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 the telescope offsets transformed to pixels.
There is no resampling, merely integer shifts of origin. The recipe makes the mosaics by applying offsets in intensity to give the most consistent result amongst the overlapping regions. The noise will be greater in the mosaic's peripheral areas, having received less exposure time. Mosaics are made and displayed for each row, except the last. At the end of each cycle of 19 frames the full mosaic of nine target frames is created and displayed instead.