[vsnet-chat 1744] Re: BVRI Photometry

[Taichi Kato <tkato>]

Re: [vsnet-chat 1742] BVRI Photometry

> What I do for them is to use the fainter stars as an 'ensemble' to obtain
> a zero point for the field.  Even though an individual star might have
> Poisson errors of 5-10 percent, if you average enough of
> them, you beat the errors down by root(n).

> Unlike visual observing, CCDs are linear and you can use
> faint stars to calibrate bright ones.

   A few caveats:

   When there is a systematic error in determining the sky background,
this effect accumulates (or can't be properly compensated) when determining
an 'ensemble' zero point.  Some CCD's (and softwares) don't have very
linear responses; the effect of incomplete charge transfer maximizes to
the lower light level (I have seen this effect up to 10%. If the stellar
image looks streaking, one may consider this effect non-negligible).
The chance of contaminating variable stars among the ensemble becomes larger.
From these reasons, I'd recommende, esp. for novice observers, to use
a few brighter set of comparison stars rather than using a large ensemble
of faint stars even if the latter ensemble count exceeds the brighter star.
(However, there is indeed number of incidences when we have to use stars
several magnitudes fainter than the variable -- U Sco at maximum was 4
mag brighter than the comparison we used).

Regards,
Taichi Kato

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