The sizes of telescope pointing offsets between successive exposures can be very different, depending on whether the purpose is “mosaicking
” or “dithering
.” Mosaicking is done with the aim of increasing the area of sky covered by a particular set of exposures, usually with the goal of providing a seamless joining of contiguous frames. The angular offsets used when mosaicking are generally large, up to the size of the field of view. Only programs observing targets larger than the field of view of the detector need to use mosaicked exposures.
Dithered and mosaicked exposures can be combined using software included in DrizzlePac
. Note that it is sometimes necessary to use software like that in DrizzlePac
to combine even CR-SPLIT or repeat exposures, when pointing drift causes slight misalignment of exposures and differences in how PSFs are pixilated, or when gradual changes in focus over the course of an orbit produce changes in the observed PSF.