A premium obstructed 8" aperture with a 0.2D obstruction and 0.98 Strehl packs 90% of the light in the central disc of about 0.68" arc. That's about 76% of the 84% total light possible. Sometimes I think we are rightfully awed by the high Strehl and fine wavefront error for premium optics and forget they are affected by diffraction, too. Only 76% of the light? Yuk!
In comparison, a nearly true "diffraction limited" 10" aperture with a 0.2D obstruction and 0.8 Strehl packs 73% of the light in its smaller central disc of about 0.55" arc. That's about 61% of the 84% light possible in the Airy Disc, about 15% less than the premium. The decrease in encircled energy is significant, especially for a same aperture comparison. But with a larger aperture, it's also delivered to a smaller angular area including the offending first diffraction ring. Especially in average seeing.
Consider most of the light on the aberrant and obstructed 10" is displaced into the first ring at 184/Dmm out to about 0.72" arc. That's pretty close to the same as the Airy Disc of the 8" premium both with a bunch of light packed into about the roughly the same 0.7" angular diameter. In seeing, I imagine this difference is negligible given the aperture difference.
The observable spurious discs of both are somewhat smaller than the Airy disc, with the still often visible 10" spurious disc being a little smaller. Consider average seeing is about Pickering 5/10 (R0 ~ 100mm), where the central spurious disc is blurred slightly, and both are just below the diffraction limit, in this case, due to seeing induced FWHM. The smaller 10" spurious disc could still prevail in terms of short exposure "snapshot" seeing.
I think, as always, the premium scope will enjoy slightly better contrast out to a couple arc seconds with less light in the rings and this is what skilled observers notice especially on nights with better seeing. But, I bet it would be hard to tell the difference in average seeing as the increased aperture makes up some of the ground lost to the deservedly premium optic. And assuming the Chinese cheapo 10" produces a relatively clean PSF without any other major faults.
I like gwlee's comment above. It makes sense. As Joe says, it does not get any better than a premium optic. That's absolutely true. As Seabee says, he has piece of mind. I've had some nights with good seeing that blew my socks off, too. No doubt a premium optic shines in this case and substantiates the anecdotal evidence in support of premium optics. As Barbie says, some Chinese producers are doing alright. I have one. And I observe in normally better than average seeing. Gimme that, anyday.
A scope with a nice Strehl of 0.95 and an obstruction near 30% can pack 80% if the light in the central disc meeting the intent of the Rayleigh criterion of (presumably smooth) 1/4 pvw LSA and a Strehl of 0.8 or near the Marechal criterion of 1/14 wave RMS. That's about 67% of the 84% possible (Double yuks! ) diffracted and aberrated light in the central disc on the focal plane. If you can get there, though, you're fine IMO and IME. Anything better is gravy and probably worth paying for, if you're so inclined. Why? Because while a good scope is good, it does not get any better than premium.
Edited by Asbytec, 14 April 2018 - 12:58 AM.