Telescope sizes

[kramenyak77]

Hi! Just a quick question. Why do telescopes come in a 9.25 size? It's such an arbitrary number and there must be a reason that they aren't just a round number like 8, 10, and 11.

[billdan]

Interesting question Mark, the only 9.25 inch scope I know of is the Celestron SCT, why they chose that size, I don't know. Maybe it is just to be different to their SCT competitor Meade.

Meade offer 6, 8, 10, 12, 14 and 16 inch SCT'S.
Celestron offer 6, 8, 9.25, 11 and 14 inch in their SCT range.

Cheers
Bill

[Wavytone]

It's normally a function of the available blanks from the glass manufacturers - both optical glass for the corrector and Pyrex or quartz for the primary.

[AstralTraveller]

Does 23cm sound less arbitrary?

[Saturnine]

Sorry to be pedantic Dave but 9.25 " is , according to my tape measure, 235 mm, but , yes it is an odd size. As said , probably just to be different to the opposition and available glass blank sizes.

[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.

[sil]

Probably just worked out a usable price point for the mirrors performing within optical tolerances. Its common practice. CPUs are made all the same speed but a certain number of defects in the silicon during manufacture means when they are tested they perform stably at various speeds, they then get separated and packaged at different lower speeds. There is no design different in the silicon chips themselves, they all have the same number of transistors etc, the only difference is not all are functional. So maybe the 9.25 mirrors get made from failed 10" mirrors? or maybe the 8" process pushed to its limit results in the 9.25"? I don't think anyone started with saying "we have 8 and 10" scopes, lets do a 9.25" one. It would have been a viable economic reason due to manufacturing processes or from the wavefront equations indicating it would be a good performer for the size, possibly the offshoot of a military/space project?

Has anyone actually dismantled one to see what is actually 9.25" in diameter in the OTA?

[raymo]

Assuming that the 9.25 is lighter, and a little smaller, than a 10",
there could be a niche market for older/frail people who would
like something bigger than an 8", but find a 10" a bit much.
I assume also that the 9.25" is cheaper than a 10, which could
be a selling point.
raymo

[kramenyak77]

Thanks guys I have learned lots from your answers!

[Wavytone]

You may find this interesting re the central obstruction sizes:

https://www.cloudynights.com/topic/583084-post-your-catcass-co-measurement/?fromsearch=1

Basically astrograph can be 40...50% by aperture,
SCTs at 30...35%
Maks around 23...32%
Mak Newtonians well below that.

There's another thread from someone testing celstrons interferometrically who stated they're all marginal to very ordinary and he has yet to see one with straight bands.

[Wavytone]

Raymo you're right... though I think a large part of the problem concerns the larger SCTs where the OTA remains attached to the fork and base.

While that's ok for an 8" it means the 9-11" scopes are a heavy lift unless you have de-forked them to use on some other mount.

And even my MK91 is a tad over 15kgs and carrying it is like hugging a water heater.