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A Schmidt-Cassegrain Telescope, invented by Bernhard Schmidt, is a catadioptric telescope. This type of telescope is advantageous because it combines the long focal length of the refracting telescope with the lower cost per aperture of the reflecting telescope.
The optical design combines elements from both the Schmidt camera and the Cassegrain reflector. In this system the parabolic primary mirror is replaced by a spherical mirror, which introduces spherical aberration. This is corrected by the Schmidt corrector plate, found in the Schmidt camera. From the Cassegrain, it inherits the convex secondary mirror, perforated primary mirror, and a final focal plane located behind the primary. Some designs add additional optical elements (such as field flatteners) near the focal plane.
Variations
While there are many variations, (both mirrors spherical, both mirrors aspherical, or one of each) they can be divided into two principal design forms: compact and non-compact. In the compact form, the corrector plate is located at or near the focus of the primary mirror. In the non-compact, the corrector plate remains at or near the center of curvature (twice the focal length) of the primary mirror. Typical examples of the compact design are Celestron and Meade commercial instruments, combining a fast primary mirror and a small, strongly curved secondary. This yields a very short tube length, at the expense of field curvature. Most compact designs from Meade and Celestron have a primary mirror with a focal ratio of f/2 and a secondary with a negative focal ratio of f/5 yielding a system focal ratio of f/10. One notable exception is the Celestron C-9.25, which has a primary focal ratio of f/2.3 and a secondary focal ratio of f/4.3, the result being a slightly flatter field and a slightly longer tube aspect ratio than most other compact designs.
Non-compact designs keep the corrector at the center of curvature of the primary mirror. One very well-corrected design example would be the concentric (or monocentric) Schmidt-Cassegrain, where all the mirror surfaces and the focal surface are concentric to a single point: the center of curvature of the primary. Optically, non-compact designs often yield better aberration correction and a flatter field than a compact design, but at the expense of longer tube length
Notable telescopes
Currently the largest production Schmidt-Cassegrain is a 20-inch model from Meade. There are, however, larger custom-built telescopes.
- 26 in (660 mm) Schmidt-Cassegrain telescope at the UCF Robinson observatory.
See also
"Schmidt-Cassegrain telescope"