Schmidt telescope

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Beam path of a Schmidt telescope

The Schmidt telescope , even Schmidt camera or Schmidt telescope called, is a specifically for astrophotography constructed reflecting telescope that a particularly large useful by its design field has. The combination of lenses and mirrors makes it a catadioptric system .


The Schmidt telescope goes back to an invention by Bernhard Schmidt around 1930 , who combined a spherical main mirror with a thin, very specially shaped correction plate ( Schmidt plate ). This is located in the center of curvature of the main mirror and eliminates its spherical aberration . The coma is avoided solely by the fact that the aperture stop is in the center of curvature of the main mirror. In order to reduce vignetting (darkening of the corners), the main mirror is made larger in diameter than the telescope opening (see illustration). The Schmidt camera was widely used in celestial photography because of the large image angle that can be captured and the highest image quality right into the corners of the photo plates . However, the Schmidt camera's field of view is curved, so a spherically curved film must be used in the focus to compensate for the curvature of the field . This can also be completely compensated by optical correctors so that flat photo sensors can be used.

Schmidt already pointed out in his original publication that it is possible to dispense with the difficult-to-produce correction plate (so-called "lensless Schmidt telescope") if the focal ratio is weaker.

Schmidt decided not to apply for a patent for his idea .

The telescope type is not suitable for visual observation (in contrast to the Schmidt-Cassegrain telescope ), but only for photography , as the focus lies within the telescope tube and only a camera or photo plate can be mounted there.

The completely differently constructed Schmidt-Cassegrain telescope guides the focus out of the tube from the back and is therefore also suitable for visual observation.

Bernhard Schmidt completed the first Schmidt mirror of the observatory in Hamburg-Bergedorf in 1930 (free opening 360 mm, mirror diameter 440 mm, focal length 630 mm). When the Hamburg observatory was looking for a new director, Walter Baade called for a large Schmidt telescope with a 1 m aperture as a candidate in 1937. The Hamburg Senate also approved the funds after Baade had canceled and Otto Heckmann had been appointed director. The construction came about after the end of the war. The Great Hamburger Schmidt (free opening 800 mm, mirror diameter 1200 mm, focal length 2400 mm) was put into operation in 1954. The originally planned survey work were now but already from Palomar been taken -Schmidt. The Great Hamburg Schmidt Mirror was brought to the Calar Alto Observatory in Spain in 1975 and remained in operation there for 25 years.

The Alfred Jensch telescope, the world's largest Schmidt camera

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

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

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

Modifications of the Schmidt mirror

The outstanding optical properties of the Schmidt camera motivated the investigation of a number of variants in order to level the field of view, to simplify the structure, to enlarge the angle of view or the aperture:

Schmidt Väisälä camera

The principle of the corrector plate was also discovered in 1924 - before Schmidt - by Yrjö Väisälä , but was rejected by him due to the curvature of the field. Väisälä later developed two-lens field planes for Schmidt mirrors, which are located close to the focal point, and built two cameras, one with a 120 mm aperture and one with a 500 mm aperture, both with an aperture ratio of 1: 2 and an angle of view of around 7 °. In 1941 he made another one with a 31 cm opening for the Kvistaberg observatory .

Two such cameras with a particularly large aperture of 63 cm and an aperture ratio of almost 1: 1 at an angle of view of 10 ° were specified by J. Hewitt in the early 1960s and built by Grubb Parsons. These devices , known as Hewitt cameras, were used for satellite observation in England and Australia.

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

Two-mirror variants

Schmidt-Cassegrain telescope for amateur astronomers, the Schmidt plate also serves as a holder for the secondary mirror, eliminating the need for a spider for mounting and avoiding the diffraction peaks caused by this.

Main articles: Schmidt-Cassegrain telescope and Schmidt-Newton telescope

James G. Baker developed an alternative to the flattening of the image field using lenses by combining the Schmidt corrector with a Cassegrain mirror arrangement with at least one slightly aspherical mirror surface. Compared to the original Schmidt camera, this results in a more accessible image plane near the main mirror and a shorter design due to a reduced distance between the corrector and the main mirror. Further variants are designs in which both mirrors are spherical, but image errors are not completely eliminated; these can then be eliminated by further correctors close to the focus. According to this principle, two large scientific instruments with an opening a little more than 80 cm have been built, as well as a number of telescopes for amateur astronomy with an aperture of about 55 cm. The combination of a Newtonian telescope with a Schmidt corrector plate in front of it is also offered for the amateur sector.

Another variant is the monocentric design, in which the center of curvature of both mirrors lies on top of one another. The symmetry formed in this way results in a large angle of view.

Super Schmidt optics

Baker-Nunn camera , it is shorter than a Schmidt camera due to the higher aperture ratio of 1: 1.

The principle of the arrangement discovered by Schmidt can be further improved by expanding the concept of a corrector in the center of a spherical main mirror through a multi-part structure of the corrector. For this purpose, the division of the correction between the Schmidt plate and meniscus lens of a Maksutov telescope has proven to be very efficient, since some of the aberrations of the Schmidt plate and meniscus cancel each other out. These optics have an angle of view of 60 ° with light intensities of around 1: 1 and were mainly used as satellite cameras around 1960 .

