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44-598: The Anglo-Australian Telescope ( AAT ) is a 3.9-metre equatorially mounted telescope operated by the Australian Astronomical Observatory and situated at the Siding Spring Observatory , Australia, at an altitude of a little over 1,100 m. In 2009, the telescope was ranked as having the fifth-highest-impact of the world's optical telescopes. In 2001–2003, it was considered the most scientifically productive 4-metre-class optical telescope in

88-420: A "wedge". Many mid-size professional telescopes also have equatorial forks , these are usually in range of 0.5-2.0 meter diameter. The English mount or Yoke mount has a frame or " yoke " with right ascension axis bearings at the top and the bottom ends, and a telescope attached inside the midpoint of the yoke allowing it to swing on the declination axis. The telescope is usually fitted entirely inside

132-562: A close second to the Víctor M. Blanco Telescope from 1976 until 1998, when the first ESO Very Large Telescope (VLT) was opened. The AAT was credited with stimulating a resurgence in British optical astronomy. It was built by the United Kingdom in partnership with Australia but has been entirely funded by Australia since 2010. Observing time is available to astronomers worldwide. The AAT was one of

176-403: A parabola, K 1 = − 1 {\displaystyle K_{1}=-1} . Thanks to that there is no spherical aberration introduced by the primary mirror. The secondary mirror, however, is of a hyperbolic shape with one focus coinciding with that of the primary mirror and the other focus being at the back focal length B {\displaystyle B} . Thus,

220-704: A port for autoguiding. A special instrument tracks a star and makes adjustment in the telescope's position while photographing the sky. To do so the autoguider must be able to issue commands through the telescope's control system. These commands can compensate for very slight errors in the tracking performance, such as periodic error caused by the worm drive that makes the telescope move. In new observatory designs, equatorial mounts have been out of favor for decades in large-scale professional applications. Massive new instruments are most stable when mounted in an alt-azimuth (up down, side-to-side) configuration. Computerized tracking and field-derotation are not difficult to implement at

264-437: A reflecting telescope included a Cassegrain configuration, judging by a convex secondary mirror found among his experiments. The Cassegrain design is also used in catadioptric systems . The "classic" Cassegrain has a parabolic primary mirror and a hyperbolic secondary mirror that reflects the light back down through a hole in the primary. Folding the optics makes this a compact design. On smaller telescopes, and camera lenses,

308-421: A right ascension axis at its base. The telescope is attached to two pivot points at the other end of the fork so it can swing in declination. Most modern mass-produced catadioptric reflecting telescopes (200 mm or larger diameter) tend to be of this type. The mount resembles an Altazimuth mount , but with the azimuth axis tilted and lined up to match earth rotation axis with a piece of hardware usually called

352-436: A single point, the focus. A convex hyperbolic reflector has two foci and will reflect all light rays directed at one of its two foci towards its other focus. The mirrors in this type of telescope are designed and positioned so that they share one focus and so that the second focus of the hyperbolic mirror will be at the same point at which the image is to be observed, usually just outside the eyepiece. In most Cassegrain systems,

396-611: A telescope design based on the American Kitt Peak telescope until its deficiencies were known. Both the horseshoe mount and the gearing system needed improvements. Although the revised gear system was considerably more expensive it was significantly more accurate, lending itself well to future applications. The mirror blank was made by Owens-Illinois in Toledo, Ohio. It was then transported to Newcastle, England, where Sir Howard Grubb, Parsons and Co took two years to grind and polish

440-405: A worm and ring gear system driven by servo or stepper motors, and the operator need not touch the instrument at all to change its position in the sky. The computers in these systems are typically either hand-held in a control "paddle" or supplied through an adjacent laptop computer which is also used to capture images from an electronic camera. The electronics of modern telescope systems often include

484-406: Is a Schmidt corrector plate . The plate is figured by placing a vacuum on one side, and grinding the exact correction required to correct the spherical aberration caused by the spherical primary mirror. Schmidt-Cassegrains are popular with amateur astronomers. An early Schmidt-Cassegrain camera was patented in 1946 by artist/architect/physicist Roger Hayward , with the film holder placed outside

