[Shiraz (Ray)]

hi Greg.
In Australian conditions, the star shape and size for all but smaller refractors is determined almost entirely by the atmospheric seeing. With anything above about 6 inches aperture, you will never get anywhere near the optics diffraction pattern. ie the stars from my 250f4 and your CDK17 will be exactly the same shape and size as those from the AAT in typical conditions. It woud be nice to have seeing of 0.5 arcsec, but I tend to jump up and down if the seeing gets below 2 arcsec and I think that even the best Australian sites rarely get down to 1 arcsec.

I am not altogether sure what you mean by bloat, but if you mean that the stars look bigger, the two main reasons for this are: 1. that the pixels are small (eg the stars from your Trius will be 4x as big as those from your 16803) 2. you may be stretching the image to get to deep features (you see more of the star skirts as you go deeper - ie they look bigger).

If you are managing to saturate the stars excessively, then the simple answer is to expose for shorter subs - not just a bit shorter, but a lot. The optimum sub exposure varies almost with the square of the read noise - at the same aperture and pixel scale, you only need subs about 1/4 as long with your Trius as with your 16803. There is a tendency to overexpose with the new cameras, because that is how we had to do it with the older Kodak ones - those cameras have relatively high read noise, so you need long subs to overcome it - and then of course you need deep wells to handle the extra signal you get from a long sub. If appropriate subs are chosen, the results from both classes of camera should be identical.

Edit: as Slawomir said - I posted before reading his summary