The SAS summary file is examined to produce a list of the OM filters that were used in the observation, and for
each filter a list of the exposures for that particular filter is also produced.
A flatfield (currently a static flat, with each pixel have the value 1, from the CAL data) is
then created by the task omflatgen.
The chain then scans the input directory to check that the necessary auxilliary
files (e.g. tracking history, window files, housekeeping files etc) are
present, exiting with an error message if any key files are absent.
