[Shiraz (Ray)]

Hi again Rolf.

Looks like I was wrong in an earlier post to suggest that optical quality was not such a major issue for DSO imaging. I did a quick and dirty analysis using Aberrator to generate PSFs for a variety of scopes and then convolved these with a 2 arc sec Gaussian (in IRIS) that represents good seeing averaged over a fairly long integration time. The top row of the diagram represents the resulting combined PSFs (star shapes) for scopes from 80 to 300 mm, with brightness normalised and assuming perfect optics and with identical angular scale. The chosen scopes are a couple of refractors, three Newts and an RC. For scale, the 300mm Newt produces pretty close to a 2 arc sec combined PSF (half power).
The two PSF images below the 250 f5 entry are a 250 f5 with a mirror that only meets lambda/4 and the lower one includes some pinching and tube currrents as well.

the whole pattern is repeated lower down, but with the synthetic stars driven into saturation by a factor of 2x.

So:
1. for 2 arc sec seeing, you get no resolution increase above about 250mm aperture, but even the 80mm refractor does a pretty good job.
2. moderate spherical aberration results in larger stars from the 250mm Newt - the half power area is ~20% larger at lambda/4 than with perfect optics. Looks like optics with low SA are definitely worth having in this application as well, if you are imaging in above average seeing conditions.
3. tube currents and improper mirror mounting can really ruin the image

These results are a bit unexpected and I am now going to re-read the relevant parts of Suiter's book to make sure I have it somewhere near right with Aberrator - maybe someone else can try something similar as a check.
Regards ray