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. Several documents provide examples of how to use this software.
WFC3 ISR 2014-23 describes the procedure by which the individual images in the Frontier Fields program have been aligned using galaxies and provides the FORTRAN source code
hst2galign that accomplishes the alignment. (Further uses of
hst2galign involving PSF-fitting and faint source location and photometry are discussed in
WFC3 ISR 2014-24.)
WFC3 ISR 2015-04 describes a methodology for optimizing the parameter
pixfrac and shows the results of tests conducted for the Frontier Fields program.
WFC3 ISR 2015-09 shows how mosaic alignment can be achieved in a single step in DrizzlePac 2.0 by building up an expanded reference catalog, using the WFC3 observations of the Eagle Nebula (M16) as an example.
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.
