Schmidt–Cassegrain telescope

The Schmidt- Cassegrain telescope is a reflecting telescope with a concave spherical primary mirror and a scattering secondary mirror, a Cassegrain arrangement are so forming behind a Schmidt corrector plate.

The tube of the Schmidt-Cassegrain telescope, the incident light is focused by a spherical primary mirror ( primary mirror ) and thrown back to the secondary mirror ( secondary mirror ). In contrast to the Newtonian telescope the light is not directed out the side of the secondary mirror of the tube, but thrown back into the center of the main mirror. At this point of the primary mirror is pierced, so that the light is guided from the tube there out to the eyepiece. The secondary mirror is mounted at the center of a thin glass plate ( Schmidt plate ) by means of a mirror frame as a separate optical element. The object of this corrector is to eliminate the spherical aberration of the primary mirror and to minimize the coma of the system. The difference with the Maksutov - Cassegrain telescope is the correction plate is very thick and arched at the Maksutov -Cassegrain ( meniscus lens).


The optician James G. Baker, described in 1940, that can eliminate the field curvature of the Schmidt telescope and reduce its overall length with this arrangement. This telescope is just like the original Schmidt telescope free from the aberrations of coma and astigmatism, and achieved so also large angle of several degrees, but it requires at least a slightly aspherical shaped mirror. If, however, both mirrors made ​​spherical, there remains a slight astigmatism, what Hermann Slevogt pointed out shortly thereafter. A more simplified production and a particularly compact design is obtained if, in addition, the secondary mirror is mounted on the corrector plate. In this case, however, the performance of the Schmidt camera is not achieved because remain aberrations due to the geometric constraint. However, this can be solved by a two - or three-lens close focus corrector.

After this two great principles, scientific instruments used, as well as a number of smaller instruments were built:

ADH Baker -Schmidt

In Kollabaration of Armagh, of Dunsink and the Harvard Observatory was due to the ADH Baker - Schmidt telescope built with a 90 -cm mirror and a Schmidt corrector with an aperture of 81 cm and put into operation in 1950. The telescope of Baker - type C4 has an angle of 4.8 °. The telescope was installed until 1981 in the Boyden Observatory in South Africa and was subsequently withdrawn from the Dunsink Observatory, where it was, however, no longer built.

James Gregory Telescope

The largest Schmidt- Cassegrain telescope is the James Gregory Telescope St Andrews University with an aperture of 37 inches. The telescope was built in 1962 and has since been used scientifically, among other things, for the SuperWASP project. It is named after the mathematician James Gregory, who taught in the 17th century in St. Andrews and also occupied himself with the construction of telescopes, although he developed Gregory mirror assembly is not used in this telescope.

Amateur Astronomy

The Company Celestron and Meade offer since the late 1960s or early 1980s, a number of smaller telescopes in Schmidt Cassegrain mirror arrangement. This optimized with regard to a simple manufacture telescopes have two spherical mirrors, one of which is held by the Schmidt plate. However, this cost structure can not correct all aberrations of the first order.

From the company Meade Schmidt-Cassegrain also a variant of 20 cm below the ACF Acronym that stands for Advanced Coma Free, offered to 50 cm aperture, wherein the secondary mirror is hyperbolic in shape and the image field is nearly leveled.

The company ASA Astro Systems offers a 3 -lens Schmidt- Cassegrain Reducer, which is the mounted near the focus and aberrations simpler reduced Schmidt -Cassegrain telescopes. In the execution of the Company designated as EdgeHD Celestron a two lens corrector near the focus is added, which eliminates also remaining aberrations 1st order, others reduced. Telescopes in this design are available with 20 to 35 cm aperture.