Images with an irregular background can result from incomplete
correction of vignetting or background colour variations. A flat field correction
is the first step to correcting both effects. Still, a large scale irregularity
in the background may exist after the flat field corrections are applied.
Furthermore, good flat field calibration images may not exist for a given
instrumental setup, or the "flats" may not provide a complete correction. In
such cases, how do we improve or, in the worst case, salvage such an image? In
this Tech Note we show how to do this using Mira.
Both luminance images and colour images usually require a flat field correction
to correct the large scale vignetting. Despite such effort, a background
irregularity may remain and, further, it is usually different in the different
colour planes of an RGB image. The image below shows a severe case of
vignetting for which no flat field calibration has been applied. This Tech Note
shows how to correct this image to obtain a background that is quite flat. The
same technique is equally well applied to colour images to remove background