I have been desperately trying to get enough signal to minimise noise in the smooth regions of nearby galaxies. Maybe that was a futile exercise, since it appears that, once there is enough signal to get rid of read and shot noise, what is left
really should still look noisy due to the star structure. Even though the stars are not resolved, they introduce a level of statistical roughness sometimes referred to as "incipient resolution", that looks like random noise, but does not smooth out with more data. This surface brightness structure has been studied in some detail and measured as a way of probing distant stellar populations and as a secondary measure of distance.
explanation in
https://ned.ipac.caltech.edu/level5/...Jacoby9_1.html
I assume that the same process applies to the underlying background galaxies in the night sky, so maybe the background sky should never actually look smooth either - perhaps smooth galaxies and smooth sky is not realistic and I have been chasing an unattainable goal
thanks for looking. regards Ray.
edit: by the way, the rest of that NED publication is an interesting read