An accurate determination of the background is extremely important for the latter analysis. Therefore, one prefers to use all pixels, which are not contaminated by sources, and fit a low order polynomial surface to the background. Non-linear filters are often used to remove stellar images and other sharp features (see Section 2.2.1) while extended objects are very difficult to eliminate automatically. If only point like objects are analyzed background following methods like the recursive filter described by Martin and Lutz (1979) can be used.
-clipping techniques are applied in an iterative
scheme where pixels with high residual compared to a low order
polynomial fit to the frame are rejected (Capaccioli and Held 1979).
In this method areas containing extended objects can be excluded
before the iteration starts. In Figure 2.8, a field with
extended objects is shown with the mask defining the areas to be
omitted in the computations.