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I am not sure if this is a factor but it could also be down to the airy disc.
The airy disc has outer rings and these are not visible in short exposures but they would start to become visible in longer exposures. When you say FWHM is nearly independent of the brightness of the stars I have not found that to be true at all. Usually the lowest FWHM is always a dim not very bright star
and the larger really bright stars have the worst FWHM using CCDstack to measure them. So much so I usually measure a centre star that is small and a bit on the dim side.
This could also relate to the above idea about the outer rings of the airy disc becoming brighter in bright stars in longer exposures. I see a similar phenomenon with small well cameras versus deep well cameras. In short well cameras you have to be alert to overwhelming the wells.
I have noticed the KAF8300 blooms a little bit in 2x2 subs at 10 minutes on a bright star on my Honders at 305mm F3.8. So 5 minutes 2x2 seems to be the go there.
Your finding adds even more value to using larger aperture and faster F ratio optics. They are more demanding in terms of alignments but the gains are there in several ways.
There is a good graphic on CCDware site about autoguding that shows how a star gets pushed around over the course of an exposure and this obviously fattens it. It can fatten in a round manner if the PE is in all directions. So that's why you see longer exposures tending to have larger sizes.
But there are times you want a longer exposure anyway like narrowband or to get the faintest detail and you have to rise about the read noise.
So we are back to the low read noise topic of the ASI1600 again and the appeal of short exposure high QE reasonably widefied astrophotography.
Greg.
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