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Illumination Corrected Flat Fields

A second method to account for illumination problems in the flat fields is to remove the large scale pattern from the flat field itself. This is useful if there are no reasonable blank sky calibration frames and the astronomical exposures are evenly illuminated but the flat fields are not. This is done by smoothing the flat field frames instead of blank sky frames. As with using the sky frames there are two methods, creating an illumination correction to be applied as a separate step or fixing the original flat field. The smoothing algorithm is the same as that used in the other illumination commands. The commands to make these types of corrections are ILLCOR/CCD and ILLFLAT/CCD. The usage is virtually identical to the previous sky illumination correction commands. Obviously, after having obtained the illumination corrected flat field it is reasonable to replace the original flat fields by the corrected flat fields in the reduction table. Like is the case of SKYCOR/CCD, by default the command ILLFLAT/CCD will replace the original flat field by the illumination corrected one.

Petra Nass