gain_scale step rescales pixel values in JWST countrate
science data products in order to correct for the effect of using
a non-standard detector gain setting. The countrate data are
rescaled to make them appear as if they had been obtained using
the standard gain setting.
This currently only applies to NIRSpec exposures that are read out using a subarray pattern, in which case a gain setting of 2 is used instead of the standard setting of 1. Note that this only applies to NIRSpec subarray data obtained after April 2017, which is when the change was made in the instrument flight software to use gain=2. NIRSpec subarray data obtained previous to that time used the standard gain=1 setting.
gain_scale step is applied at the end of the
pipeline, after the
ramp_fit step has been applied. It is applied
to both the
rateints products from
ramp_fit, if both
types of products were created. The science (
SCI) and error (
arrays are both rescaled.
The scaling factor is obtained from the
GAINFACT keyword in the
header of the gain reference file. Normally the
ramp_fit step will
read that keyword value during its execution and store the value in
the science data keyword
GAINFACT, so that the gain reference file
does not have to be loaded again by the
gain_scale step. If, however,
the step does not find that keyword populated in the science data, it
will load the gain reference file to retreive it. If all attempts to
find the scaling factor fail, the step will be skipped.
Gain reference files for instruments or modes that use the standard
gain setting will typically not have the
GAINFACT keyword in their
header, which will cause the
gain_scale step to be skipped. Alternatively,
gain reference files for modes that use the standard gain can have
GAINFACT=1.0, in which case the correction will be benign.
Upon successful completion of the step, the
S_GANSCL keyword in the
science data will be set to “COMPLETE.”