[Aavso-photometry] [AAVSO-DIS] Differential photometry again

[arne]

Dr. Tiziano Colombo wrote:
> Dear All,
> 
> I read in a previous e-mail of the group the willing of calculating color 
> coefficients once a time and apply them successively.
> 
> I think you could obtain a correction for your V-filter measurement 
> comparing R-filter for the set of comparison stars in the chart of the 
> measured var star: the known V-R values. Making a plot and a regression of 
> V-R versus the v-r measured values, you get the V-R value for the variable 
> star at the instant of measurement, considering the fact that V-R could 
> change during variability time. Then, making another plot and regression of 
> V-v versus v-r values you can get the corrected V-filter magnitude with 
> respect to set of comparison stars used.
> 
> For instance, AG Peg on 13th November 2006: I got V=8.9281 with the square 
> of residual quadratic sum with a value 0.0226 only with the V-filter 
> measurements of variable and all comparisons. Having used R-filter also and 
> applying the previous relations I obtained a value of V=8.728 with a square 
> mean error 0.0158. We can see therefore that though a little error of 2/100 
> of magnitude in the mean error with a V-filter, we have an error of still 
> 2/10 of magnitude if you don't take into account the color of the comparison 
> stars.
> 
> Would be correct the procedure?
> 
I'm cross-posting this to aavso-photometry, as that is where
questions like this should be posted.

I think you are suggesting that you can use the comparison stars in
the field of a variable to determine the transformation coefficients.
I recommend against this for several reasons:
(1) you need to use true Standards for determining transformation
coefficients.  Right now,the best UBVRcIc standards are from Landolt
(1987, 1992).  This is the only way to avoid systematic errors.
(2) if you use local comparison stars, they are *rarely* calibrated
at the level of a Landolt standard.  There are fewer measures; they
are at least one step removed from Landolt, so systematic errors can
occur; they will usually have a limited color range.  An example of
a systematic error is a flatfielding problem, so that stars further
from the field center are fainter (or brighter).
(3) Transformation coefficients remain constant for long periods of
time, so you can certainly calculate your coefficients on a different
night (or nights) and then apply them to the variable star field.  However,
I have used local comparison stars on rare occasion to calculate
coefficients - sometimes you obtain images from someone else and
need to determine their coefficients; sometimes you are on an
observing run at a different observatory and cannot get sufficient
observing time or good weather to image Landolt fields, etc.  I only
use such coefficients for differential work, and add extra error
to my results.
Arne