[Aavso-photometry] Differential transformation

[Geir Klingenberg]

Hi

The methods I have seen for estimating the transformation coefficients
involves using instrumental magnitudes in a LS solution. But - if one
is only doing differential photometry and don't need to be concerned
with extinction - is there anything wrong in using a differential
approach? More specifically, if we ignoring extinction the relation
between the stars magnitude, instrumental magnitude and color can be
written in the usual form as

V = v + a*(v - r) + b
V - R = c*(v - r) + d

This is for the V and R band - capital letters are magnitudes and
small letters are instrumental magnitudes. So, subtracting magnitudes
of star 1 and 2 and rearranging we get

(V1 - V2) - (v1 - v2) =  a*( (V1 - R1) - (V2 - R2) )          
(V1 - R1) - (V2 - R2) = c*( (v1 - v2) - (r1 - r2) )             

which can be used to estimate a and c. 

Since all instrumental magnitudes in these equations are on
differential form  between stars on the *same* image, won't the
solution be more robust against sneaking cirrus clouds and changing
atmospheric conditions? And can measurements from different nights be
used to improve the coefficient estimates?

Geir Klingenberg