I had star tested my new dob when I go it, and SA was actually really nicely corrected. This is not a premium quality mirror, but way better than I had prepared for.
My 10" f/4.8 Dob is a Celestron C10NGT OTA ported to a Dob mount. (I can't stand any Newt OTA larger than a 6" on a GEM.) The optics are very nice. I see no difference in the diffraction pattern in-focus and out-focus.
I am going to qualify this statement before I even make it. I was not using a 33% obstruction, so the test may not be fully accurate.. Anyway, I decided to rack though focus and to my surprise, the secondary shadow ratio was a lot different on the other side of focus!
This is the classic sign of spherical aberration.
Again, it is important to mention that I did not use a 33% obstruction. The problem is perhaps not really serious and maybe is some higher order spherical aberration (and that is why you use a 33% obstruction rather than the native 23%.. It greatly reduces any influence of HSA).
A larger CO reduces HSA? I had not heard of this. Maybe I should go back and reread my Suiter's?
But I would think that any advantage to a larger CO - and I'm really skeptical that there is any practical advantage, except maybe for double stars - would be offset by the diminished contrast for planet/lunar. Did you raise the primary in an attempt to binoview without an OCS, and so had to change to a larger secondary? Otherwise, I can't imagine willingly changing from a 23% CO to a 33%.
By the way, my 10" Dob also has a native 23% CO. I like it that way, because IME it places the scope in a Goldilocks Zone making it good for planet/lunar and deep sky.
Will try to remember to follow up.
Please do, even if you have to start another thread to show your binoviewer experiences.
Scope seems to perform well on the moon though. I would say at least ad good as my 6" APO and I think perhaps a bit better, but I really haven't done a side by side, so just working from memory of detail around the Triesnecker Rille. I know this area well from the C14 and the 6" APO, and I felt that it was showing detail right between the C14 and the 6" APO.
Sounds very much like my 10" Dob. That's why I'm not in such a hurry to upgrade to a 14" Dob - which I do want to do eventually. I'll just have to continue to ignore the condescending remarks and glances from my fellow observers at the dark site. It seems like for deep sky you're not really a member of the club unless you have at least a 14" Dob.
I love the Mark V and coma corrector though.
But the coma corrector extends into the focuser tube 92mm. This is not much different than the OCS on the Binotron.
So, this may not work for you either..
Yep, this definitely needs more thought before I pull the trigger. I might even wait on a binoviewer upgrade until I get that 14" Dob. Looks like I'll have to design the Dob around the binoviewer so everything will work optimally.
I'm also concerned that the Mark V coma correction might not be as good as a TV Paracorr. After all, the Paracorr has built-in tuning to optimise coma correction. AFAIK, there is no accommodation like that for the Mark V corrector OCS. Unless the coma correction is just right, there is a definite possibility of making the image worse rather than better. That may be what you are finding out with your Mark V and Dob.