This recipe works well for faint sources and for moderately crowded fields. 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. 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. The mosaic is not trimmed to the dimensions of a single frame, thus the noise will be greater in the peripheral areas having received less exposure time.