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Celestron Schmidt-Cassegrain Telescopes
Some discontinued items included for public information purposes only
The Schmidt-Cassegrain telescope is a "Catadioptric" design.
This design employs the features of both refracting and reflecting (all-mirror)
designs. A primary, and a secondary mirror work with a thin aspheric corrector lens (the glass plate at the front) gather and reflect light to focus at a point behind
the rear of the telescope. An eyepiece for viewing, or a camera (film or video) for
photography may be attached to the focuser. These telescopes are portable,
well suited for astrophotography, and due to their popularity a
wide selection of photo-visual accessories are available. With its
short, compact design (compared to other mirror or refractor
telescopes) the Schmidts are very popular with "suburbanites" who
travel from the city for better viewing conditions, and those who prefer the simpler to use fork mount.
The combination of individually hand matched optics, with precise drive
systems incorporating features of practical value, and sound mechanics
make the Celestron Schmidt-Cassegrain telescope (SCT) the best available. Celestron has long been recognized as the leader in fairly consistent quality, mass produced Schmidt-Cassegrain optical technology. From compact, multi-purpose sizes of as small as 5 inch up to an impressive 14 inch aperture the Celestron SCT's are internationally appreciated.
All of the primary optics, and all mechancial components of Celestron Schmidt-Cassegrain telescopes are made and assembled in the United States:
- Each optical system is individually HAND FIGURED and guaranteed to be at least diffraction limited (Strehl 0.8 or better). Fine corrections are made on the secondary mirror that smooths the optical figure. Hand figuring of an optical system is a complex optical procedure done on a commercial basis soley by Celestron. The art of hand figuring takes time but the results are easily evidenced by the surprisingy good resultant astrophotographs and viewing obtained. Company Seven does test and inspect every system that we recieve, and not every telescope meets our requirements. However, one of the better C-8 systems we delivered tested on an interfereometer at 1/4.6 wave Peak to Valley (1/22 wave RMS) with a Strehl Ratio of 92.4; this was tested at 543.3 nanometers (more demanding than testing at common helium neon lines).
- Optical testing is performed on double-pass auto-collimators in a
temperature controlled environment. This ensures reasonably high quality on a
fairly consistent basis. Celestron is working towards providing the most stringent
optical fabrication, testing and quality control in the mass production SCT industry.
- All Celestron Schmidt-Cassegrains are furnished with the multi-coated
Starbright coatings group - Multi-Coated, High Reflectivity enhanced Aluminum coatings on the mirrors, with durable Anti-Reflection coatings on the correcting
lens for increased light transmission - the best cumulative transmission of
any commercially made Schmidt-Cassegrain telescope.
In a published "Sky & Telescope" magazine review, the Celestron SCT
had the best light transmission by a large margin over the competition.
- The optical systems are baffled to reduce off axis stray light thereby helping to ensure the good contrast levels.
- Basically, Celestron Schmidt-Cassegrains are f/10 designs; although the Fastar 8 can operate at f1.95 or f10, while the Fastar C-14 is an
f/11 system. This design allows Celestron to maintain a relatively small central obstruction. With optional telecompressor optics threaded onto the telescope rear cell, the f10 telescopes can operate at f6.3. At f6.3 these systems will actually yield apparently brighter images, and better contrast than competing f6.3 designs which have larger secondary obstructions than the Celestron systems.
In fact, everything you see at similar magnification - stars, galaxies, nebulae - appears just as bright at f/10 as it does when using a telecompressor at f/6.3. While when using the same magnification, the image in the f/6.3 SCT is fainter. Celestrons Focal Reducer/Corrector (#94175) is our favorite optional SCT telecompressor. This accessory permits an f/10 system function at f/6.3 (f/7 on a C14). And it incorporates a field flattening element to maintain image quality across the entire field of view. This is almost a necessary accessory or those who prefer a faster system for certain astrophotography and CCD applications, and for economical fantastic wide field observing with 1.25" oculars; the results are astonishing!
And a standard feature for all worm gear driven C-8, and the C-14 optical tubes made since the summer of 1998 is the "Fastar" concept. This is where the optical tube has an easily interchanged secondary mirror housing that will accept an optics set and adapter to accommodate a CCD camera for operation at about f2!
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