unchanged.
If the subarray mode of the superbias reference file matches that of the science exposure, the reference data are directly subtracted. If the superbias reference file contains full-frame data, while the science exposure is a subarray mode, the corresponding subarray is extracted from the superbias reference data before being subtracted.
No special handling is necessary for science data taken in the IRS2 readout mode, because matching IRS2 superbias reference files are supplied in CRDS.