The source centre is also used to determine the off-axis angle for the PSF.
Spatial regions may be entered in detector (DETX, DETY), sky (X/Y) or celestial (RA, DEC, FK4 2000) coordinates. If the coordinates of the region are not the same as those of the image, e.g. a sky pixel region on a detector coordinate image, then the task will still function but will run more slowly. An error will result if the spatial region is not within the image. Any shape conforming to the selectlib rules may be used.
