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Hi,I'm following the Massey-Davis guide to perform CCD photometry. In this guide there's a 'sigma' value mentioned (actually the greek letter for sigma is assigned to this value, but I don't know how to insert greek letters), which is calculated like so:sigma = sqrt(s * p + r^2) / pwhere: s : sky value in ADU; p: number of photons per ADU and r: read-noise in units of electrons (I obtain the 'sky value' doing imexa + 'm' key over several sky patches of a single frame and then averaging the 'MEAN' values shown on screen).Now: the package 'daophot' has a task 'datapars' and inside this task there's a parameter called 'sigma' which, if I understood correctly, is the the amount by which the parameter 'threshold' in the task 'findpars' will be multipled by.
Usually the value I input in daophot's 'datapars.sigma' is obtained doing imexa + 'm' key over several sky patches of a single frame, and then averaging the 'SSTDEV' values shown on the screen.My question is: is this last value (obtained through imexa + 'm' key and averaging the 'STTDEV' values shown) the same as the first 'sigma' value (calculated with 'sky value' + 'number of photons' + 'read-noise')?Thanks!

The STDEV value reported by the IMEXAM 'm' key is simply the standard deviation of the pixel values, it does not take into account read-noise and such so is not the same as the sigma in the Massey doc.Cheers,
-Mike

Thanks for the answer!Is there a task that can retrieve the MEAN and SSTDEV values, that can be obtained through IMEXAM + 'm' key, automatically?
It would really help me if there was such a task!Cheers,
Gabriel

The 'm' key simply does pixel statistics of a rectangular region, you can do this using IMSTAT and an image section just as easily. For example,[code:1:9acf6f01d7]
cl> imstat dev$pix[100:120,200:220] fields="mean,stddev"[/code:1:9acf6f01d7]would print only the MEAN and STDDEV field for that section. For multiple sections you could specify an @file containing the image sections. Depending on what you're doing, a small script task to automate all this may be more convenient. -Mike