[Aavso-photometry] xform question

[Radu Corlan]

>  > 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?
> 
> The normal all-sky photometric method, as described in my book as well
> as other texts, is to first solve for the standard (B-V) color.
> This only requires the instrumental (b-v) color.  Then you use the
> calculated standard (B-V) color in the (Vstd-Vins) equation.  If
> you have a priori knowledge of the (B-V) color, you can of course use
> that, especially on nights when you are observing in only one filter.

Thanks, that makes sense -- it's the same thing as solving the B and V 
system directly, only easier to apply (because solving for B-V first 
decouples the equations). 

I've been trying to get your book, but it seems that it's only available 
used right now, and Amazon won't ship used book over here ;-(. Hopefully 
we will have a new edition...

Radu

> 
> Arne
> 
> Michael Koppelman wrote:
> > If I call the slope term 'm' and the intercept term 'b' (and dropping 
> > airmass for now) you could also write this equation as:
> > 
> > Vstd-Vins = b + m*(B-V)
> > 
> > m and b are both things that I calculate when I calculate my 
> > transformation coefficients. Then I also calculate a zeropoint for the 
> > night (call this zp). The way I have been applying my transformation the 
> > equation looks like this:
> > 
> > Vstd-Vins = zp + m*(B-V)
> > 
> > My question being: the b that I calculate with my transforms is not 
> > really ever used, right? I always use the nightly zeropoint and never 
> > the averaged intercept coefficients.
> > 
> > Thanks!
> > Michael
> > 
> > On Dec 11, 2004, at 12:40 PM, 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)
> > 
> > 
> > 
> > 
> 
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Radu Corlan       Snail Mail: Bucuresti sect. 1, 
rcorlan at pcnet.ro  str. Argentina nr. 28, Romania

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