FWIW, it looks like many of us have a similar view of what constitutes an acceptable amount of noise in a finished "pretty pic" type astro image. I just did a quick check on 5 good quality images from IIS and a couple of my own better ones - in all of them, the noise level in the darker regions was consistently around 300-500DN RMS in a 16 bit representation. Also interestingly, the same sort of noise level seems to be acceptable in brighter regions - the extra signal gives better SNR, but the acceptable noise still seems to around 400DN(16bit). Would be nice if that could be checked by others on their true 16 bit data, rather than me using JPEGs off the web.
Of course, the signal level in any part of an image can be set at any desired value for aesthetic purposes, so measuring SNR is only of value for comparative testing or for validating theoretical approaches to system design - however, since there is some evidence that "good quality" images all have similar noise levels, measuring just the final noise may well be a useful thing to do as one part of "pretty pic" image quality assessment. Good thinking Dunk
. cheers Ray