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58-550: The Hobby–Eberly Telescope is one of the largest optical telescopes in the world. It combines a number of features that differentiate it from most telescope designs, resulting in lowered construction costs: The telescope is named for former Texas Lieutenant-Governor Bill Hobby and for Robert E. Eberly , a Penn State benefactor. Three instruments are available to analyze the light from the targets. All three instruments are spectrographs . The instruments work at high, medium and low spectral resolution. The low-resolution spectrograph

116-484: A coudé train , diverting the light to a fixed position to such an instrument housed on or below the observing floor (and usually built as an unmoving integral part of the observatory building) was the only option. The 60-inch Hale telescope (1.5 m), Hooker Telescope , 200-inch Hale Telescope , Shane Telescope , and Harlan J. Smith Telescope all were built with coudé foci instrumentation. The development of echelle spectrometers allowed high-resolution spectroscopy with

174-476: A spherical or parabolic shape. A thin layer of aluminum is vacuum deposited onto the mirror, forming a highly reflective first surface mirror . Some telescopes use primary mirrors which are made differently. Molten glass is rotated to make its surface paraboloidal, and is kept rotating while it cools and solidifies. (See Rotating furnace .) The resulting mirror shape approximates a desired paraboloid shape that requires minimal grinding and polishing to reach

232-439: A combination of curved mirrors that reflect light and form an image . The reflecting telescope was invented in the 17th century by Isaac Newton as an alternative to the refracting telescope which, at that time, was a design that suffered from severe chromatic aberration . Although reflecting telescopes produce other types of optical aberrations , it is a design that allows for very large diameter objectives . Almost all of

290-423: A concave bronze mirror in 1616, but said it did not produce a satisfactory image. The potential advantages of using parabolic mirrors , primarily reduction of spherical aberration with no chromatic aberration , led to many proposed designs for reflecting telescopes. The most notable being James Gregory , who published an innovative design for a ‘reflecting’ telescope in 1663. It would be ten years (1673), before

348-455: A corrector plate) as primary optical elements, mainly used for wide-field imaging without spherical aberration. The late 20th century has seen the development of adaptive optics and lucky imaging to overcome the problems of seeing , and reflecting telescopes are ubiquitous on space telescopes and many types of spacecraft imaging devices. A curved primary mirror is the reflector telescope's basic optical element that creates an image at

406-402: A different meridional path. Stevick-Paul telescopes are off-axis versions of Paul 3-mirror systems with an added flat diagonal mirror. A convex secondary mirror is placed just to the side of the light entering the telescope, and positioned afocally so as to send parallel light on to the tertiary. The concave tertiary mirror is positioned exactly twice as far to the side of the entering beam as

464-412: A given size of primary, and is popular with amateur telescope makers as a home-build project. The Cassegrain telescope (sometimes called the "Classic Cassegrain") was first published in a 1672 design attributed to Laurent Cassegrain . It has a parabolic primary mirror, and a hyperbolic secondary mirror that reflects the light back down through a hole in the primary. The folding and diverging effect of

522-571: A more traditional design. As reported in Nature of 28 November 2012, astronomers have used the Hobby–Eberly Telescope to measure the mass of an extraordinarily large black hole (with mass approximates 17 billion Suns), possibly the largest black hole found so far. It has been found in the compact, lenticular galaxy NGC 1277 , which lies 220 million light-years away in the constellation Perseus . The black hole has approximately 59 percent of

580-526: A much more compact instrument, one which can sometimes be successfully mounted on the Cassegrain focus. Since inexpensive and adequately stable computer-controlled alt-az telescope mounts were developed in the 1980s, the Nasmyth design has generally supplanted the coudé focus for large telescopes. For instruments requiring very high stability, or that are very large and cumbersome, it is desirable to mount

638-408: A narrower field of view than a Nasmyth focus and is used with very heavy instruments that do not need a wide field of view. One such application is high-resolution spectrographs that have large collimating mirrors (ideally with the same diameter as the telescope's primary mirror) and very long focal lengths. Such instruments could not withstand being moved, and adding mirrors to the light path to form

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696-477: A narrower range of observations. When the two mirrors are on one mount, the combined mirror spacing of the Large Binocular Telescope (22.8 m) allows fuller use of the aperture synthesis. Largest does not always equate to being the best telescopes, and overall light gathering power of the optical system can be a poor measure of a telescope's performance. Space-based telescopes , such as

