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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.