Relatively short exposures for the WFPC2 show evidence of shutter shading, resulting from the finite time of flight of the shutter blade. This gives rise to uneven illumination across the field of view. The input science image is corrected for the shutter shading by means of an additive correction that varies spatially according to the shutter shading reference file which is scaled for the appropriate exposure time. The name of the shutter shading reference file must be provided in the keyword SHADFILE.
