I wonder if under near perfect seeing conditions a good (not excellent) SCT will out perform an excellent, but substantially smaller aperture telescope of another design.
Absolutely. I have had the great pleasure of owning an Astro-Phycics 6" triplet and as one would expect, the optics were textbook. Perfect spherical aberration correction in green (tested with a narrow pass green filter and 33% obstruction) no zones, turned edge, and a figure so smooth that it has to be believed to seen how perfect a star test can look.
As perfect at it was, and as unobstructed as it was, my C14 easily showed a huge amount of detail that was out of the reach of the 6" Apo. In one of my posts, I described how I was able to actually resolve surface detail on Ganymede, and could easily see polar shading on Io using my C14. I was never able to duplicate these observations using the 6" Astro-Physics. In fact, the only refractor observation I have ever read where I felt the observer may have seen a similar amount of actual detail (not just albedo shading) was done with hugely expensive 8" Apo.
And considering that a C14 not only costs a tiny fraction of a high quality 8" apo (about $62,000 less for the C14), and is lighter (an 8" Apo weighs 61 lbs, vs 52 for a C14), more compact, easier to mount, and more comfortable to use, it makes one wonder why someone would buy an 8" Apo for planetary observing????
And heck, my mass market 12" dob does better on planets than the 6" Apo did. Sure, it takes a night of good seeing, but here is what I have to say about that. Even when seeing is perfect, you are not going to see the actual shape of Galileo Regio on Ganyemende using a perfect 6" apo. Never-ever did I achieve that kind of result with the AP. It does not have enough resolution and contrast transfer to do this. So, the choice is to have as good or even better view in a 6" Apo on nights of poor or bad seeing, where there is little point in even bothering to try to look at solar system objects, or have ocassions where you can see far more with the less expensive, easier to mount, and more comfortable to use large SCT?
The break-even on winning out against a top quality 6" Apo is the C11, but to do this, the C11 is going to have to be at the upper end of the quality range, and yes, the seeing will have to be very good, and if less than excellent, the observer will have to be very patient.
To answer your question though, it is not at all difficult for a larger, less perfect, obstructed aperture to beat out a meaningfully smaller perfect aperture. I have done it with a very wide variety of comparisons. Seeing is seeing, but patience will almost always give the significantly larger aperture the upper hand.
(Roland Christen once told me that if my main goal of being on the 155EDF wait list was to do planetary observing, he suggested that I drop off the list and get a high quality 10" reflector because it could do a better job. He very politely explained that where the big advantage of the 155ED would be realized was when one was imaging with large sensors. I took his word for it, and have since concluded that he was correct).
Edited by Eddgie, 23 June 2019 - 12:43 PM.