[Aavso-photometry] Aavso-photometry Digest, Vol 49, Issue 3

[Brad Walter]

1. I don't understand why the STDEV of K-C is added in quadrature to the
Poisson noise of the target star. When you measure the target star in
differential photometry you are actually measuring K-T. When you measure K-C
you are including all of the stochastic error elements that are in the K-T
measurements if C and T are the same magnitude. 
2. How do you correct the error estimate for differences in magnitudes
between C and T? It seems to me you have to do a calculation something like
the following:
STDEV(k-t) = SQRT((STDEV(k-c)^2 - SE(k)^2)*SE(t)^2/SE(c)^2 + SE(k)^2)

Where SE(x) = -2.5*Log(1+1/SNR(x))

Usually you work with three stars for time series: the target (V),
the comparison star (C), and the check star (K).  Usually you
obtain measures for all three stars on every image.  If you
form the difference between the check and the comparison star (K-C),
this represents the major part of the error in estimating the
target, since typically these two stars are similar in magnitude
to the target and their difference roughly matches the error of
a constant star with the target's brightness.

So the usual way of handling a time series is to calculate the
mean and standard deviation of (K-C), and report the standard
deviation of (K-C), added in quadrature to the Poisson noise
of the target, as the error for the target star.

If your sky conditions are varying, you may want to break up the
calculation of the standard deviation of (K-C) into smaller time
intervals, and modify the reported error for each of those intervals.
Arne


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End of Aavso-photometry Digest, Vol 49, Issue 3
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