Houghton Telescope

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The original design of the Houghton telescope is a powerful mirror lens telescope with a short overall length for astrophotography, with exclusively spherical surfaces, in which two, rarely three correction lenses with about the diameter of the main mirror are arranged centrally in the beam path in front of it. The flat image is created inside the tube, just in front of the corrective lenses, and can only be used there photographically with a photo plate or CCD sensor. By using mirrors and lenses, it is a catadioptric telescope.

James Houghton, who worked for Kodak , filed a patent application for the telescope or lens type on March 7, 1941 in Great Britain . In the same year, on August 30, 1941, Robert Richter and Hermann Slevogt , both employed by Carl Zeiss , patented a similar system, which is why it is also known as the Richter Slevogt lens .

The corrector, which consists of the lenses, is afocal and, in terms of its optical principle, resembles the Schmidt plate of the Schmidt camera developed around ten years earlier and the meniscus lens of the Maksutov telescope developed around the same time . They are afocal lens systems of small thickness and large radii of curvature, consisting of a converging and a diverging lens and therefore only have a low level of chromatic aberration , but they cause coma and spherical aberration that compensate for the same aberrations of the spherical main mirror. In contrast to the other two variants mentioned, the image field is flat and the lenses are much easier to manufacture. Since they do not generate chromatic aberration due to their long focal length system, they can be made from the same type of glass, although slightly different types of glass improve the optical properties of the telescope somewhat. The shape of the lenses can also be varied; symmetrical convex and concave or plano-convex and plano-concave lenses each with the same radii are possible, with asymmetrical lenses again slightly improving the optical properties. Another advantage of the corrector is, in particular, that it is easier to manufacture, which makes it ideal for the amateur sector, if an amateur has the option of having an anti-reflective coating applied to the four lens surfaces. The good correction of aberrations, a flat image field and an almost perfect spot diagram result in compact photographic telescopes with high luminosity and a large angle of view, typical values ​​are from f / 3 to f / 5 and 2 ° with a lens and mirror diameter of 200 mm.

Further development

The use of correction lenses with the diameter of the telescope aperture does not make the application for larger telescopes economical. However, the good optical properties inspired a number of further developments.

Newton and Cassegrain beam path

Richter and Slevogt already showed in their patent specification that these corrective lenses and spherical primary mirrors are also suitable for the beam paths of Newtonian telescopes and similarly of Cassegrain telescopes . The first arrangement is occasionally used with amateur telescopes; the last, mostly further developed, into mirror lens lenses for photographic purposes. Both versions have remarkable optical properties, since negative effects of the position shift of the corrector along the optical axis can be compensated for by degrees of freedom in the lens system.

Baker-Nunn camera

The Baker-Nunn camera picks up the three-lens corrector suggested by Houghton in his patent specification and makes it aspherical and with special types of glass. This further reduces image errors and an aperture ratio of 1: 1 is possible with an image angle of 30 ° and an aperture of 50 cm.

Lurie Houghton Telescope

The Lurie-Houghton telescope is based on the principle of the Houghton telescope with two correction lenses, but with a beam path as in the Newton telescope. The position of the correction lenses is chosen so that they can also serve as a mirror holder for the secondary mirror. For a low-vignetting image, its diameter must be around 30 to 40% of the main mirror diameter, so that the Lurie-Houghton can only be used to a limited extent for visual applications. The geometry of the correction lenses was optimized for this position by Robert J. Lurie, using a parabolic primary mirror. Often, however, telescopes with a spherical main mirror and two-lens corrector are - inappropriately - also called that.

literature

credentials

  1. ↑ James Leonard Hougthon: LENS SYSTEM , patent specification , 1942
  2. ^ Robert Richter, Hermann Slevogt: Lens , patent specification, 1941
  3. ↑ a b c full-aperture Houghton corrector (Engl.)
  4. ↑ Werner Eich: DIY 8-inch Lurie-Houghton optics Part 1 ( Memento from July 17, 2012 in the web archive archive.today ), Part 2 ( Memento from July 13, 2012 in the web archive archive.today ), Journal for Astronomy , Association of Star Friends , 2007
  5. ↑ Rick Scott: Lurie-Houghton Telescope Design on astronomy.net ( engl )
  6. ↑ Aki Lötjönen: Optics of Lurie-Houghton Newtonian ( Memento of April 29, 2010 in the Internet Archive ) (Engl.)
  7. ↑ Robert J. Lurie: Anastigmatic catadioptric telescopes , J. Opt. Soc. At the. 65, 261-266 (1975) doi : 10.1364 / JOSA.65.000261 .