[Aavso-photometry] Atmospheric extinction questions

[Jim Roe]

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.

Jim Roe [ROE]