[Shiraz (Ray)]

FWIW, I think that you can probably get a bit of a handle on the significance of guiding etc. using a simple approximation that assumes that the FWHM terms add in quadrature.

FWHM = SQRT(seeingFWHM^2 + scopeFWHM^2 + (2.35*guidingRMS)^2)

there are some complications : the scope and camera FWHM is not exactly the same as the scope resolution, since the camera will introduce more spreading via sampling point variations and crosstalk. In addition, the guiding RMS is definitely not independent of the seeing (as assumed in the above equation) and it will also vary with guide update rate and Dec drift rate (polar alignment accuracy). However, the above still gives some idea.

In general, all of our imaging will be dominated by seeing, often to the extent that guiding/scope FWHM contributions may not be all that significant, so there may not be too much point in aiming for perfection in mount and scope performance. In particular, on a night of good seeing, the mount guiding will be better than on a poor night, so the mount/guiding error tends to automatically keep itself within the seeing FWHM. Although of course, systematic errors (mainly backlash and residual PE) that result in non-round stars are generally not acceptable and will be more noticeable in good seeing. With appropriate sampling, a big scope will not really resolve much better in Australian seeing - compare your images with those from the 3.9m AAT, there will not be much in it. The main advantage of a big astrograph is in how quickly it can get to a desired SNR.