Association of images

The association of scientific frames with suitable calibration images is achieved by selecting and ranking all calibration frames which for a given scientific exposure match a set of user defined selection criteria. One may expand the search by submitting a ``second choice'' set of criteria when too few frames match the original one. For instance, this may in natural language be expressed as: ``Find for each scientific frame 2 dark exposures which have been observed within the same night. If not found, look for dark exposures observed within 3 days.''