[Aavso-photometry] Differential transformation

[Arne Henden]

Radu Corlan wrote:
>>   Transformation coefficients remain constant over long periods of
>>time; I am not quite sure what you mean about not using coefficients
>>obtained on different nights since this is a very common practice.
>>In fact, the best coefficient determination method is to use the
>>handful of really good, photometric nights for your coefficient
>>determination and then average these determinations to give you
>>mean coefficients.  These can then be applied to future nights for
>>long periods of time.
> 
> 
> I always wondered how much would the extinction coefficients change with 
> airmass, or between a good photometric night and a run-of-the-mill average 
> one. There must be a small variation, but i have no feeling for the order 
> of magnitude. 
> 
I consider constant items like transformation coefficients separate
from nightly changing ones like extinction and zeropoint.  These
latter you always have to determine for all-sky work; the former
stay fixed for months.
   Look at papers by Lockwood from Lowell in the 1980's and 1990's
regarding extinction changes.  There are seasonal variations as
well as isolated events like volcanic ash.  My new book has some
nice figures from Flagstaff.
Arne