[Aavso-photometry] Red star transformations

[Michel Bonnardeau]

I wonder if it is worth transforming photometry measurements for red stars.
The problem comes from the large color difference with the comparison star
(which should not be red itself as most red stars are variable). Any
error/uncertainty on the transformation coefficients translates into a huge
error in the transformed magnitudes.

More precisely, the transformed I magnitude is computed from the measured
v,i magnitudes as:
I = i + TiTvi*[(v-i)-(Vc-Ic)]
where Vc and Ic are the magnitudes of the comparison star and TiTvi the
transformation coefficient.
The term [(v-i)-(Vc-Ic)] is very large for a red star, so any error on TiTvi
translates into a large error on I.

I have the problem with KU Her (Alert 351) for which I measure v-i=3.466 and
the comparison 119 has Vc-Ic=0.878. I have a standard deviation on TiTvi of
0.073. This gives a wooping error on I...

Any thoughts?

Michel
http://mbond.free.fr