[Aavso-photometry] Transformation equaions for only B, V and I filters.

[arne]

Lionel Catalan wrote:
> Transformation equations are usually written as 4 equations involving the
> terms V, B-V, V-R, and R-I. Hence, they assume that one uses the set of
> B,V,R, and I filters. However, when I use only 3 filters (B, V and I), it
> would seem logical to use only 3 equations giving V, B-V and V-I to
> calculate the transformation coefficients. Is this permissible? If so, is
> there more uncertainty on the transformed I magnitudes than if the R filter
> was also used? 
>  
V, (B-V), (V-Ic) are fine for transformation coefficients; you don't need
Rc.  In most cases, you will get superior results for Ic if you solve
directly for Ic using your instrumental magnitude and (V-Ic) than by
solving for V, (V-Rc), (Rc-Ic) and obtaining Ic from
   Ic = V + (V-Rc) + (Rc-Ic)

Arne