Corrector (telescope)

from Wikipedia, the free encyclopedia

A corrector is a special arrangement of lenses or mirrors that reduces imaging errors of a telescope objective or eliminates imaging errors due to atmospheric dispersion ( chromatic aberration ), whereby the beam path of the telescope is only slightly changed. Ralph Allen Sampson gives a definition of a lens system:

"Corrector [...] introducing very little deviation in the ray but an arbitary amount of aberration [...]."

There are different types that differ in the position of the corrector in the optical system and consequently also the corrector diameter in relation to the aperture (opening of the objective).

Aperture position

Schematic representation of the corrector position and the beam path of the Schmidt camera
Schematic representation of the meniscus lens and the folded beam path of the Maksutov-Cassegrain telescope

If the corrector is in the aperture position, it also forms the diaphragm of the telescope. Significant examples are

These enable the use of a spherical main mirror with a relatively large aperture ratio (in the range 1: 3 to 1: 2) and result in a usable angle of view of several degrees. The complex Baker-Nunn camera, which uses special types of glass and aspherical lenses, achieves an angle of view of 30 ° with an aperture ratio of 1: 1 and a lens diameter of 50 cm.

The basics of these proofreaders were developed in the 1930s and 40s. They are afocal lenses that are achromatic due to their special shape and produce an aberration that is opposite to that of the main mirror and compensates for it. It was found that the optically most advantageous position of the corrector is in the center of the curvature of the main mirror, twice the focal length. Despite the high aperture ratios, this results in relatively long telescope dimensions and the main mirror must have a larger diameter than the aperture or the corrector in order not to vignette the image . This and the lenses, which are difficult to manufacture and stabilize with larger diameters, typically limit the aperture to a diameter of about one meter. The largest apertures are achieved with the Schmidt plate, as a lens with a diameter of 134 cm in the Alfred Jensch telescope or as an oblique mirror in the LAMOST with an aperture of 4 m.

Although it has a disadvantageous effect on the optical properties, a whole series of shortened versions have been developed based on this, as implemented in the Maksutov-Cassegrain telescope and Schmidt-Cassegrain telescope , such as the Wright telescope or with beam paths similar to the Newton telescope are. The Houghton corrector is particularly advantageous here, as it can better compensate for this position shift due to the further degrees of freedom through the use of two lenses. Further advantages of the Houghton corrector are the simpler manufacture and the avoidance of an image field curvature , which is inherent in the other correctors from the geometry and is compensated by a curved film or an additional image field flattener near the focal point.

The performance of the correctors in the aperture position can be improved by using additional correctors, usually close to the focus.

Focus position

Ray tracing a Rosin corrector for hyperbolic mirrors; the spot diagrams are determined in the center, at 0.35 ° and 0.5 °.
Including a Jones-James corrector for spherical mirrors and the spot diagrams in the center, at 0.175 ° and 0.25 °.
For reference the spot diagram of a parabolic mirror in the center and at 0.05 °; all with marked Airy discs for a mirror with a diameter of 600 mm and an aperture ratio of 1: 5.6. The beam paths shown correspond to the last 500 mm to the focus.

There are also correctors close to the image plane, which are referred to in English as sub-aperture correctors . Its principles were published by Ralph Allen Sampson in 1912–1914 and have since been developed in many variants:

  • Sampson's corrector for Newtonian telescopes and for Cassegrain telescopes
  • Gascoigne , aspherical , single lens, for example in the Irenée du Pont and Henrietta Swope telescopes ,
  • Rosin, two-element,
  • Ross , two- or three-element, used in the 60- and 100-inch telescopes of the Mount Wilson Observatory ,
  • Wynne , three-element or four-element, for example in the 5-m Hale telescope and in the 2.3-m telescopes of the Vainu-Bappu Observatory and the Wyoming Infrared Observatory
  • Paul, a convex and a concave spherical mirror correct the coma and astigmatism of a parabolic primary mirror. Further developed by Baker, who also eliminates the curvature of field in the Paul Baker telescope using aspherical mirror surfaces. This configuration came in a 1.8 m telescope and is used in a modified form in the LSST .
  • Paracorr, two achromatic lenses for parabolic primary mirrors

These correctors are used, for example, in Newtonian telescopes to expand the field of view. It is particularly advantageous to combine correctors with a main mirror specially designed for this purpose and, if necessary, a secondary mirror, the former then having a hyperbolic shape instead of the parabolic one ; corresponding telescopes are also referred to as hypergraph .

