Why do Schmidt-Cassegrain telescopes have focal ratios of f/10?

Why do most if not all of the Schmidt-Cassegrain telescope I see advertised have focal ratios of f/10?

During the past 20 years or so, amateurs have favored shorter tubes for both refractors and reflectors, even if it meant paying for more costly optics. Portability is the chief motivation - the days of the cumbersome f/11 reflector are pretty much gone. But a Schmidt-Cassegrain, with its folded light path, already has quite a short tube, and a benefit of the f/10 design is that you get high powers for lunar and planetary viewing with medium-focus eyepieces rather than ultrashort ones.

Still, there are times when observers really would prefer an instrument with a focal ratio lower than f/10. The angular field of view would be wider, which helps in viewing or imaging extended dep-sky objects. That's why telecompressors have become important accessories for Schmidt-Cassegrains.

The main reason you don't see Schmidt-Cassegrains (and Cassegrains in general) made much faster than f/10 has to do with the size of the secondary mirror. For the same overall tube length, an f/5 Cassegrain needs a much larger secondary than an f/10, and that would boost diffraction problems and light loss.

—Dennis Di Cicco

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