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celestron 9.25 OTA. Why so special?

Started by robert_rule, 01/30/2003 10:00PM
Posted 01/30/2003 10:00PM Opening Post
I have been looking for something to complement a Traveler on an AP 600 mount. I've been reading a lot about this OTA. I am sure that this has come up before but what makes it appear to be so special? Also, is there a great range in quality now that the company has been taken back over by professionals? Finally, any particular dealers that will make sure that everything is in excellent shape? I am leaning towards Company Seven.

Thanks for your time,

Bob
Posted 01/31/2003 12:11AM #1
Optically, advantages of the c9.25 come from its slower primary (~f/2.4). For one, it makes it easier to produce aspheric corrector within certain tolerances, the way it is manufactured. Then, its field curvature is milder, nearly 300mm, comapred to ~170mm in a "regular" 8" SCT. This makes for a better field performance.

There are speculations that its primary is aspherical, in which case secondary could be spherical (that would be a plus, since small aspheric convex mirrors are also tricky to manufacture) but it's not so. I knife-edged it at the radius of curvature, and the shadow is perfectly straight (characteristic of a spherical mirror). Secondary is more strongly aspherized than in a "regular" design, possibly even mild hyperbola, but it's more likely that they kept it a strong ellipse.

Also, due to lower secondary magnification, chromatism in the c.9.25 is about twice lower than in a "regular" sct. That shouldn't be a dramatic advantage by itself (due to relatively low chromatism in a "regular" sct), but combined with better, on average, corrector figure and milder field curvature, it is probably enough to give noticeably better impression.
Posted 01/31/2003 04:28PM #2
One thing I've always wondered was: if the 9.25 is so superior optically, why doesn't Celestron adopt it's unique design for ALL of it's line of SCTs?