The OM flatfield is currently maintained as a unit image in the OM_PIXTOPIXSENS CCF file. It was originally intended that OM flatfield exposures would be held in an archive and that users would be able to extract and combine sets of flats of their choice from this archive. This would permit users to generate flatfields temporally close to their observations if desired and also allow one to monitor the evolution of the flatfield.
In practice, the complexity of deriving a meaningful flatfield for the
photon-counting OM detector has led to the installation of a unit flatfield
image. In fact, evidence indicates that there are no large scale variations
within the statistical uncertainties of the measurements and that the image is
flat on large scales to better than 
5%. Evidence also suggests that pixel to
pixel variations are probably constrained to a similar level. Thus, currently,
the unit flatfield is deemed adequate for OM applications.
The omflatgen task simply retrieves the flatfield image from the CCF file and creates a flatfield file for use by omflatfield.