For seated Dobsonian viewing, 6" f7 sounds better than f5, 6, or 8. But for a tripod, f5 is better but likely heavy regardless.
7" f6 would be nice. Or f6.3.
Faster mirrors (f4-4.5) with CC are nice for rich field views. Just costly and more skill to collimate.
Interesting observations about your Milky Way results. Hard for me to comment on those. I can say for sure that an F/5 120mm (Synta) and F/5 5” (Jaegers) achromats do better than a 4” F/7 ED scope for the Milky Way & Andromeda, at least to my eye.
But Stargazer 193867, my point about the beautiful aspect of the 6” F/6 dob is mostly focal length in absolute terms, F/whatever. I can squeeze 2.42° out of my widest field ep (the APM 30mm UFF), but if one goes much above that, there goes the wider TFOV. So a 130mm F/7 is fine (910mm, not much of a hit on TFOV), but a 6” F/7 will only have a tiny amount more field than my C102GT, which at 2-1/3° isn’t horrible, but under where I’d like to be. Of course, 2.5° and better is preferable (why I’d love a 130mm F/6), but staying within spitting distance of that is critical to me. The 6” F/6 certainly gets me there, but even at F/7, it’s too 4” F/10 refractor, if you see what I mean.
Look, I’ve been to dark sky sites with guys with 14” and larger dobs oogling over the TFOV my rather modest C102GT can yield, which can indeed seem considerable compared to those, but a 6” F/6 dob would have them oogling all the more, and certainly could be as portable with easier setup, even than my C102GT on the excellent GSO SkyView Deluxe alt-az mount (original AstroTech with genuine AstroTech Pier Extension) on Ho wooden legs, or about as easy as a good 4” F/10 refractor setup as it gets. That’s what I’m talking about.
I’d never argue about F/7 being somehow worse, optically than F/6. About that I’m with you from an optical perspective, but in terms of field of view and functionality, a 6” F/7 wouldn’t quite do it for me, taking me too far, no longer spitting distance, from 2.5° TFOV.
Edited by CollinofAlabama, 28 November 2020 - 05:13 PM.