[Aavso-photometry] xform question

[Arne Henden]

Michael, zeropoint and intercept are related.  If you look at
a typical transformation equation,
   Vstd-Vins = zeropoint + extinction*airmass + coeff*(B-V)
That zeropoint changes with time, and is a combination of
several effects: basic transparency of the sky (i.e., the constant
part of the extinction), throughput of your optical system, any
aging of components in your signal-chain electronics, etc.
A plot of zeropoint changes is in my older book, and Larry
Wassermann and crew from Lowell have a couple of papers of
how the "constant" part of extinction varies by season, volcanic
ash, etc.
   Be careful when using weighted fits for anything.  You need to
understand what the weights mean and what the proper size of those
weights should be.
   Hope that answers things a bit.
Arne

Michael Koppelman wrote:
> So when we do our transformation coefficients we end up with basically a 
> slope coefficient and an intercept coefficient for each transformation. 
> In theory the slope coefficients don't change much over time for a given 
> telescope/camera/filter combination. The zeropoint, though, which needs 
> to be determined each night, is basically the intercept coefficient, 
> right? The question I'm leading up to is: you don't really use the 
> intercept coefficient that you calculate when you are calculating your 
> slopes, right? When you actually transform unknown fields you just use 
> the slopes and the zeropoint, right?
> 
> I'm thinking a guy should also use weighted fits when determining these 
> coefficients as well?
> 
> For me my slope coefficients after 8 nights over 2 years are:
> 
> Vr      0.017  (V from V-R)
> Vb      0.007  (V from B-V)
> B-V     1.060
> V-R     1.097
> R-I     0.947
> V-I     1.016
> 
> Cheers,
> Michael Koppelman
> http://www.lolife.com/astronomy/
> 
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