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Hi,I'm writing my PhD and I'm looking for a [b:d95d1a586b]full description of the errors[/b:d95d1a586b] IRAF attachs to the final 'mag' values of the CCD photometry process. I followed Massey's '[b:d95d1a586b][i:d95d1a586b]A User's Guide to CCD Reductions with IRAF[/i:d95d1a586b][/b:d95d1a586b]' + Massey & Davis '[b:d95d1a586b][i:d95d1a586b]A User's Guide to Stellar CCD Photometry with IRAF[/i:d95d1a586b][/b:d95d1a586b]'.Is there a manual/guide/text-of-any-kind where I can find the trail of the errors from the first task used in the reduction process to the last one used in the photometry process?I've read a couple of things here and there but none seem to be thorough in that aspect. Since my PhD is in physics I need to be very specific with errors and where they come from.Thanks a lot!Regards, Gabriel

There is no automatic or any type of error propagation in IRAF for a sequence of reduction steps. You can carry along an error array using standard error propagation but you would need to do things with basic image arithmetic (e.g. imexpr). In astronomy the starting point for CCDs is the gain/read noise model for the raw pixels.In a task like the photometry that can produce errors the input form for the pixel uncertainties is similar, a constant sigma and a gain term that scales with the square root of the value (Poisson noise). So if you can define such a simple noise model for your input data the output errors will be based on the combination of the pixel uncertainties as described in the help page (hopefully).Frank Valdes