[Aavso-photometry] Atmospheric extinction questions

[arne]

Jim Roe wrote:
> I'm doing differential photometry.  Ie, my comp stars are on the same 
> image as my target and, usually only a few arc minutes away.  Ergo, I 
> expect delta X to be zero.  According to Hall & Genet, the second order 
> extinction coefficient in V band is essentially zero.  So, should I see 
> a brightening of my target as it rises from a low elevation (High air 
> mass) to a high elevation (low air mass) (in V)?  The B-V values of the 
> target and comp star are very close, too.
> 
This problem is not stated concisely enough to have a good answer.
In general, stars will get brighter as they go from high airmass to
low airmass.  In general, blue stars will get brighter quicker than
red stars, in the same filter.  Beyond that, the amount of change
depends entirely on your circumstances.  Which star is redder than
the other.  Which star is at higher altitude in your frame.  Whether
seeing is important for the problem.  And remember, you are phrasing
the problem in terms of differential photometry, not raw instrumental
values.

The easiest solution is to use two comparison stars of similar color
to the variable, and see if that (K-C) difference changes systematically
the way the (V-C) difference does.  In all photometric projects, you
look for systematics - they guide you as to whether a given feature
is possibly real or an artifact.
Arne