Re: Optical Conditions

[Michael Richmond <richmond@stupendous.cis.rit.edu>]

> I have been looking at some strange results.  Threre seem to be conditions
> where the atmosphere is opaque to the V band and quite transparent to the
> I band.  This results in wildly varying detection ratios.  This seems to
> take place over a relatively short period of time, and is most aparent for
> the tom1 telescope which is looking through the most atmosphere.

  Well, the Earth's atmosphere _does_ block more the V-band
light from a star than the I-band light ... but the difference
of extinction is roughly

           V:  0.25 mag/airmass
           I:  0.08 mag/airmass
        -------------------------
       diff    0.17 mag/airmass

That means that _roughly_ 20 percent more of the V-band
light is blocked than the I-band light, at the zenith.
At airmass 2 (roughly 30 degrees above the horizon),
this factor would roughly double.

 That doesn't explain the very large differences in
the number of stars detected in the two bands which
Tom describes, though.  
 
  If that camera sometimes does detect roughly equal
numbers of stars in V and I, and other times detects
many fewer stars in V, I would suspect some sort
of condensation in the V-band optical system.

                             Michael Richmond