[Aavso-photometry] xform question

[Arne Henden]

ok, I think I understand you now.  On those nights on which you
determine your transformation coefficients, the intercept is the
same as the zeropoint for those nights.
   For all other nights, you use your predetermined transformation
coefficents, but re-derive the zeropoints(intercepts) for those
new nights.  Only the transformation coefficients from the earlier
nights are reused.
   Radu's question:
 > 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.

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)
> 
> 
> 
>