Some newer correctors are used to upgrade existing large telescopes in order to carry out sky surveys with these . Due to the large main mirror and its shape not adapted to the corrector, they require large, spaced-apart lenses. The following table gives an overview of their dimensions. A detailed discussion can be found in

Surname telescope Aperture Field of view lenses diameter length camera date
WFI 2.2-m MPG / ESO 2.2 m 0.6 ° 6th ≈ 40 cm ≈ 35 cm 67 Mpix 1995
Megacam CFHT 3.6 m 1.4 ° 4th 81 cm 190 cm 340 Mpix 2003
WFCAM UKIRT 3.8 m 1.0 ° 3 + 1 mirror 81 cm 16 Mpix 2004
DECam Victor M. Blanco telescope 4.0 m 2.2 ° 5 93 cm 520 Mpix 2011
Hyper Suprime Cam Subaru telescope 8.2 m 1.5 ° 7 (including ADC) 82 cm 185 cm 889 Mpix 2011
WIYN-ODI WIYN-KPNO 3.5 m 1.4 ° 2 (+ 4 ADC) 63 cm 1024 Mpix 2011
Integrated corrector
SDSS 2.5 meter telescope 2.5 m 3 ° Cassegrain + 2/3 72 cm - 126 Mpix 1998
Pan-STARRS 1.8 m 3 ° Cass. + 3 50 cm - 1400 Mpix 2006
VISTA 4 m 1.4 ° Cass. + 3 52 cm - 67 Mpix 2008
VST 2.6 m 1.4 ° Cass. + 1 (+ 4 ADC)
or Cass. + 2		 46 cm
44 cm		 - 268 Mpix 2011
Space Surveillance Telescope 3.5 m 3.5 ° Paul-Baker +? ? - ? 2011
LSST 8.4 m 3.5 ° Paul-Baker + 3 162 cm - 3200 Mpix 2014

Spherical primary mirror

Another group of correctors is designed for spherical primary mirrors that are easier to manufacture:

  • meniscus
  • Jones-Bird, Doublet,

It turns out that with the simple lenses a good correction is only possible with smaller mirrors or apertures, for example for a diameter of 200 mm and an opening of 1: 5; with larger mirrors, for example an 800 mm F / 4, the image in the center will not be sharp either. While these lenses correct for spherical aberrations, they usually induce additional astigmatism , coma, and increased field curvature . These imaging errors that limit the image field can be corrected or reduced by using additional, spatially separated elements:

  • Jones-James
  • a diffusing and focusing lens or achromatic lens
  • Pankratz triplet and doublet, for a mirror with an aperture of 1: 2.13 and a diameter of 75 cm, achieves a focal length extension of 3.
  • GAnAs consisting of two aspherical plates and a meniscus lens for a 1 m mirror with 5 m focal length and an image field of 0.5 °.

For telescopes with a Cassegrain beam path, corrector lenses are also used near the secondary mirror, as already shown in the work of Sampson, in the Klevtsov telescope and in the Argunov telescope. A number of correctors have also been developed for Gregory telescopes with spherical mirrors. A simple spherical aberration corrector consists of a lens at the focus and a slightly tilted ellipsoidal secondary mirror that projects the image near the focus; In this arrangement, which is similar to Schupmann Medial telescopes , coma, astigmatism, and image field curvature and distortion are also eliminated. The Mertz corrector of the Arecibo radio telescope, constructed from two mirrors, and the correctors of the Hobby-Eberly telescope and the Southern African Large Telescope constructed from four mirrors also serve to compensate for the spherical aberration ; If necessary, this is followed by a seven-lens corrector to expand the image field. A newer design for the Hobby Eberly telescope promises a Mertz corrector, followed by an inverse Cassegrain arrangement, a correction over an image field of 18 arc minutes by 4 mirrors. An overview and further configurations are given by Ackermann et al.

Using a completely different approach, very large image angles can also be achieved with spherical main mirrors. The corrector consists of many small segments, which are made up of two aspherical mirrors arranged in pairs, segments of a Mertz corrector, and which compensate for the image errors in their small area. This makes it possible to construct a 30 m telescope with a 3 ° angle of view. Allan David Beach shows a post-focal lens corrector for the same mirror arrangement, which creates a new image and removes the spherical aberration with a meniscus lens and inserts an image lens like in a Schuppmann telescope. The relay optics for spherical mirrors in Cassegrain arrangement and in Newtonian telescopes by Michael Paramythioti, which are used in the Clavius ​​telescope, work in a similar way.

