[Aavso-photometry] xform question

[Radu Corlan]

On Sat, 11 Dec 2004, Arne Henden wrote:

> Michael, zeropoint and intercept are related.  If you look at
> a typical transformation equation,
>    Vstd-Vins = zeropoint + extinction*airmass + coeff*(B-V)

Arne,

in the above equation, are the B and V in coeff*(B-V) standard 
magnitudes? If this is the case, then one needs to build a system of 
equations to determine B and V (this is the approach i took in gcx).

Or are they instrumental? if so, you need to fit extinction before doing
the transformation (and assume a photomteric night). Or am i missing
something?

Radu


> 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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> > Aavso-photometry at mira.aavso.org
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> > 
> > 
> 
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Radu Corlan       Snail Mail: Bucuresti sect. 1, 
rcorlan at pcnet.ro  str. Argentina nr. 28, Romania

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