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528-407: Is a specialized Cassegrain reflector which has two hyperbolic mirrors (instead of a parabolic primary). It is free of coma and spherical aberration at a flat focal plane, making it well suited for wide field and photographic observations. It was invented by George Willis Ritchey and Henri Chrétien in the early 1910s. This design is very common in large professional research telescopes, including

572-436: Is installed. Equatorial telescope mounts come in many designs. In the last twenty years motorized tracking has increasingly been supplemented with computerized object location. There are two main types. Digital setting circles take a small computer with an object database that is attached to encoders. The computer monitors the telescope's position in the sky. The operator must push the telescope. Go-to systems use (in most cases)

616-468: Is narrow. The dome is required to move with the telescope to avoid obstruction. The top of the dome is 50m above ground level. The telescope tube structure is supported inside a massive 12m diameter horseshoe, which rotates around the polar axis (parallel to Earth's axis) for tracking the sky. The total moving mass is 260 tonnes. The telescope has various foci for flexible instrumentation: originally there were three top-end rings which can be exchanged using

660-528: Is the Schiefspiegler telescope ("skewed" or "oblique reflector"; also known as the "Kutter telescope" after its inventor, Anton Kutter ) which uses tilted mirrors to avoid the secondary mirror casting a shadow on the primary. However, while eliminating diffraction patterns this leads to several other aberrations that must be corrected. Several different off-axis configurations are used for radio antennas. Another off-axis, unobstructed design and variant of

704-666: The Hubble Space Telescope , the Keck Telescopes , and the Very Large Telescope (VLT); it is also found in high-grade amateur telescopes. The Dall-Kirkham Cassegrain telescope design was created by Horace Dall in 1928 and took on the name in an article published in Scientific American in 1930 following discussion between amateur astronomer Allan Kirkham and Albert G. Ingalls, the magazine's astronomy editor at

748-781: The Ritchey–Chrétien design ); and either or both mirrors may be spherical or elliptical for ease of manufacturing. The Cassegrain reflector is named after a published reflecting telescope design that appeared in the April 25, 1672 Journal des sçavans which has been attributed to Laurent Cassegrain . Similar designs using convex secondary mirrors have been found in the Bonaventura Cavalieri 's 1632 writings describing burning mirrors and Marin Mersenne 's 1636 writings describing telescope designs. James Gregory 's 1662 attempts to create

792-463: The focal point at a convenient location behind the primary mirror and the convex secondary adds a telephoto effect creating a much longer focal length in a mechanically short system. In a symmetrical Cassegrain both mirrors are aligned about the optical axis , and the primary mirror usually contains a hole in the center, thus permitting the light to reach an eyepiece , a camera , or an image sensor . Alternatively, as in many radio telescopes,

836-595: The 2dF instrument and its later enhancements AAOmega and HERMES. The AAT is equipped with a number of instruments, including: The newest instrument, HERMES, was commissioned in 2015. It is a new high-resolution spectrograph to be used with the 2dF fibre positioner. HERMES is mainly being used for the 'Galactic Archaeology with Hermes' (GALAH) Survey, which aims to reconstruct the history of our galaxy's formation from precise multi-element (~25 elements) abundances of 1  million stars derived from HERMES spectra. Equatorial mount In astronomical telescope mounts ,

880-546: The Cassegrain is the ' Yolo ' reflector invented by Arthur Leonard. This design uses a spherical or parabolic primary and a mechanically warped spherical secondary to correct for off-axis induced astigmatism. When set up correctly the Yolo can give uncompromising unobstructed views of planetary objects and non-wide field targets, with no lack of contrast or image quality caused by spherical aberration. The lack of obstruction also eliminates

924-649: The Southern Hemisphere in 1959. In 1965, Macfarlane Burnet , president of the Australian Academy of Science , wrote to the federal education minister John Gorton inviting the federal government to support a joint British-Australian telescope project. Gorton was supportive, and nominated the Australian National University and CSIRO as Australia's representatives in the joint venture; he was unsuccessful in his attempts to induce NASA to join

