This recipe is suitable for faint objects or objects within a comparatively bright core embedded in faint extended emission, e.g. a quasar. If there are other objects of comparable brightness to the principal target in other quadrants, they will introduce artifacts into the flat field. 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 common point sources in the overlap regions.
If the recipe cannot identify sufficient common objects, it matches the centroid of the central source. If this fails, 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.