Greg if it was easy it would not be worthwhile mastering or at least attempting to master it.
Yes Mike the lens is much better at f/3.5. The Canon 300mm F2.8L has a clear aperture of 104mm and I assume at f/2.8 the center of the sensor 'sees' the lot. I was never happy with the diffraction spikes the iris of the lens gave so I never stopped it down for astro.
The earlier manual super telephoto lenses had far better iris's with far more leaves so the hole was very close to a perfect circle. The 300mm F2.8 lens Scott owns has this very high quality iris. I can only assume that a manual lens can afford to do this as a tiny motor has to set the aperture almost instantly for taking the image from wide open so the autofocus works. By the way this lens autofocuses almost instantly and silently when used for terrestial photography even in very low light. It will autofocus even on bright stars! It is worth every cent of the seemingly exhorbitant price.
The diameter of the aperture in front of the lens is 83.5mm to give a calculated f ratio of f/3.5. The lens actually has a measured focal length of 291mm.
Making mosaics will be much easier as all the stars are essentially the same at the corners as at the center. The vignetting is also far less severe than at f/2.8. Gradients will also be far less of a problem than at f/2.8.
If only those clouds would go away for a bit I could find out.
