I just came across this old thread, and in going over it today, there's a few things that I can add to this that will help a lot, especially with resolution.
I too have seen the Cassini Division in a modest ED80. I too was very surprised considering its angular size.
In the last few years I've also been noticing a few other items that seemed to defy the "resolution" limits of various scopes I've used. At the same time, I've also failed to see the same features with other scopes despite the same aperture!
Seeing is one factor, but certainly NOT the only one. I'll leave eyesight out of this post.
First, optical quality. Not all scopes are created equal, even from the same factory and design. I've looked through many different 8" SCTs, new and old, and their optical quality varies TREMENDOUSLY! From superb (very few), to mediocre and unfortunately crap. Age has nothing to do with it as the worst I've seen was a brand new one!!! I will never buy a new or used SCT without star testing it first, and NO retailer will let that happen
But this doesn't explain why an 80mm refractor can show the Cassini Division when its angular size suggests it is too small for an 80mm scope.
The problem here has to do with the accepted notion of "resolution" for scopes, and we need to rethink this.
First we need to understand how we get the Dawe's limit and Rayleigh limit in relation to Airy Disks. The key is the Airy Disk itself. It is an actual disk, not a pinpoint of light. It has a dimension. And the Dawe's limit is the smallest separation that can be made out between two stars of equal magnitude. And this separation is not a complete separation with a black gap between the two, but one that resembles a dog biscuit, two distinct lobes, that the observer can say with confidence that they see these two lobes. Curiously enough, the smallest gap that can be see between two stars is still smaller than the Airy Disks themselves, yet no one seems to think about that!
And that is a clue that no one picks up on...
When looking at extended objects, such as the Moon & planets, things are VERY DIFFERENT. Here there is no Airy Disks and the diffraction pattern of a point source of light is totally disrupted. The result is the resolution limit of any scope is actually much smaller. More than 1/10th smaller than the Dawe's limit for a given scope. This goes for both visual and photo.
This is why a 7" scope can resolve the Encke Division in Saturn's rings, a division that has an angular size of 0.05 arcsec, more than 1/10th smaller than the Dawe's limit for a 7" scope.
Remember, this assumes very good optical quality as some scopes are not even able to resolve to the resolving power of their aperture.
I've seen the Encke Division in a 7" Skywatcher Mak. And this explains my seeing the Cassini Division in an 80mm refractor & why the Cassini Division was discovered by a 2.5" scope. Also a major testament to the superb quality of that 2.5" refractor considering when it was made and the quality of eyepieces available then too!
I've also seen 8" SCTs struggle to show the Cassini Division. And of course the Encke Division was out of the question...
Alex.