Wow , I'm overwhelmed by the time you all have given to this topic. The quest for a large FOV from a long Focal length ( Large diameter) scope is not at all an easy proposition when factoring limitations such as exit pupil, TFOV, large diameter eye piece/focuser combinations. As Alex has attested to, he loves his 8" with a 3° TFOV in a dark sky. (I'm with you Alex and thank you for your contribution....) and yes, a 25" F4 with a 55mm Plossl will see your retina only receive 1/4 of the primary's light but hey , I love looking around in there , trying to count numerous galaxies that an 8" would have no chance .
The 20" isn't the scope for sky scanning !With Big aperture comes fainter and brighter visual objects . Reducing the F ratio still evokes the tyranny of exit pupil. By the time we reach an age where a Large Dob is possible, if you are anything like me you have a shed filled with your previous favourite telescopes in descending aperture diameters. So for wide field I'm dusting off my 10" F5 ATM and doubling the FOV to 2°.
Observing through The Mt Wilson 100" Hooker Telescope was an interesting read which I thought would fuel Mega aperture fever. Well, it DIDN’T. The author referred to the poor seeing conditions described by Norm Vargas , when looking at Neptune as "We don't get excited unless the seeing is better than 1 arc second". Robert Naeye ( Author )put it this way." From a pure observing perspective, I've had better views of many of these objects through much smaller scopes operating under darker and steadier skies...".From personal experience when comparing views at this year’s QLD Astrofest between Zamar, my 20" F5 and Peter R's 30" F4.8 at similar magnifications I found the views through the 30" to be softer having been affected more by the seeing. (Larger may not be better except on rare occasions)Even the Mt Wilson 60" isn't the scope to view Galaxies!!
So rest assured that in Australia if you have a large Dob in a Dark Sky with sub arc seeing, your views may surpass that of the mighty Mt Wilson 100" Hooker telescope !! And forget about the quest for a Big Dob with an ultra-wide FOV It just ain't going to happen.
