It should not be used for frames where the telescope guided on the moving object. The flat field is created iteratively. First an approximate flat-field is created by combining normalised object frames using the median at each pixel. This flat field is applied to the object frames. The first stage is repeated but applied to the masked frames to create the final flat field.
Registration is performed using common point sources in the overlap regions.
If the recipe cannot identify sufficient common objects, the script resorts to using the telescope offsets transformed to pixels. Once the offsets are determined, they are adjusted for the motion of the target, so that the final mosaic registers the target, not the background stars. The resampling applies non-integer shifts of origin using bilinear interpolation. There is no rotation to align the Cartesian axes with the cardinal directions. The recipe makes the mosaics by applying offsets in intensity to give the most consistent result amongst the overlapping regions.