The corrected sky frames are combined pixel by pixel using a median of the values in each frame. The resultant frame is normalised by its median to form the flat field. This frame median is subtracted from the source frames after they have been flat-fielded. A flat field is created for each row of the grid of target frames, and applied only to that row of target frames. The sky subtraction comes from linear interpolation of the sky modal values of the two sky frames which immediately bracket the target frame.
Registration is performed using common point sources in the overlap regions.
If the recipe cannot identify sufficient common objects, it then tries the crosshead offsets. If these are null, the script resorts to the telescope offsets. 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.