Re: TN89 and hints on optical vignetting and stray light

["Richard Miles" <rmiles@baa.u-net.com>]

Just one additional point re. the following comment of mine in the foregoing
note:

"However, applying the methodology
 used in TN89 but applying it to only the dark-subtracted images entirely
avoids this approximation of averaged sky flats since it is based directly
on point-source signal data.  Furthermore, if the TN89 methodology (applied
to only dark-subtracted image data) can utilise an even greater number of
frames and star measures, then coverage may be sufficient to even correct
for response variations having relatively high spatial frequency such as
caused by dust located near to, or on the window of the CCD chip."

The point is that if CCD chips show increasingly large variations in
intrinsic pixel-to-pixel response then this will increasingly compromise the
proposed methodology for flat-fielding since it is not really possible to
calibrate down to the pixel level.  (Am not sure what this pixel-to-pixel
characteristic is as used in the MkIV camera.)
However, it can in principle get down to a level comparable in size to the
stellar images themselves.

Richard Miles