Primary mirror
In a mirror telescope or a catadioptric telescope, the main mirror (also known as the primary mirror ) is the first optically effective mirror that the light coming from the object hits. Usually it is ground as a paraboloid , as with the Newton telescope , with special optics such as the Schmidt camera it is also as a spherical mirror or in a hyperbolic shape.
In telescopes for optical wavelengths (light, UV, near infrared), main mirrors are mostly made of glass or glass ceramic today . Until around 1900, metal mirrors were mainly used because the problem of streaks was still unsolved in larger glass molds. Parabolic mirrors can also be implemented as liquid mirrors .
In SLR cameras , the primary mirror is the flat, partially transparent mirror that directs the light into the viewfinder and, when folded away, onto the film or the CCD array. The auxiliary mirror for the autofocus is located behind him .
history
In the early years of the mirror telescopes invented by Newton , mirrors were made of mirror metal. However, because the metal oxidized quickly, these mirrors often had to be repolished. The smooth surface produced was easily deteriorated and the exact surface shape was changed. That is why glass was used as a carrier, which was mirrored with silver. Today's telescope mirrors are vaporized with a thin aluminum layer in a high vacuum and provided with a quartz protective layer to protect against rapid blindness.
The huge mirrors of today's large telescopes (up to 10 meters in diameter) are no longer cast in one piece, but are made up of hundreds of computer-controlled segments. The largest individual mirrors are those on Mount Palomar , USA (5 meters, 1947) and at the Selentschuk Observatory , Russia (6 meters, 1975/78).
Beam path and imaging errors
The mirrors of very small telescopes are concave mirrors in the form of pure spherical surfaces . A spherical mirror does not collect parallel light rays exactly in one point, but in a spatial extension along the longitudinal axis of the focal point (so-called "focal line" ). For larger mirrors, a paraboloid of revolution is therefore produced, which really collects the light rays in one point. Nowadays, very large telescopes are usually built as Ritchey-Chrétien telescopes , in which the main mirror is hyperbolically deformed - the secondary mirror, by the way, in addition to the hyperbolic shape that it already needs for its task in the Cassegrain system.
Manufacturing
In amateur astronomy, borosilicate glass , which has a very low coefficient of expansion , is usually used as the mirror material . The glass blanks used to be made by pressing or casting in metal molds. Today, borosilicate float glass (e.g. Borofloat ) with a thickness of 25 mm is produced from which the glass blanks are cut.
The large mirrors used in astronomical research today, on the other hand, are mostly made of glass ceramic . The mirror blanks are melted into shape directly from broken glass in special rotary kilns . The furnace rotates at the defined speed that creates the desired paraboloid shape. When the molten glass is cooled, the temperature profile is regulated in such a way that ceramic crystallization results in a mixture of 60 percent ceramic and 40 percent glass. The negative expansion coefficient of the ceramic cancels out the positive of the glass, so that there is practically no thermal expansion at all. In order to achieve freedom from tension and to crystallize out the ceramic component, the cooling process takes a correspondingly long time.
Once the mirror has cooled down, the final shape can be ground and polished. When polishing , a surface accuracy below lambda / 2 (half the wavelength in which the observation is to be carried out later), but usually better than lambda / 8, must be achieved. Professionally used mirrors are manufactured with an accuracy of up to 20 nanometers.
Small primary mirrors, which can be manufactured with a diameter to thickness ratio of 10: 1, are inherently dimensionally stable. From a diameter of 50 cm, however, such mirrors are quite heavy. However, if you make them thinner, they bend due to their own weight when the position changes. The effect is less than with lens deflection , but it is noticeable.
Larger mirrors used to be drilled out from the rear to reduce weight. Today the mirrors are cast with a honeycomb structure on the back, which reduces the weight by over 60%. The Richard F. Caris Mirror Lab in Arizona produces a large number of the mirrors with a diameter of 8.4 m and a honeycomb structure that are used in large telescopes today. The mirrors are made of borosilicate glass and are cast while rotating to create a paraboloid surface, which reduces the grinding process.
Segmented mirrors
Solid mirrors with a diameter of more than 6 m (as most recently at the Selentschuk Observatory in 1978 ) are no longer manufactured due to the deformation caused by their own weight. That is why, in the 1980s, several mirrors with an aperture of 8 to 10 meters were initially made from dozen segments. These hexagonal , up to 1.5 m large segments were positioned by the bracket (statically) in such a way that a flawless image is created. Today there are active bearings that dynamically support the mirror at many support points and thus compensate for the bending caused by its own weight or assembly errors. This correction is also dependent on the respective elevation angle of the telescope. In addition, the adaptive optics was developed to compensate for interference caused by the unrest in the air .
literature
- Seb.Hoerner, Karl Schaifers : Optical systems . Chapter 2.1 in Meyer's handbook on space , Bibliographic Institute, Mannheim 1960
- W. Jahn: The optical observation instruments, pp. 9–79 in the manual for star friends , Springer-Verlag 1981
- Rudolf Brandt : The telescope of the star friend . Kosmos-Verlag, Stuttgart 1958
- Detlev Block: Astronomy as a Hobby , Section Telescopes (pp. 144–156). Bassermann-Verlag, Munich / Tešin 2005
- Günter D. Roth : Cosmos of astronomy history - astronomers, instruments, discoveries . 190 p., Franckh-Kosmos, Stuttgart 1989, ISBN 978-3-440-05800-8 .
- Bernhard Mackowiak: The new super telescopes - subtitle: To be able to look deeper and deeper into space, the telescopes have to get bigger and bigger. But there are technical limits when it comes to lenses. The telescopes of the future will be composed of many smaller ones . Welt Online, September 30, 2006, Axel Springer, Berlin 2006.
Individual evidence
- ↑ Liquid telescope mirrors . In: Spectrum of Science . April 1994, p. 70 ( Spektrum.de ).
- ↑ Concave mirror, basis of modern telescopes ( Memento of the original from October 15, 2012 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.
- ↑ Aluminum vapor deposition for telescope mirrors , Berlin planetarium
- ↑ Differentiation of the mirror materials
- ↑ on alluna-optics.de ( 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.
- ↑ People: Physics and its Applications in Technology and Environment , page 385
- ↑ Jürgen Gobrecht, Erhard Rumpler: Material Technology - Metals , page 261
- ↑ Quality mirror production on alluna-optics.de ( Memento of the original from January 11, 2011 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.
- ^ Richard F. Caris Mirror Lab