Atmospheric dispersion

Atmospheric dispersion at different tilt angles of a telescope
Atmospheric dispersion corrector through rotatable Amici prisms

If observations outside the zenith are carried out with a telescope , this leads to a splitting of the light by the atmosphere similar to the splitting of colors of a prism . The picture on the right illustrates this effect for three wavelengths. Atmospheric dispersion was observed by Airy in 1869 ; he and his assistant suggested various countermeasures. In modern telescopes, straight vision prisms according to Amici are often used, which by mutual rotation cause the opposite dispersion and compensate for the effect. In order to allow a small size, these are arranged close to the focus and are optionally combined there with further correction lenses.

credentials

  1. ^ A b c Ralph Allen Sampson: On a Cassegrain Reflector with Corrected Field. (PDF; 565 kB).
  2. ↑ Mark R. Ackermann, John T. McGraw, Peter C. Zimmer: An Overview of Wide-Field-Of-View Optical Designs for Survey Telescopes , Advanced Maui Optical and Space Surveillance Technologies Conference, 2010
  3. ^ Ralph Allen Sampson: A New Treatment of Optical Aberrations . In: Philosophical Transactions of the Royal Society of London. Series A, Containing Papers of a Mathematical or Physical Character . tape 212 , 1913, pp. 149–185 , JSTOR : 91051 ( rsta.royalsocietypublishing.org [PDF; accessed December 17, 2011]).
  4. ^ Ralph Allen Sampson: On correcting the field of a Newtonian telescope. bibcode : 1913MNRAS..73..524S
  5. ^ Sidney Charles Bartholemew Gascoigne: Some Recent Advances in Astronomical Optics. 1968, Quarterly Journal of the Royal Astronomical Society , vol. 9, p. 98. bibcode : 1968QJRAS ... 9R..98G
  6. ^ Seymour Rosin: Cassegrain tele-objective. , Patent US3274886.
  7. ^ Seymour Rosin: Ritchey Chrétien Corrector System . In: Applied Optics . tape 5 , no. 4 , March 1, 1966, p. 675-676 , doi : 10.1364 / AO.5.000675 .
  8. ↑ a b c d Sub aperture corrector (English).
  9. ↑ Frank Elmore Ross: LENS SYSTEM FOR CORRECTING COMA OF MIRRORS , Astrophysical Journal , Volume 81, p. 156. bibcode : 1935ApJ .... 81..156R
  10. ↑ Charles Gorrie Wynne: Ritchey-Chrétien Telescopes and extended Field Systems , 1968, Astrophysical Journal, Volume 152, p. 675, bibcode : 1968ApJ ... 152..675W
  11. ↑ CG Wynne: A new wide filed triple lens parabolid field corrector , 1974, Monthly Notices of the Royal Astronomical Society , Volume 167, pp. 189-198, bibcode : 1974MNRAS.167..189W
  12. ^ Maurice Paul: Systèmes correcteurs pour réflecteurs astronomiques , Revue d'optique, Volume 14, no. 5, pp. 169-202, May 1935.
  13. ↑ a b Mark R. Ackermann et al .: The Unique Optical Design of the NESSI Survey Telescope (PDF; 2.2 MB), 2006
  14. ↑ Tele Vue Optics: Paracorr
  15. ↑ Hypergraph ( Memento of the original from October 12, 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. @1@ 2Template: Webachiv / IABot / www.astrooptik.com
  16. ↑ The Wide Field Imager at the 2.2-m MPG / ESO Telescope: First Views with a 67-Million-Facette Eye. bibcode : 1999Msngr..95 ... 15B
  17. ↑ eso.org: WFI - Wide Field Image
  18. ↑ Mega Prime - Instrument Description
  19. ↑ UKIRT WFCAM - Optical Design
  20. ↑ K. Honscheid et al .: The Dark Energy Camera (DECam) , 2008, arxiv : 0810.3600
  21. ↑ Satoshi Miyazaki: Hyper Suprime-Cam (PDF; 4.7 MB), Cosmology Near and Far: Science with WFMOS, 2008
  22. ↑ George H. Jacoby et al .: The WIYN One Degree Imager Optical Design  ( page no longer available , search in web archives )  Info: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice. (PDF; 1.2 MB), 2008@1@ 2Template: Dead Link / www.wiyn.org  
  23. ↑ THE 2.5 m TELESCOPE OF THE SLOAN DIGITAL SKY SURVEY (PDF; 2.1 MB) Description of the SDSS telescope (English).
  24. ↑ What is Pan-STARRS? ( MS PowerPoint ; 11.8 MB) Discussion of the properties of the corrector of the Pan-STARRS and a further draft. (English).
