The name of the flat-field frame.
The package uses this frame to calculate balance factors on a per-pixel basis. If a balance frame is already available, then the frame should be supplied instead of a flat field, and the parameter TUNE_PREBAL should be set TRUE.
If you do not wish to supply a flat field at all, then you may either reply NONE to this prompt (must be upper case) or set the parameter TUNE_NOFLAT=YES.
If no flat-field frame is supplied then the balance factors will be set to unity for all pixels.
Martin Clayton