FAQ on FOC Flat Fields

Q:   My image does not seem to been flat fielded properly.
     What's the problem?

A:  First of all, the flat field applied is heavily smoothed
and so fine scale features like scratches, reseau, blemishes,
and so on will not be flat fielded out. (The reason is that the
distortion is not perfectly stable and a drift of a pixel or so
can result in more damage being done by trying to flat field
out a scratch than not). This can often be remedied by flat
fielding yourself (see next question) with an appropriate
registration of the flat field.

Second, the current pipeline scheme assumes the same flat field
applies for all formats. In actuallity, each format has its own
flat field signature, particularly near the beginning of the
scan line. The pipeline will in the next few months be modified
to allow these format dependent corrections to be applied.