If the object is not isolated, there will be artifacts introduced into the flat field. This arise from the contribution of sources outside the quadrant containing the primary object. The flat field is created in two steps. The quadrant containing the object is masked in each object frame. Then the recipe combines the normalised and quadrant-masked object frames using the median at each pixel.
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 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. The full signal will be in the central ninth containing the main object.