more flatfields

[aah@nofs.navy.mil]

Glenn G. wrote about Chromey's CCD Astronomy article.  Highly recommended.
As I wrote a few months ago, he subsequently published his data in
"The Flat Sky: Calibration and Background Uniformity in Wide Field Astronomical
Images", PASP 108, 944, 1996.

Mike G. wrote:
>....  I have found that the resulting
>flat vectors are fairly stable over many weeks.
  While this is generally true, I strongly caution against using a 'master
flat vector' over a period of weeks.  We take flats here every night to
guard against changes, such as dust/pollen/hairs.  If you break the flat-taking
process up into two, by using a 'dome flat' for high frequency stuff
and an 'adjustment flat' for low frequency stuff (like gradients in the
dome flat), then the adjustment flat should certainly be stable over long
periods and need only be calculated infrequently.
  I still distrust sky flats with wide-field cameras.  As Chromey showed,
there are natural gradients in the sky, much less the influence of city
and local lights, and these are additive effects rather than multiplicative.
  The bottom line is that the better you calibrate your images, the more
useful they become.  If you want to do 0.01mag photometry, then many
experiments need to be performed to find systematic errors and improved
extraction/reduction techniques.  Tom spent a fair amount of time building
these cameras; why not take an equivalent amount of time to learn the
best way to use them?

Arne