[Aavso-photometry] Differential transformation

[Radu Corlan]

On Thu, 30 Dec 2004, Arne Henden wrote:

> 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.

After some thought, i realised where the confusion comes from. When you 
work all-sky, you take care of the color-dependent part of exctinction 
with the second order extinction coefficient - so your transformation 
coefficients are indeed constant. 

Now, when working differentially, the first-order exctinction is taken 
care of by the differtial process itself. But just applying the 
(constant) transformation coefficients would still leave the second-order 
exctinction effect unaccounted for. Algebraically, it's the same think as 
adding an airmass-dependent term to the transformation coefficient - which 
is what made me think of the transformation coefficients changing.

Thanks for your patience, and have a happy new year.

Radu


>    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
> 
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Radu Corlan       Snail Mail: Bucuresti sect. 1, 
rcorlan at pcnet.ro  str. Argentina nr. 28, Romania

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