Schmidt camera

The Schmidt telescope, also Schmidt camera or Schmidt telescope called, is a specially designed for astrophotography telescope with a particularly large field of view. The combination of lenses and mirrors is a catadioptric system.

Development

The Schmidt telescope is based on an invention Bernhard Schmidt in 1930, which combined a spherical primary mirror with a thin, very specially shaped corrector plate ( Schmidt plate ). This is located in the center of curvature of the primary mirror and the spherical aberration is eliminated. Coma is only avoided in that the aperture stop is in the center of curvature of the primary mirror. To reduce the vignetting (darkening of the corners ), the main mirror is made ​​larger in diameter than the telescope aperture ( see illustration). Because of the large detectable angle of view and the highest image quality in the corners of the photo plates, the Schmidt camera was in the sky Photography widespread. The image field of the camera, however, Schmidt- curved so that the focus of a spherically arched film must be used to compensate for the curvature of field. However, Schmidt had already in his original publication indicates that it is possible to dispense with a fainter opening ratio on the corrector plate very difficult to prepare (so-called " lensless Schmidt telescope ").

The type of telescope is not suitable for visual observation (such as the Schmidt- Cassegrain telescope ), but only for photography, as the focus is within the telescope tube and there only one camera can be mounted.

Schmidt chose not to announce his idea as a patent.

The Schmidt- Cassegrain telescope takes the focus back side of the tube out and is thus also suitable for visual observation.

The first Schmidt telescope of the observatory in Hamburg- Bergedorf presented Bernhard Schmidt in 1930 (Clear Aperture 360 mm mirror diameter 440 mm, focal length 630 mm) finished. As a new director was sought at the Hamburg Observatory, Walter Baade called as a candidate in 1937 a large Schmidt telescope with 1 m aperture. Although the Hamburg Senate approved the funds even after Baade had canceled and Otto Heckmann was appointed manager. The building came into being but only after the end of the war. The Greater Hamburg Schmidt (Clear Aperture 800 mm mirror diameter 1200 mm, 2400 mm focal length ) was put into operation in 1954. The originally planned survey work had now but already accepted by the Palomar Schmidt. The Great Hamburger Schmidt telescope in 1975 brought to the Calar Alto Observatory in Spain and stayed there for 25 years in operation.

The following instruments are of particular importance for astronomical research, sorted by size:

• Alfred Jensch telescope in the Thuringian State Observatory in Tautenburg near Jena ( free aperture 1.34 m, 2.00 m diameter mirror, focal length 4.00 m, the world's largest Schmidt telescope )
• Oschin Schmidt telescope at Palomar Observatory in 1948 (Big -Schmidt, clear aperture 1.26 m, 1.83 m diameter mirror, focal length 3.07 m )
• UK Schmidt Telescope of the Anglo - Australian Observatory / Siding Spring Observatory, Australia ( clear aperture 1.24 m, 1.83 m diameter mirror, focal length 3.07 m )
• More with approximately 1 meter in diameter are at the Kiso Observatory in Byurakan Observatory, the observatory Kvistaberg, the Observatorio Nacional de Llano del Hato Astronómico and out of service since 1998, at the La Silla Observatory ( ESO Schmidt).
• The Kepler spacecraft, which was launched on March 6, 2009, essentially consists of a Schmidt optics ( clear aperture 0.95 m mirror diameter 1.4 m).

The Big Schmidt Palomar Observatory was the first large Schmidt telescope, which was used for a complete Himmelskartografie of the northern sky. The map series of POSS ( Palomar Observatory Sky Survey) has long been the reference source of observational astronomy. It was repeated in the 1980s. In southern latitudes the ESO Schmidt was used to photograph the sky Südsternhimmels.

The UK Schmidt Telescope, the Oschin Schmidt telescope and the ESO Schmidt have a special feature on an achromatic, made ​​of two types of glass corrector plate, which was prepared for the former Grubb Parsons.

