MCT questions

Why does every review rate them so high? I saw on ITE's site they had sent a random one from Intes to an optical test facility and it came out as 1/6 wave but something about some micron specs or something being 14 microns (close to apo at 10) versus an 8" sct which was 30 microns, but I have no idea what they were talking about.

The gist was that a 6" MCT was markedly better than an 8" SCT. apples to apples, is that about right? either for scientific reasons or I'd settle just for your opinion...so I'll ask again:

In your opinion is a 6" (let's say Intes MK 67) MCT the equal or better than a Celestron (let's say average NexStar 8i type) 8" SCT?

The "micron story" should be related to spherochromatism.
I don't think they talk about spot size of actual instruments, because level of correction/optical quality varies. So these spot sizes are design-related. A near-perfect MCT has very low spherochromatism, so that its spot size in all colors is within the Airy disc diameter (theoretical diameter 13.4 microns for green/yellow light at f/10). An SCT has, due to higher spherochromatism, nearly twice larger spot in red light, and somewhat larger than that in violet. This is still at a level of an apo, so that real consequences in regard to image quality are much less than what the bare numbers imply.

Also, claim of 13 or so microns spot doesn't agree with the usual Strehl # that comes with MCTs. A 95%-96% Strehl indicates 1/30 wave RMS wavefront, which for very smooth optics comes down to ~1/6 wave pv wavefront. This level of correction would still have ~8% of the energy lost from the Airy disc, and a faint blur at the best focus about twice the Airy disc diameter. In order to have a 13 microns or less spot size, an f/10 instrument would have to be ~1/12 wave pv wavefront, and 98%-99% Strehl. I don't think they are that good.

The difference in performance between the two comes mainly from the production quality. What makes an SCT harder to manufacture at the same level of quality is the two aspheric surfaces - corrector and secondary. Still, with the SCT production quality improved so that it nearly matches MCT's price-wise, I don't think there would be so much of difference to talk about.