754-560: A shadow on the primary. However, while eliminating diffraction patterns this leads to an increase in coma and astigmatism. These defects become manageable at large focal ratios — most Schiefspieglers use f/15 or longer, which tends to restrict useful observations to objects which fit in a moderate field of view. A 6" (150mm) f/15 telescope offers a maximum 0.75 degree field of view using 1.25" eyepieces. A number of variations are common, with varying numbers of mirrors of different types. The Kutter (named after its inventor Anton Kutter ) style uses

812-458: A single combined image are ranked by their equivalent aperture. Fixed altitude telescopes (e.g. HET) are also ranked by their equivalent aperture. All telescopes with an effective aperture of at least 3.00 metres (118 in) at visible or near-infrared wavelengths are included. (256 in) There are only a few sites capable of polishing the mirrors for these telescopes. SAGEM in France polished

870-450: A single concave primary, a convex secondary and a plano-convex lens between the secondary mirror and the focal plane, when needed (this is the case of the catadioptric Schiefspiegler ). One variation of a multi-schiefspiegler uses a concave primary, convex secondary and a parabolic tertiary. One of the interesting aspects of some Schiefspieglers is that one of the mirrors can be involved in the light path twice — each light path reflects along

928-500: A small diagonal mirror in an optical configuration that has come to be known as the Newtonian telescope . Despite the theoretical advantages of the reflector design, the difficulty of construction and the poor performance of the speculum metal mirrors being used at the time meant it took over 100 years for them to become popular. Many of the advances in reflecting telescopes included the perfection of parabolic mirror fabrication in

986-400: A spectacle correcting lens is added between the secondary mirror and the focal plane ( catadioptric Yolo ). The addition of a convex, long focus tertiary mirror leads to Leonard's Solano configuration. The Solano telescope doesn't contain any toric surfaces. One design of telescope uses a rotating mirror consisting of a liquid metal in a tray that is spun at constant speed. As the tray spins,

1044-463: Is 30 arcminutes for comparison). The telescope mirrors are aligned within a fraction of a wavelength of visible light by actuators under each segment. The tower next to the telescope, called the Center of Curvature Alignment Sensor Tower (CCAS), is used to calibrate the mirror segments. One of the advantages of this telescope design is that it was over 5 times more cost efficient for its aperture size than

1102-558: Is housed at the prime focus, while the medium and high-resolution spectrographs reside in the basement and the light is fed into them via a fiber-optic cable. Since achieving first light in 1996, the telescope has been used for a wide variety of studies ranging from the Solar System to stars in our galaxy and studies of other galaxies . The telescope has been used successfully to find planets orbiting around other stars by measuring radial velocities as precisely as 1 m/s. Using

1160-512: Is larger than 10 meters; it is actually about 11 m by 9.8 m. Upon first light, the usable optical aperture at any given time was 9.2 m. After a multi-year upgrade completed on July 29, 2015, the usable optical aperture was increased to 10m. The mirror itself is composed of 91 hexagonal segments, a segmented mirror design like the Keck telescopes . Updates to the telescope have increased its field of view from 4 arcminutes to 22 arcminutes (a full moon

1218-413: Is reduced by deformation of the secondary mirror by some form of warping harness, or alternatively, polishing a toroidal figure into the secondary. Like Schiefspieglers, many Yolo variations have been pursued. The needed amount of toroidal shape can be transferred entirely or partially to the primary mirror. In large focal ratios optical assemblies, both primary and secondary mirror can be left spherical and

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1276-578: Is replaced by a metal surface for reflecting radio waves , and the observer is an antenna . For telescopes built to the Cassegrain design or other related designs, the image is formed behind the primary mirror, at the focal point of the secondary mirror . An observer views through the rear of the telescope, or a camera or other instrument is mounted on the rear. Cassegrain focus is commonly used for amateur telescopes or smaller research telescopes. However, for large telescopes with correspondingly large instruments, an instrument at Cassegrain focus must move with

