[Aavso-photometry] Re: Calculating mag errors for filter bands from color indices

[Arne Henden]

Wolfgang Renz wrote:
> 
> I know that the non-photometry astronomers prefere color indices
> to ease their work.
> Calculating a color index and a rms of a color index is a pretty simple
> task But keeping combined error estimates as small as possible is
> not.
> 
> Shouldn't the photometrists use the original values with the smaller
> error estimates instead of color indices with already combined error
> estimates ?
> 
If your goal is to report separate magnitudes for all filters
(B,V,Rc,Ic for example), then yes, calculating each magnitude
from its transformation will yield the lowest errors.  However,
in practice, you usually have to calculate all of the color
indices anyway since they are used in subsequent magnitude-transformation
equations, such as
      V-v = coef*(B-V) + zeropoint
which requires knowledge of (B-V) before you can get V.  Therefore,
the usual practice is to determine all of the possible color indices
along with one of the magnitudes, usually V.  When you have, say, 4 filters'
worth of information, there are many ways the data can be combined,
and people usually take the most common way.  There is nothing to
prevent you from calculating your values in an alternative way if
you feel your results will be improved.
Arne