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IRAF’s reject parameter: crreject or avsigclip ?

From “A User’s Guide to CCD Reductions with IRAF” by Philip Massey (iraf’s photometry docs) we read at page 19 §3.8 that there are two useful options to set the reject parameter:

> reject parameter = crreject
when we know the gain and read noise of our camera

> reject parameter = avsigclip
where the “typical” signal is determined from the data, so no need (or lack of) gain and read noise