[Aavso-photometry] Negative CCD observations

[Tomas L. Gomez]

To measure the limiting magnitude I do the following:

First I measure the sky noise, for the aperture size I am using.
I multiply it by 3, and then convert it to magnitude, using
a comparison star.

Note that the noise depends on the aperture size, and,
therefore, for a given image, the limiting magnitude
depends on the chosen size of the aperture!.
Of course, reducing the aperture size to get a better limiting
magnitude is "cheating" and will give unrealistic results,
because to do good photometry one has to choose an
aperture size of 2-3 times FWHM.

To measure the sky noise, I first calculate the standard deviation of the
pixels in the annulus of the variable star (actually, this is
done after performing a mild sigma-clipping on the pixel of the
annulus, to avoid contamination by other stars and cosmic
rays). And then I multiply this standard deviation by the square
root of the number of pixels in the aperture.

By the way, I find the postings in this mailing list extremely useful,
so I would really like to know if anyone has any comment about this
method. Let me know if this sounds good or if I am making some
mistake.

-Tomas Gomez (GOT)