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thanks Sil. agree with your comments on noise reduction.
However, I have been trying to get enough signal so that I do not use any noise reduction at all - with a lot of signal, the shot noise is inherently low re the signal and noise reduction should not be needed. However, for nearby galaxies, I have been finding that the galaxy itself does not ever get to look smooth, even when there is buckets of signal and a high signal to noise ratio. The linked article points out that this is exactly how it should look - there is a lumpiness in the signal from nearby galaxies that makes them look noisy, even when there is no shot or read noise to speak of. The distribution of the stars within pixels results in signal roughness, even when the stars are not resolved.
this does not apply to far off galaxies or to true smooth extended objects (eg nebulae) - it only applies to nearby galaxies where there is some variation in the number of included bright galactic stars from pixel to pixel - and I suspect that it probably applies to the background sky when going deep and where the numbers of unresolved deep galaxies and dim milky way stars in the pixels will vary.
from the linked article: "An image of an elliptical galaxy with milli-arcsec resolution would look like an enormous globular cluster, but even when the resolution is a thousand times worse, the discreteness of the stars causes measurable bumpiness in its surface brightness. This phenomenon, dubbed ``surface brightness fluctuations'', has been recognized for many years and is sometimes referred to as ``incipient resolution''.
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