A flat-field file required by the Fast chain. It was originally envisaged that
flat-filed images, generally obtained during spacecraft slews or periods when
the OM is inactive, would be stored in an archive. This was intended, partly to
monitor the evolution of the detector response and also to allow users to
accumulate a statistically good flat-field from flats taken close in time to
their observations. However, due to the complexities of analysing and
constructing a meaningful flat-field for the OM, at the current time a single,
unit OM flat-field is stored as a component of an OM Calibration File
(OM_PIXTOPIXSENS). The unit flat has been found to be adequate to better than
5% (in fact flat within the statistical uncertainties) in terms of large-scale
sensitivity variations. Pixel-to-pixel sensitivity variations are suspected to
be of a similar magnitude. At present a warning message is issued by the chain
noting that a unit flat-field is being used. This flat-field
is created automatically within omfchain.