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968-457: The classical Cassegrain has ideal focus for the chief ray (the center spot diagram is one point). We have, where Actually, as the conic constants should not depend on scaling, the formulae for both α {\displaystyle \alpha } and K 2 {\displaystyle K_{2}} can be greatly simplified and presented only as functions of the secondary magnification. Finally, and The Ritchey-Chrétien

1012-715: The classical Cassegrain secondary mirror is replaced by a sub-aperture corrector consisting of three air spaced lens elements. The element farthest from the primary mirror is a Mangin mirror , which acts as a secondary mirror. The Klevtsov-Cassegrain, like the Argunov-Cassegrain, uses a sub-aperture corrector consisting of a small meniscus lens and a Mangin mirror as its "secondary mirror". Cassegrain designs are also utilized in satellite telecommunication earth station antennas and radio telescopes , ranging in size from 2.4 metres to 70 metres. The centrally located sub-reflector serves to focus radio frequency signals in

1056-461: The common Dobsonian telescope type, to overcome that type of mount's inability to track the night sky. Cassegrain reflector The Cassegrain reflector is a combination of a primary concave mirror and a secondary convex mirror , often used in optical telescopes and radio antennas , the main characteristic being that the optical path folds back onto itself, relative to the optical system's primary mirror entrance aperture. This design puts

1100-447: The diffraction associated with Cassegrain and Newtonian reflector astrophotography. Catadioptric Cassegrains use two mirrors, often with a spherical primary mirror to reduce cost, combined with refractive corrector element(s) to correct the resulting aberrations. The Schmidt-Cassegrain was developed from the wide-field Schmidt camera , although the Cassegrain configuration gives it a much narrower field of view. The first optical element

1144-405: The distance to the focus behind the primary mirror, b {\displaystyle b} , then D = f 1 ( F − b ) / ( F + f 1 ) {\displaystyle D=f_{1}(F-b)/(F+f_{1})} and B = D + b {\displaystyle B=D+b} . The conic constant of the primary mirror is that of

1188-422: The dome crane during the daytime. One was for f/3.3 prime-focus, with corrector lenses and a cage for a human observer taking photographs (rarely used after the 1980s); one has a large secondary mirror giving an f/8 Cassegrain focus; and a third top-end has smaller f/15 and f/36 secondary mirrors. A fourth top-end was built in the 1990s to give a 2-degree field of view at prime focus, with 400 optical fibres feeding

1232-411: The equatorial axis (the right ascension ) is paired with a second perpendicular axis of motion (known as the declination ). The equatorial axis of the mount is often equipped with a motorized " clock drive ", that rotates that axis one revolution every 23 hours and 56 minutes in exact sync with the apparent diurnal motion of the sky. They may also be equipped with setting circles to allow for

1276-421: The final focus may be in front of the primary. In an asymmetrical Cassegrain, the mirror(s) may be tilted to avoid obscuration of the primary or to avoid the need for a hole in the primary mirror (or both). The classic Cassegrain configuration uses a parabolic reflector as the primary while the secondary mirror is hyperbolic . Modern variants may have a hyperbolic primary for increased performance (for example,

1320-551: The fork, although there are exceptions such as the Mount Wilson 2.5 m reflector , and there are no counterweights as with the German mount . The original English fork design is disadvantaged in that it does not allow the telescope to point too near the north or south celestial pole. The horseshoe mount overcomes the design disadvantage of English or Yoke mounts by replacing the polar bearing with an open "horseshoe" structure to allow

1364-404: The last large telescopes built with an equatorial mount . More recent large telescopes have instead adopted the more compact and mechanically stable altazimuth mount . The AAT was, however, one of the first telescopes to be fully computer-controlled, and set new standards for pointing and tracking accuracy. British astronomer Richard van der Riet Woolley pushed for a large optical telescope for

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1408-437: The location of objects by their celestial coordinates . Equatorial mounts differ from mechanically simpler altazimuth mounts , which require variable speed motion around both axes to track a fixed object in the sky. Also, for astrophotography , the image does not rotate in the focal plane , as occurs with altazimuth mounts when they are guided to track the target's motion, unless a rotating erector prism or other field-derotator

