The flat-fielding procedure for any image data serves to accomplish two goals:
Traditionally, these components are measured simultaneously
by imaging an assumed uniformly illuminated surface with the
detector-camera-telescope system.
Because the night sky background levels for broadband near infrared
measurements are so high, the sky itself is taken to be the uniformly
illuminated ''screen,'' and the flat-field image is constructed from
the astronomical observations. 2MASS and 2MAPPS currently employs such a
technique. ''Dark-sky'' flat-field images are constructed using
the trimmed average of a subset of frames (
30) taken from
each scan, and that flat is applied to each of the frames in the subset.