[Aavso-photometry] Time series help needed

[arne]

Yenal Ogmen wrote:
> Hi all,
> 
> I see that sometimes "time-series" observations are
> needed. I would like to do them therefore I need to
> know essential things about these observations.
> Especially about statistical error calculation. Now I
> take 5 composite images and calculate standard
> deviation of them to submit as error. But how this can
> be done for time-series?
> 
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