1452-420: The mirror's surface. Mitsubishi Electric built the mount which was constructed by August 1973. First light occurred on 27 April 1974. The telescope was officially opened by Prince Charles on 16 October 1974. The telescope is housed within a seven-story, circular, concrete building topped with a 36m diameter rotating steel dome. It was designed to withstand the high winds prevailing at that location. The slit

1496-444: The other. An equatorial platform is a specially designed platform that allows any device sitting on it to track on an equatorial axis. It achieves this by having a surface that pivots about a "virtual polar axis". This gives equatorial tracking to anything sitting on the platform, from small cameras up to entire observatory buildings. These platforms are often used with altazimuth mounted amateur astronomical telescopes, such as

1540-405: The professional level. At the amateur level, however, equatorial mounts remain popular, particularly for astrophotography. In the German equatorial mount , (sometimes called a " GEM " for short) the primary structure is a T -shape, where the lower bar is the right ascension axis (lower diagonal axis in image), and the upper bar is the declination axis (upper diagonal axis in image). The mount

1584-462: The project. Gorton brought the proposal before cabinet in April 1967, which endorsed the scheme and agreed to contribute half the capital and running costs. An agreement with the British was finalised a few weeks later and a Joint Policy Committee started work on construction planning in August 1967. It took until September 1969 for plans to be finalised. The agreement initially committed the specification to

1628-443: The secondary (the spider) may introduce diffraction spikes in images. The radii of curvature of the primary and secondary mirrors, respectively, in the classic configuration are and where If, instead of B {\displaystyle B} and D {\displaystyle D} , the known quantities are the focal length of the primary mirror, f 1 {\displaystyle f_{1}} , and

1672-513: The secondary is often mounted on an optically flat, optically clear glass plate that closes the telescope tube. This support eliminates the "star-shaped" diffraction effects caused by a straight-vaned support spider. The closed tube stays clean, and the primary is protected, at the cost of some loss of light-gathering power. It makes use of the special properties of parabolic and hyperbolic reflectors. A concave parabolic reflector will reflect all incoming light rays parallel to its axis of symmetry to

1716-412: The secondary mirror blocks a central portion of the aperture. This ring-shaped entrance aperture significantly reduces a portion of the modulation transfer function (MTF) over a range of low spatial frequencies, compared to a full-aperture design such as a refractor or an offset Cassegrain. This MTF notch has the effect of lowering image contrast when imaging broad features. In addition, the support for

1760-417: The telescope to access Polaris and stars near it. The Hale Telescope is the most prominent example of a horseshoe mount in use. The Cross-axis or English cross axis mount is like a big "plus" sign ( + ). The right ascension axis is supported at both ends, and the declination axis is attached to it at approximately midpoint with the telescope on one end of the declination axis and a counter weight on

1804-557: The telescope. The Maksutov-Cassegrain is a variation of the Maksutov telescope named after the Soviet / Ukrainian optician and astronomer Dmitri Dmitrievich Maksutov . It starts with an optically transparent corrector lens that is a section of a hollow sphere. It has a spherical primary mirror, and a spherical secondary that is usually a mirrored section of the corrector lens. In the Argunov-Cassegrain telescope all optics are spherical, and

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1848-405: The time. It uses a concave elliptical primary mirror and a convex spherical secondary. While this system is easier to polish than a classic Cassegrain or Ritchey-Chretien system, the off-axis coma is significantly worse, so the image degrades quickly off-axis. Because this is less noticeable at longer focal ratios , Dall-Kirkhams are seldom faster than f/15. An unusual variant of the Cassegrain

1892-467: The world based on scientific publications using data from the telescope. The telescope was commissioned in 1974 with a view to allowing high-quality observations of the sky from the Southern Hemisphere. At the time, most major telescopes were located in the Northern Hemisphere, leaving the southern skies poorly observed. It was the largest telescope in the Southern Hemisphere from 1974 to 1976, then

1936-519: Was developed by Joseph von Fraunhofer for the Great Dorpat Refractor that was finished in 1824. The telescope is placed on one end of the declination axis (top left in image), and a suitable counterweight on other end of it (bottom right). The right ascension axis has bearings below the T-joint, that is, it is not supported above the declination axis. The Open Fork mount has a Fork attached to

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