Re: Using flat field image

[<aah@nofs.navy.mil>]

Michael,
>  Just a caution that 15 or 20 or 30 different night-sky
>fields may be necessary to get rid of stellar residuals
>in night-sky flats.

  That is one reason that I usually mix programs with
the Mark IV, and make sure it does a ScnSky sequence
sometime during the night.  The flats turn out better
when both RA and DEC are varied with reasonably long
exposures.
  As for the faint stars -- that could be a flatfielding
problem, since any residual stars would decrease the
brightness of any frame subsequently flatfielded.
The Tycho catalog *does* fail on the faint end, so
that is also suspicious.  Remember the transformations
use BT _and_ VT, and one is often very marginal for
fainter stars.  Other than these two points, I'd have
to wait for the diagnostics.
Arne