Flat creation

Frames are optionally cleaned to remove extreme outliers ($\pm$3 or 6$\sigma$ about the mean in 15$\times$15-pixel neighbourhood) iterated three times, except for thermal recipes NOD_SELF_FLAT_NO_MASK and variants, and NOD_SKY_FLAT_THERMAL. The data are then normalised, combined pixel by pixel using a broadened median, and the combined array normalised to a have a mean of one. It is possible to use another statistic for the combination, such as a clipped median.
[_MAKE_FLAT_FROM_GROUP_, _MAKE_FLAT_FROM_NORMALISED_GROUP_,
_MASK_DEVIANTS_]

There are variants for certain families of recipes, which marshall the various required subgroups of frames.
[_MAKE_FLAT_CHOP_SKY_, _MAKE_FLAT_FROM_NORMALISED_CHOP_SKY_,
_MAKE_FLAT_EXTENDED_, _MAKE_FLAT_FROM_NORMALISED_EXTENDED_,
_MAKE_FLAT_QUADRANT_JITTER_]

Flats are filed with the calibration system.
[_FILE_FLAT_]