The Auto Flatten Background command can be used when the image background level across the frame varies in a systematic way, and not due to the subject itself. Causes of these kinds of variation include a radial falloff in brightness due to instrumental vignetting, the proximity or inclusion in the image of bright stars or planets, or gradients due to sky glow, moonlight or light pollution.
Such problems can be reduced or even eliminated in many images by using this command. It estimates the background level in various regions of the image and then fits a two-dimensional polynomial surface to the background points. The resulting model is subtracted from the image, thus reducing the variations.
Note that the algorithm works best on star fields; images with a large foreground object consuming a significant part of the field can bias the background extraction.
Availability of this feature depends on Product Level.