At its basic level this step flat-fields an input science data set by dividing by a flat-field reference image. In particular, the SCI array from the flat-field reference file is divided into both the SCI and ERR arrays of the science data set, and the flat-field DQ array is combined with the science DQ array using a bit-wise OR operation.
MultiSlit data models are handled as follows. First, if the flat-field reference file supplied to the step is also in the form of a MultiSlit model, it searches the reference file for slits with names that match the slits in the science exposure (e.g. ‘S1600A1’ or ‘S200B1’). When it finds a match, it uses the flat-field data for that slit to correct the particular slit data in the science exposure. If, on the other hand, the flat-field consists of a single image model, the region corresponding to each slit in the science data is extracted on-the-fly from the flat-field data and applied to the corresponding slit in the science data.
Multiple-integration datasets (the _rateints.fits products from the ramp_fit step) are handled by applying the flat-field to each integration.
For pixels whose DQ is NO_FLAT_FIELD in the reference file, the flat value is reset to 1.0. Similarly, for pixels whose flat value is NaN, the flat value is reset to 1.0 and DQ value in the output science data is set to NO_FLAT_FIELD. In both cases, the effect is that no flat-field is applied.
If any part of the input data model gets flat-fielded (e.g. at least one slit of a MultiSlit model), the status keyword S_FLAT will be set to COMPLETE in the output science data.
The difference with NIRSpec data is that the flat field array that will be divided into the SCI and ERR arrays of the input science data set is not read directly from CRDS. This is because the flat field varies with wavelength, and the wavelength of light that falls on any given pixel depends on mode and on which slit or slits are open. The flat-field array that is divided into the SCI and ERR arrays is constructed on-the-fly by extracting the relevant section from the reference files, and then – for each pixel – interpolating to the appropriate wavelength for that pixel. See the Reference File section for further details. There is an option to save the on-the-fly flat field to a file.
This step handles input science exposures that were taken in subarray modes in a flexible way. If the reference data arrays are the same size as the science data, they will be applied directly. If there is a mismatch, the routine will extract a matching subarray from the reference file data arrays and apply them to the science data. Hence full-frame reference files can be used for both full-frame and subarray science exposures, or subarray-dependent reference files can be provided if desired.