[Aavso-photometry] Identifying a bad (varying) comparison star whendoing ensemble photometry

[Wolfgang Renz]

Hi Lionel

If you measure and plot the differential mag between pairs of stars
(comp-check1), (comp-check2), (check2-check1), ...., the differences
of pairs that include a variable should vary, while the differences of
pairs of constant stars should stay pretty constant. So you need at
least 3 stars to compare against each other. But there is also a
chance that you might have picked more than one varying star. Then
you need of course more stars to compare against each other.

If you have measured the differential mags just against a single star
(comp-check1), (comp-check2), (comp-check3), ... and all plots of
the differences show the same variation, its pretty probable that the
comp star is varying.

With ensemble photometry using several standards it gets more
complicated as the ensemble algorithm should minimize the differ-
ences between the standard stars and therefore will adapt the zero
point. If one of the standards of the ensemble varies, the residuals
of the varying one should be off in one direction while the residuals
of the constant ones should be slightly off in the opposite direction
when comparing it with the usual/mean residuals.

If you want to be 100% sure, measure and plot it as described in
the first paragraph and try to find at least 3 constant stars.

Clear skies
 Wolfgang

-- 
Wolfgang Renz, Karlsruhe, Germany
Rz.BAV = WRe.vsnet = RWG.AAVSO



----- Original Message ----- 
From: "Lionel Catalan" <lcatalan at lakeheadu.ca>
To: <aavso-photometry at mira.aavso.org>
Sent: Friday, June 13, 2008 6:07 PM
Subject: [Aavso-photometry] Identifying a bad (varying) comparison star whendoing ensemble photometry


> I have collected photometry data on LPH058 (a high mass X-ray binary in
> Monoceros) using ensemble photometry with 5 comparison stars and a check
> star. For several months, the check star was constant (+ or - 0.015 mag).
> However, over a period of a few days, the check star suddenly dropped in V
> magnitude by 0.04 and in I magnitude by about 0.1. I suspect that the
> problem is not with the check star itself, because the calculated magnitudes
> of the comparison stars (using Maxim DL photometry tool) also varied during
> these few days. So, I suspect that one or more of the comparison stars is
> actually varying. Can somebody suggest the most effective method of finding
> which one(s)? 
> 
> Lionel Catalan CTE