Modifications of the Schmidt mirror

The outstanding optical properties of the Schmidt plate motivated the investigation of a number of variants in order to pave the image field, to simplify the structure to increase the angle of view or the aperture:

Schmidt- Väisälä camera

The principle of the corrector plate is 1924 - was discovered by Yrjö Väisälä, but was rejected by him because of the field curvature - before Schmidt. Väisälä later developed two-lens field flattener for Schmidt mirror, sitting close to the focal point, and built two cameras, one with a 120 mm aperture and a 500 mm aperture, both with an aperture ratio of 1:2 and a field angle of about 7 °. In 1941 he made ​​another one with 31 cm opening for the observatory Kvistaberg.

A lens for field flatness was later used in the Oschin Schmidt telescope and the Observatorio Nacional de Llano del Hato Astronómico to operate the telescopes with flat CCD image sensors. In the Schmidt telescope of the Kiso Observatory, however, a suitably shaped CCD chip is used.

Two mirror variants

Main article: Schmidt- Cassegrain telescope and Schmidt- Newtonian telescope

James G. Baker developed an alternative to field flatness by lenses by the Schmidt corrector combined it with a Cassegrain mirror assembly having at least a slightly aspherical mirror surface. This results in a - compared to the original Schmidt camera - more accessible image plane near the main mirror and a shorter design with a reduced distance of the corrector to the primary mirror. Other variants are versions in both mirrors are spherical aberrations are not completely eliminated; these can then be eliminated by further focus close correctors. According to this principle two large scientific instruments have been built with more than 80 cm aperture, and a number of telescopes for amateur astronomy at about 55 cm aperture. Also for the amateur, the combination of a Newtonian telescope with a front sitting Schmidt corrector plate is offered.

Another variant is the single-center design in which the center of curvature of both mirrors each other lies. Due to the symmetry thus formed results in a larger angle of view.

Super -Schmidt optics

The principle discovered by Schmidt arrangement can be further improved by the concept is expanded by a corrector in the center of a spherical primary mirror by a multi-unit structure of the corrector. For this purpose, in particular the division of correction on Schmidt plate and meniscus lens of a Maksutov telescope has proven to be very powerful because some aberrations of Schmidt plate and meniscus cancel. These lenses have image angle of 60 ° at light intensities of approximately 1:1 and were used in 1960 primarily as a satellite cameras.

Examples of Super -Schmidt optics are:

• Meniscus Super -Schmidt camera. Developed in England camera uses two menisci that surround an achromatic Schmidt plate; it has an aperture of 30 cm and a nominal aperture ratio of 1:0,63.
• The Baker developed parallel super -Schmidt, who designed by James G. Baker and built by Perkin Elmer 1950 camera for meteor watching, had a similar structure and similar properties.
• Soviet FAS -camera consisting of corrector plate and meniscus lens.
• VAU- camera, a constructed in the Soviet Union camera with a lens diameter of 650 mm. It was installed in 1969 in the Observatory of Zvenigorod. It is based on a 1953 developed by Maksutow and Sosnina Astrodar lens, in which there is a meniscus lens behind the aperture.
• A constructed in Poland and installed in the observatory Poznan Poznan camera -2, which has a corrector of five lenses.
• The Baker - Nunn satellite tracking camera with an aperture of 50 cm and an aperture ratio of 1:1. The arrangement of the three -lens corrector and the spherical mirror resembles a Houghton telescope, the corrector lenses are thinner and aspherical but as in a Schmidt camera.

Corrector mirror

Main article: Large Sky Area Multi -Object Fiber Spectroscopic Telescope

The light refraction at the glass exported from Schmidt plate causes a wavelength-dependent splitting of the light. This chromatic aberration can be avoided by an equivalent shaped, slightly slanted mirror that replaces the glass plate. A Schmidt telescope with corrector mirror was implemented for research purposes for the first time in 2007 by the Chinese LAMOST. Since the corrector mirror also can be extensively supported against deformation, a clear opening of 4 m was achieved, and the avoidance of chromatic errors allows a field of view of 5 °.