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.