  25. ↑ The VISTA IR Camera  ( page no longer available , search in web archives )  Info: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice. Description of the camera and the corrector of the VISTA@1@ 2Template: Dead Link / www.roe.ac.uk  
  26. ↑ VST FINAL OPTICS DESIGN SUMMARY FOR THE WHOLE SYSTEM ( Memento of the original from July 18, 2004 in the Internet Archive ) Info: The archive link was automatically inserted and not yet checked. Please check the original and archive link according to the instructions and then remove this notice. ( MS Word ; 735 kB) @1@ 2Template: Webachiv / IABot / www.astro-wise.org
  27. ^ Large refractive corrector producing a 3.5 degree field of view , corrector of the Large Synoptic Survey Telescope
  28. ↑ Thomas Bird, Alfred V. Bowen: A Compact All-Spherical Catadioptric Newtonian Telescope ( Memento of the original from December 16, 2009 in the Internet Archive ) Info: The archive link was automatically inserted and not yet checked. Please check the original and archive link according to the instructions and then remove this notice. , Telescope Making No. 03 (Spring 1979) @1@ 2Template: Webachiv / IABot / www.kalmbachstore.com
  29. ↑ a b c Herbert Gross, Fritz Blechinger, Bertram Achtner: Handbook of Optical Systems, Survey of Optical Instruments p. 846 ( limited preview in the Google book search)
  30. ^ Damien J. Jones, William E. James: Prime focus correctors for the spherical mirror . In: Applied Optics . tape 31 , no. 22 , 1992, pp. 4384-4388 , doi : 10.1364 / AO.31.004384 .
  31. ↑ Patent US4881801 : Fast, aberration-free flat field catadioptric telescope. Inventor: Rolin J. Gebelein. ‌
  32. ↑ Patent DE60121561T2: reflecting telescope , inventor: Peter Wise.
  33. ↑ Michael Pankratz: Icarus750 - Optical Design
  34. ↑ F. Della Prugna, H. Schenner: GANAS: A HYBRID ANASTIGMATIC ASPHERICAL PRIME-FOCUS CORRECTOR (PDF; 88 kB), 2009RMxAC..35..259D
  35. ↑ PS Argunov: isochromatic telescope designs with spherical optics. Astron. Vestn. 6 (1), 52 (1972)
  36. ↑ Pavol A. Dubovský: Astronomical Observatory on Kolonica Saddle ( Memento of the original from February 22, 2014 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. (English). @1@ 2Template: Webachiv / IABot / www.astrokolonica.sk
  37. ↑ Peter Ceravolo: All-Spherical Catadioptric Gregorian Optical Designs for Meter Class Telescopes (PDF; 169 kB)
  38. ^ John J. Villa: Adaptation of the Schupmann Medial Telescope to a Large Scale Astronomical Optical System . In: Applied Optics . tape 11 , no. 8 , August 1972, ISSN  1539-4522 , p. 1814-1821 , doi : 10.1364 / AO.11.001814 .
  39. ↑ Michael Bass (Ed.): Handbook of optics. Volume 2.
  40. ↑ Handout (PDF; 6.5 MB) p. 7 (English).
  41. ↑ SALTICAM (English).
  42. ↑ John A. Booth et al .: The wide field upgrade for the Hobby-Eberly Telescope (PDF; 1.6 MB), Proc. SPIE, volume 6267, 2006
  43. ↑ Mark R. Ackermann, John T. McGraw, Peter C. Zimmer: Improved Spherical Aberration Corrector for fast spherical primary mirrors  ( page no longer available , search in web archives )  Info: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice. (PDF; 1.2 MB).@1@ 2Template: Dead Link / 144.206.159.178  
  44. ↑ Burge, J. H., Angel, J. R. P .: A 30 meter Cassegrain telescope with spherical optics and a 3 ° field ( Memento of the original from July 23, 2010 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. , Proc. SPIE Volume 4840, 2003. @1@ 2Template: Webachiv / IABot / www.optics.arizona.edu
  45. ↑ Beach, AD: KiwiStar: a design system for ultrafast high-resolution broad-spectrum wide-angle catadioptric lenses , bibcode : 1997SPIE.3130 ... 13B .
  46. ↑ Allan David Beach: Reflector Telescope Lens System , patent US5734496.
  47. ↑ Michel Paramythioti, Paul-Louis Vinel: Clavius: an operational concept in relay telescopes , bibcode : 2003SPIE.4842..106P
  48. ↑ Michel Paramythioti: Aberration correcting optical relay for optical system, in particular mirror telescope , patent US2003021024.
  49. ↑ CG Wynne: An new Form of Atmospheric Dispersion Corrector , bibcode : 1993MNRAS.262..741W
  50. ↑ eso.org: FEROS - The Fiber-fed Extended Range Optical Spectrograph

Web links