[vsnet-chat 1122] Re: Photometric errors

[Taichi Kato <tkato>]

Re: [vsnet-chat 1113] Photometric errors

> Consider this example. I'm following a variable star, using two
> reference stars. The magnitude difference between these stars is not too
> large, over all the observed period.
> 
> I draw the light curves V-C1 and C1-C2 (differential photometry). I find
> that the C1-C2 curve shows value between -0.05 and -0.13. 
> 
> Can I assume, as a rough estimate, that the error is (0.13-0.05)/2=0.04
> mags or I have absolutely to go deeper, considering the details of noise
> in the measures?

   If the magnitude of C2 is roughly equal to the variable, your guess may
be an acceptable approximation.  More exactly, you may use the standard
deviation of C1-C2 measurements.  In practical photometry, a relatively
bright star is selected as C2, in order to check the constancy of C1 (and
C2), and select another star, C3, whose magnitude is close to the target
variable.  The inclusion of a set of faint check stars will significantly
facilitate the (experimental) error-estimating procedure.

Regards,
Taichi Kato

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