I have an 8" F/6.3... it performs quite well TYVM. They weren't as highly sought because they were built more specifically for imaging than visual. Back in the 90s we mostly had film cameras for the general masses and would use the F/6.3 scopes coupled with even a further reducer at times.
The CO has pretty much nothing to do with their unpopularity except for in the off-hand conversations on forums. Those of us who own them are glad we do because we can image quite successfully with the shorter FL much easier today with CMOS cameras than years ago with film.
Yep. In addition, when the Meade F/6.3 SCTs originally were released, there were some significant quality control problems, and as a result the F/6.3 scopes got a bad reputation. Also, Celestron came out with their F/6.3 Reducer/Corrector and boasted in advertising that with the R/C you could have F/6.3 AND a smaller obstruction. This probably helped to spread the belief that the obstruction size was a major culprit in the performance issues. Meade eventually got the quality problems sorted, but by then the damage was done to the reputation. The F/6.3 OTAs lasted into the LX200 era, but were eventually dropped. But I know there are some people out there with good ones who don't ever plan on giving them up. My understand is that they do have a lot of field curvature, though.
Higher focal ratio SCTs are a very specific target instruments therefore the market is minimal putting them in the realm of specialty one-off designs. With the myriad of choices today and capability of imaging equipment it is still the same reasoning. There is no lucrative market for such an instrument.
Today we have strong segments for short FL refractors, large aperture fast dobsonians, and in the middle sit the F/8-F/15 SCT/MAKs. If you wish to spend $15-20K on a single F/20-F25 SCT most would question the logic for any amateur purpose.
I agree. An 8" SCT is a wonderfully compact and portable OTA for something that gives you 8" of aperture. However, after that, they get big and heavy in a hurry. It's hard for them to compete against dobs in the larger sizes.
And at 6" or 8" apertures, these hypothetical planetary SCTs would have to offer a better value proposition than slightly smaller APOs. That means a 6" premium SCT with a long focal ratio needs to offer significantly better performance than a $1400 127mm APO triplet, or cost substantially less. An 8" premium SCT would have to out-perform (or be significantly cheaper) than a $4000 150mm APO. As well as being better than a 7" mass-produced Mak, and either better or cheaper than a 180mm Tak Mewlon at $2300. That is a TALL order. And even then, it's a niche market. I can't see it happening.
On the other hand, Questar still manages to get people to shell out almost $5K for a 3.5" Mak, though, and over ten grand for a 7" OTA. So maybe there is a small market. However, on the OTHER other hand, it's not like anyone is rushing out to compete with Questar over their niche.