The actual pixels PHOTOMed in high dispersion reside within a region defined by edge-detection algorithms on raw, well-exposed UV Flood images. These pseudo-circular regions coincide with the target boundary. For low dispersion, only a diagonal swath of data encompassing both the large and small apertures, as well as a generous portion of the background, is PHOTOMed.
The PHOTOM procedure accesses explicitly determined displacements between the raw science image and the ITF to align the two images and to convert the DN to FN. These displacements have been derived using the image registration technique as described in Chapter 5.