change of airmass across can be important

[Stupendous Man <richmond@a188-l009.rit.edu>]

  Jure brought up the issue of airmass:

> One thing that might be useful is the airmass.  This would normally 
> be in the header but the field of view is so big that it maybe 
> makes a difference.

  I looked at this a few weeks ago, and it _can_ make a significant
difference.  Assuming typical values of the atmospheric extinction,
k=0.20 in V and k=0.08 in I, and assuming that a Mark IV frame is 
oriented in the worst possible way (tipped diagonally relative to the
horizon), I find the following amount of differential extinction 
from the bottom of a frame to the top:

     angle from zenith             V-band               I-band
---------------------------------------------------------------------
      10 degrees                    0.004 mag            0.001 mag
      20                            0.008                0.003
      30                            0.014                0.006
      40                            0.023                0.009
      50                            0.039                0.016
      60                            0.073                0.029

  In other words, if one observes a field 50 degrees above the horizon,
and examines two stars which are intrinsically identical, then the 
star near the top of the frame (farthest from the horizon) will appear
to be brighter by 0.023 mag in V, or 0.009 mag in I.  For some
purposes, this must be corrected.

  We now return you to your regularly scheduled discussion of 
raw star list formats ....

                                         Michael Richmond