Examples of Super Schmidt optics are:

  • Meniscus-Super-Schmidt-Camera. This camera, developed in England, uses two meniscuses surrounding an achromatic Schmidt plate; it has an aperture of 30 cm and a nominal aperture ratio of 1: 0.63.
  • The Baker super-Schmidt developed at the same time , the camera for meteor observation designed by James G. Baker and built by Perkin Elmer around 1950, had a similar structure and properties.
  • The Soviet FAS camera, consisting of a corrector plate and meniscus lens.
  • VAU-Camera, a camera designed in the Soviet Union with a lens diameter of 650 mm. It was in the Observatory from 1969 Zvenigorod installed. It is based on an Astrodar lens developed by Maksutow and Sosnina in 1953 , in which a meniscus lens is located behind the aperture.
  • A Poznan-2 camera constructed in Poland and installed in the Poznan Observatory , which has a corrector made up 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 is similar to a Houghton telescope , but the corrector lenses are made thinner and aspherical, like in a Schmidt camera.

Corrector mirror


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

The refraction of light on the Schmidt plate, which is made of glass, causes a wavelength-dependent splitting of the light. This chromatic aberration can be avoided by using an aspherically shaped, slightly inclined mirror that replaces the Schmidt plate. For research purposes, a Schmidt telescope with a corrector mirror was realized for the first time in 2007 by the Chinese LAMOST . Since the corrector mirror can also be supported flat against deformation, a free opening of 4 m could be realized and a chromasia-free image field of 5 ° was achieved.


Web links

Commons : Schmidt cameras  - collection of images, videos and audio files

Individual evidence

  1. Bernhard Schmidt: A light-intensive, coma-free mirror system , bibcode : 1938MiHam ... 7 ... 15S
  2. Atmospheric Extinction Coefficients and Night Sky Brightness At the Xuyi Observational Station , arxiv : 1211.4672
  3. ^ Charles Gorrie Wynne : The Optics of the Achromatized UK Schmidt Telescope Quarterly Journal of the Royal Astronomical Society, Vol. 22, p. 146, 1981, bibcode : 1981QJRAS..22..146W
  4. Willstrop, RV: Wide-field conversions for reflecting telescopes , bibcode : 1987MNRAS.229..143W
  5. ^ E. Öpik: Yrjö Väisälä , Irish Astron. Journ., Vol. 11, p. 159, bibcode : 1973IrAJ ... 11R.159O
  6. ^ Yrjö Väisälä: Anastigmatic mirror telescope of the observatory of the University of Turku , Astr. Message 254, 361, 1935, bibcode : 1935AN .... 254..361V
  7. Yrjö Väisälä: About mirror telescopes with a large field of view , Astr. To. 259, 197, 1936. bibcode : 1936AN .... 259..197V
  8. Nobunan Itoh, Takao Soyano, Ken'rchi Tarusawa, Tsutomu Aoki, Sigeomi Yoshida, Takashi Hasegawa, Yasushi Yadomaru, Yoshikazu Nakada, Satoshi Miyazaki: A Very Wide-Field CCD Camera for Kiso Schmidt Telescope ( memento of the original from October 31, 2008 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. (PDF file; 739 kB), Publ. Natl. Astron. Obs. Japan Vol. 6. pp. 41-48, 2001 @1@ 2Template: Webachiv / IABot /
  9. ^ J. G. Baker: A family of Flat Field Cameras, Equivalent in Performance to the Schmidt Camera. Proceedings, American Philosophical Society (pp. 339, vol. 82, 1940), ISBN 1-4223-7224-3 .
  10. James G. Baker : Schmidt image former with spherical aberration corrector , USPTO, priority June 19, 1945
  11. ^ Albert Bouwers: Schmidt Type Image Former With Negative Meniscus Lens Spherical Aberration Corrector , USPTO, priority October 16, 1945
  12. ^ DG Hawkins, EH Linfoot: An improved type of Schmidt camera , Monthly Notices of the Royal Astronomical Society, Vol. 105, p. 334, 1945, bibcode : 1945MNRAS.105..334H
  13. A history of Keele Observatory: The meniscus Super-Schmidt meteor camera  ( page no longer available , search in web archivesInfo: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice. , engl.@1@ 2Template: Dead Link /  
  14. ^ A b J. Davis: The Design and Performance of three Meniscus Schmidt Meteor Cameras , Quarterly Journal of the Royal Astronomical Society, Vol. 4, p. 74, 1963, bibcode : 1963QJRAS ... 4 ... 74D
  15. ^ Fred L. Whipple : The Baker super-Schmidt meteor cameras. , bibcode : 1951AJ ..... 56..144W
  16. a b A. G. Massevitch, AM Losinsky: Photographic Tracking of Artifical Satellites , Space Science Review, 1970, bibcode : 1970SSRv ... 11..308M
  17. Nail Bakhtigaraev, Alexandr Sergeev: New instruments in Zvenigorod and Terskol observatories
  18. AG Masevich, AM Lozinskiy: New Soviet Cameras For Photographic Observations Of Artificial Heavenly Bodies ( Novyye Sovetskiye kamery dlya fotonablyudeniy iskusstvennykh nebesnykh tel , Vestnik Akadeinii Nauk SSSR, Vol. 57, No. 2, 1970, pp. 38-44, translated by Walter L. Burton, 1972)
  19. ^ Lewis C. Epstein: An All-Reflection Schmidt Telescope for Space Research , bibcode : 1967S & T .... 33..204E
  20. ^ Dietrich Korsch: Reflective Schmidt corrector. , bibcode : 1974ApOpt..13.2005K