[Aavso-photometry] Time series help needed

[Michael Newberry]

Arne and Gary have said most everything that needs to be said on this topic, 
but I thought I would add my 2 cents worth just to refine one of Arne's 
points.

There is nothing special about the check star except that it is another 
star, hopefully of similar magnitude to the variable, that you believe is 
non-variable on the timescale involved in the time series of the images. The 
check star can be another standard that you do *not* use to compute the 
photometric zero point for the images, or it can be a "field star" that you 
know to be (or believe to be) non-variable.

Michael Nwberry

----- Original Message ----- 
From: "arne" <arne at aavso.org>
To: "Yenal Ogmen" <yenalogmen at yahoo.com>
Cc: <Aavso-photometry at mira.aavso.org>
Sent: Monday, December 03, 2007 9:10 AM
Subject: Re: [Aavso-photometry] Time series help needed


> 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
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