[Aavso-photometry] Negative CCD observations

[Michael Koppelman]

Alternatively you could do photometry on all objects with a signal >  
3 sigma of the sky and plot the uncertainty vs. the magnitude. (You  
could use the magnitudes of a few stars to set the zeropoint.) When  
the magnitude equals the sky's magnitude or when the uncertainties  
become ridiculous, you have your limiting magnitude.

Just a thought.

Michael

On Jan 25, 2006, at 5:30 AM, Robert J. Modic wrote:

> I was wondering what the best method is for making negative
> CCD observations.  The eyeball method would be the easiest,
> but somewhat subjective.  I'm looking for a quantitative
> method to find the limiting magnitude for a particular frame.
> I read that the rule-of-thumb is that a SNR of 3 is roughly
> the threshold brightness for a star.  Can I just measure a
> comp star with a SNR of ~20 to 50, divide the comp star SNR
> into 3, convert this result into a differential magnitude
> and then add it to the comp star magnitude?
>
> Something like this:
>
> STD+(-2.5*Log(SNR3/SNR20))
>
> I could then average the results for a few comp stars to get
> a better result.
>
> Does this make sense or is there a better way to do this?
>
> Bob Modic
>
>
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