# Master dark frame

A master dark frame is created from a set of raw dark frames. The robust mean algorithm (see Robust mean) is applied on a pixel-by-pixel basis, while leaving out bad and overexposed pixels. If there are enough source frames, the robustness of the algorithm ensures that an accidental artifact (e.g. cosmic ray particle) is ruled out and does not affect the mean value. If a pixel has an invalid value on all source frames, it is marked as a bad pixel on the resulting frame.

The source dark frames must have the same exposure duration, otherwise the resulting frame is not correct.

A scalable dark frame is computed in the same way as a master dark frame. The program checks that the bias-frame correction was applied to all source frames and sets the SCALABLE flag in the file’s header. The status of this flag is checked in the dark correction, see chapter Dark-frame correction.