Classified price: [CN for sale $900.00]
Weight: 20 pounds (9.4 Kg) :: Max.: 14.5 mag :: Resolution: 0.52" :: RMS: 1/19th wave :: Strehl: 0.90
9.25" Schmidt-Cassegrain, 235mm/2350mm focal length (f/11) Optical Tube Assembly, without the mount.
Celestron C925 Schmidt-Cassegrain OTAC-9.25 is more than merely an intermediate step between the C-8 and the C-11. This telescope provides a higher contrast and better sharpness than comparable Schmidt-Cassegrain OTAs. This makes the C-9.25 of particular interest for the observation of the moon and planets. Celestron C-9.25 yields images with sharpness and contrast close to Maksutov/Refractor systems.
Celestron has achieved this through a complete recalculation of the optics. The C-9 is about as long as the C-11. Nevertheless, the focal ratio is 1:10. Primary and secondary mirrors are not as extreme focal ratio, making it more accurate than other Schmidt-Cassegrain. The resulting image quality is better in focus.
Most SCT compact designs have a primary mirror with a focal ratio of f/2 and a secondary with a negative focal ratio of f/5. That yields a system focal ratio of f/10. Celestron C-9.25 is a notable exception, which has a primary focal ratio of f/2.3 and a secondary focal ratio of f/4.3, resulting a slightly flatter field and a slightly longer tube aspect ratio than other similar designs.
C9.25 Optical QualityThe results of optical interferometry are around an RMS of 1/19 wavefront and a very good Strehl of 0.90.
The exceptional high quality optics of C9.25 are here analyzed in depth through a Foucault test as well as a Ronchi test: "It is a Celestron C9.25 - ie a obstructed system in which a portion of light energy (Strehl value and obstruction) moved more into the diffraction rings is. A system with 235 mm aperture and 2350 mm focal length system, a so-called Schmidt-Cassegrain system with a Schmidt plate, usually made of float glass. In the C 9:25, which I have already examined, it is one of the good systems, which can be proved in the course."
- "Planetary images are absolutely comparable with those obtained through a 10-inch ACF-SCs and significantly better than the results of a 8-inch SCs."
- "The C9.25 Strehl ratio, meaning the ratio of energy measured at the center of the Airy disk, makes C9 a perfect instrument."
- "Images of Moon and Saturn are pleasing at 600x."
- "After the replacement of my first C9.25 tube, I could record planets at 11 meters of focal length."
- "It is known that optical design of Schmidt-Cassegrain is not optimal for deep sky astrophotography (focal is too large). In this context, the images delivered by C9 fall in the average."
- Some owners have measured their C9.25 RMS lambda, and report quite different values: 1/9.8, 1/12.6, 1/15 and even 1/24. It seems C9 optics is always good, but if you are lucky it could be even better.
- [Book] Lunar and planetary webcam user's guide
- [Book] A buyer's and user's guide to astronomical telescopes & binoculars
- [Book] Star Ware: The Amateur Astronomer's Guide to Choosing, Buying, and Using Telescopes and Accessories
- [Book] The backyard astronomer's guide
- [Book] Choosing and Using a New CAT: Getting the Most from Your Schmidt Cassegrain or Any Catadioptric Telescope
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