Re: change of airmass across can be important

[<aah@nofs.navy.mil>]

Thanks to Michael for doing the differential airmass calculation.
You also have second order extinction effects, etc.  Lots
of things affect the photometry in wide-field images taken
off the zenith.  All I was saying is that the raw starlists
don't need these derived parameters taking up space.
  On the other hand, refraction does rear its ugly head
to affect the astrometry.  It is a color dependent effect
and so you get differential problems based on the star
color as well as the zenith distance.  Great fun!  That is
one reason why I suggest using UCAC as the basis for the
star coordinates rather than the astrometry from the Mark IV
frames.
Arne