[astro744]

When first released by Celestron the C9.25 was noticeably different as it looked a little longer than its aperture and f ratio would otherwise suggest. The reason for this was the f ratio of the primary being around f2.3 and not f2 as is the case for the other Celestron offerings. The system f ratio is still f10 though. The amplification factor is then 10/2.3 and not 10/2 so any wavefront errors on the primary will be less magnified or in other words the accuracy of the primary need not be as great and therefore a little easier to produce consistently, (or so I've read somewhere a long time ago). This gave the C9.25 a reputation for being supposedly better optically than other SCTs especially noticeable on planets.

I have one of the first C9.25 and whilst it performs well on most things including planets it still has a large central obstruction that reduces contrast and this is noticeable when compared to other optical designs of lesser or no obstruction. There could be a number of reasons as to why 9.25" though and perhaps design parameters and limits were set to keep the tube a certain length or simply and aperture increase of 33% was sought over that of the C8.