Ed, I offer my heartfelt gratitude for building the DNR and the fascinating discussion that spawned from it.
In your article, you wrote one line that I think tells it all,
"In my discussion, I argue that only one additional element, related to aperture determines optical performance – the quality of a stellar diffraction image."
And somewhere above, you said,
"all else follows."
That would include light scatter and such.
I have often argued, the diffraction limit is nothing to sneeze at. By that, I guess we mean something better than the equivalent of 1/4 P-V primary spherical aberration giving an operational peak intensity of a star image of about 0.8 or better, or about 80% of the 84% possible light in the central disc and the resulting intensity distribution.
The true performance of a refractor, outside the appeal of the FOV and the reason for "tight" star images, is they tend to operate at their Strehl ratio, be it polychrmoatic or otherwise. It seems the true strength of optical performance of a Newtonian is it's closely parabolic primary. This is the perfect form for focusing light rays from optical infinity. The bane of the Newtonian is the obstruction, and of the APO refractor some residual higher order SA and a very slightly caustic focus.
Both designs, including your DNR, should have a very high Strehl ratio. Let's say they are equal, both at around 0.98. The 13.7% obstruction still imparts some additional diffraction and light into the rings thus reducing the operational peak intensity of the optical train. Without doing the math, the peak intensity is not going to be 0.98 as the APO is. Not anymore. It'll be something less, say around mid 0.90s thereabouts. What this boils down to is not so much the performance of the DNR being as good as a premium APO, but likely observationally undetectable in the field. In otherwords the difficulty discerning between 0.98 and mid 0.90 peak intensities.
This seems to be in line with the school of thought that an ~20% or less obstructed aperture is very hard to distinguish. That is, thanks to both your skill with pushing glass, scaling the obstruction (adding the Barlow was a neat idea), and tackling other gremlins, to my mind, you succeeded - with equal aperture where many of us cannot.
No doubt an operating peak intensity (distribution) of about 0.90, or better, is amazing, especially with a smooth and highly corrected primary mirror. Still, how do those images compare to ones that are at least diffraction limited at 0.80 peak intensity on the focal plane?
My own 6" scope is only about 1/6th P-V of primary spherical (pretty typical of advertised stats and consistent with the star test.) And I have seen some amazing things in mild tropical climates (ambient cooling) with laminar air mass (great seeing) and nearly perfectly collimated thanks to the excellent seeing. Albedo on Ganymede at 70x per inch, soft contrast detail on Jupiter and Mars (including detail in the NPC), and a tiny crater form on Plato's dark floor at an angular diameter near the Sparrow limit (~0.72" arc), or about the size of a middling magnitude spurious disc. (How in the heck is that possible?) The tightest star I've split is 7 Tau at about 0.75" arc, conservatively a tad less that Dawes, and have elongated stars below 0.5" arc. I have observed the 'apparent' elongation of Io (when near Europa for reference), and to date have bagged 7 craterlets on Plato's floor. Sirius B has been pretty difficult and mostly elusive, though. Still, got it once (along with about 10 diffraction rings around the primary LOL.) .
How much better can it get? Is a 6" APO gonna beat that or match it? (I mean, sure, no doubt small bright low contrast planetary detail is probably close to a 4" APO, by all accounts, but resolution is right up there with the big boys.) Point being, even in a modest fairly well corrected, smooth optic, the universe drops our jaw. So long as it is at least diffraction limited on the focal plane, I am guessing, in terms of seeing, thermal equilibrium, figure, and obstruction - you're not gonna be disappointed. I have not been.
"I have a 12" f/6.25 in my garage, just a plain vanilla Newt with no bells, whistles or otherwise."
So do most other folks. What I am driving at is, to my knowledge, there is only one DNR in existence. A few of us may, one day, make one. So, how can the rest of us schmucks (and I mean that with endearment, myself included
) optimize our Run-of-the-mill Newtonian Reflectors (RNRs) or current scope to obtain refractor like images short of grabbing an additional 2 inches of aperture? To me, since none of us has a DNR, this seems to be part and parcel to the debate above. How can we make our Dob deliver the goods, or do we even have to? I think we should, but it may not even have to be a premium optic.
It may just have to be good enough, say the diffraction limit or better - on the focal plane - which includes the obstruction and all other gremlins. Because, as you said above, it all boils down to the diffraction image. The diffraction image, on the focal plane, simply has to be good enough. Yea?
The answer seems to be, short of grinding an optical window, to do many of the things you've done to include flocking, extended light shielding, tackling thermal gremlins, get the best optic within reach, optimize the obstruction, collimate, and pray for an excellent night of superb seeing. Doing so stands the best chance of getting the diffraction image up to spec for some not so disappointing views.
My avatar is one of my better Jovian observations, colors a little more saturated that real world. That's 1/6th PV primary SA, excellent observing conditions, and patient and determined observation. And a narrow FOV, the last remaining gremlin.
Edited by Asbytec, 20 September 2015 - 11:11 PM.
Does anyone think I'm through? 
Don't count on it! Just wait until I'm 90!! 
Oh yes, my doc tells me I'm gonna die if I keep up smoking my pipe. 
If I stop smoking, I don't die, right?
