Simple datapars.sigma question
Gabriel wrote on Jun 01, 2009
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) / p
where: 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!
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) / p
where: 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!
Mike Fitzpatrick wrote on Jun 01, 2009
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
Cheers,
-Mike
Gabriel wrote on Jun 01, 2009
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
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
Mike Fitzpatrick wrote on Jun 01, 2009
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,
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
cl> imstat dev$pix[100:120,200:220] fields="mean,stddev"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
Last post on Jun 01, 2009