You are asking the wrong question but I'll come back to that in a moment. Celestron makes telescopes that meet a diffraction limited "target." Diffraction limited means that the image quality is limited more by the properties of light than by optical aberration. That implies that the imaging performance of a diffraction limited system is very close to being indistinguishable from a theoretically perfect system. The problem with Celestron as a company is that they don't have a very good way to tell if any telescope meets any quantitative spec--much less to tell if any given scope is within the theoretical diffraction limit. As evidenced by the generally high optical quality that they ship, the performance targets are pretty good; but the final QC process is still mostly subjective and not very precise. The controls are generally good enough to ship scopes that are diffraction limited but that's the reason why Celestron (and most amateur) scopes vary a bit in optical quality.
The thing that most amateurs don't appreciate is that it's actually pretty hard to accurately measure wavefront errors--quantitatively. Simple tests like the star test or the knife edge test can provide some good information but neither is very quantitative and each can suffer from significant errors in reproducibility as well as repeatability. In my view any notion that it's possible to visually detect the difference between a system with a Strehl of 0.8 and a system with a Strehl of say, 0.9 is urban myth. With apologies to any radiologists out there, I suggest that you should be a bit suspicious of any test that requires an "expert" to interpret. Reproducibility demands that two independent operators produce the same results. There are so many subtle issues that can affect image quality such as micro-ripple, stray light, obscuration ratio, tube currents, and overall seeing that it's impossible to sort all that stuff out in the field. It takes a controlled measurement of each parameter in a lab to accurately measure the factors that affect image performance.
Getting back to your question, I suggest that if you want a good telescope, you should be asking, "How do I get the highest quality optical system possible? I'd answer that question by telling you to go buy something like a RCOS system. RC Optical System had two things going for them. First, they used ion-beam figuring to achieve very high surface accuracy and second, they used a 4D Technology Phasecam interferometer to measure both the components and the final system performance. RCOS also knew what they were doing in terms of mechanical design. These are superb systems that included an interferometric test report with each system (though from what I've seen, the report was not from the Phasecam.) There are other companies that also use good, quantitative test procedures. PlaneWave uses a double pass Shack-Hartman test that seems to work well. The folks at AG Optical have an interferometer (though I don't know much about their process.) Any of those scopes have well documented optical performance. (Most of them also have fairly large obscuration ratios so they may not be very well suited to planetary imaging.) You'll pay more, but you'll also have a better idea of what you are getting.
Instead, I suggest that you try to find just a "good" Celestron--they are everywhere. I've measured a couple of C11s and a C8 Edge systems on a 4D FizCam and all three showed single pass wavefront performance with a Strehl>0.8. But, I agree with others that if you want the single best scope out of 10,000, your odds are somewhere north of 1:10,000 of finding that one. Of course, you'll need to understand how to test it. It might take a lifetime of looking and maybe a few million dollars to find that one. I also suspect that when you find that one, you'll find that it might only be different from the other 5,000 at the top of the heap by a milli-wave or two. On the other hand, if you want an inexpensive system and you don't want to search forever, buy a Celestron, try it and if you don't like it sell it and try again. I suspect that Damian Peach and Christopher Go probably just ordered a system and then went about learning out how to get the best imaging performance out of it. Otherwise, pony up and pay for a "high-quality" system that's been manufactured and measured to real specs.
John
Edited by jhayes_tucson, 01 September 2016 - 11:40 AM.
Finding one that is defined as "very good" at higher strehls (1/6 to 1/8 wave, better than diffraction limited)...not so good! 