1334-572: Is sorted by aperture , which is a measure of the light-gathering power and resolution of a reflecting telescope . The mirrors themselves can be larger than the aperture, and some telescopes may use aperture synthesis through interferometry . Telescopes designed to be used as optical astronomical interferometers such as the Keck I and II used together as the Keck Interferometer (up to 85 m) can reach higher resolutions, although at

1392-721: The Hubble Space Telescope , take advantage of being above the Earth's atmosphere to reach higher resolution and greater light gathering through longer exposure times. Location in the northern or southern hemisphere of the Earth can also limit what part of the sky can be observed, and climate conditions at the observatory site affect how often the telescope can be used each year. The combination of large mirrors, locations selected for stable atmosphere and favorable climate conditions, and  active optics and adaptive optics to correct for much of atmospheric turbulence allow

1450-493: The Ritchey–Chrétien telescope ) or some form of correcting lens (such as catadioptric telescopes ) that correct some of these aberrations. Nearly all large research-grade astronomical telescopes are reflectors. There are several reasons for this: The Gregorian telescope , described by Scottish astronomer and mathematician James Gregory in his 1663 book Optica Promota , employs a concave secondary mirror that reflects

1508-465: The 18th century, silver coated glass mirrors in the 19th century (built by Léon Foucault in 1858), long-lasting aluminum coatings in the 20th century, segmented mirrors to allow larger diameters, and active optics to compensate for gravitational deformation. A mid-20th century innovation was catadioptric telescopes such as the Schmidt camera , which use both a spherical mirror and a lens (called

1566-547: The Petzval surface which is gently curved. The Yolo was developed by Arthur S. Leonard in the mid-1960s. Like the Schiefspiegler, it is an unobstructed, tilted reflector telescope. The original Yolo consists of a primary and secondary concave mirror, with the same curvature, and the same tilt to the main axis. Most Yolos use toroidal reflectors . The Yolo design eliminates coma, but leaves significant astigmatism, which

1624-518: The center of the mirror, a defect called spherical aberration . To avoid this problem most reflecting telescopes use parabolic shaped mirrors , a shape that can focus all the light to a common focus. Parabolic mirrors work well with objects near the center of the image they produce, (light traveling parallel to the mirror's optical axis ), but towards the edge of that same field of view they suffer from off axis aberrations: There are reflecting telescope designs that use modified mirror surfaces (such as

1682-402: The exact figure needed. Reflecting telescopes, just like any other optical system, do not produce "perfect" images. The need to image objects at distances up to infinity, view them at different wavelengths of light, along with the requirement to have some way to view the image the primary mirror produces, means there is always some compromise in a reflecting telescope's optical design. Because

1740-496: The experimental scientist Robert Hooke was able to build this type of telescope, which became known as the Gregorian telescope . Five years after Gregory designed his telescope and five years before Hooke built the first such Gregorian telescope, Isaac Newton in 1668 built his own reflecting telescope , which is generally acknowledged as the first reflecting telescope. It used a spherically ground metal primary mirror and

1798-454: The focal plane. The distance from the mirror to the focal plane is called the focal length . Film or a digital sensor may be located here to record the image, or a secondary mirror may be added to modify the optical characteristics and/or redirect the light to film, digital sensors, or an eyepiece for visual observation. The primary mirror in most modern telescopes is composed of a solid glass cylinder whose front surface has been ground to

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1856-616: The four VLT mirrors, the two Gemini mirrors, and the 36 segments for GTC. The Steward Observatory Mirror Lab cast and polished the two LBT mirrors, the two Magellan mirrors, the MMT replacement mirror, and the LSST primary/tertiary mirror. It is currently making the mirrors for the Giant Magellan Telescope. The Keck segments were made by Schott AG. The SALT and LAMOST segments were cast and polished by LZOS. The mirror for Subaru

1914-655: The image back through a hole in the primary mirror. This produces an upright image, useful for terrestrial observations. Some small spotting scopes are still built this way. There are several large modern telescopes that use a Gregorian configuration such as the Vatican Advanced Technology Telescope , the Magellan telescopes , the Large Binocular Telescope , and the Giant Magellan Telescope . The Newtonian telescope

1972-480: The incoming light by eliminating the secondary or moving any secondary element off the primary mirror's optical axis , commonly called off-axis optical systems . The Herschelian reflector is named after William Herschel , who used this design to build very large telescopes including the 40-foot telescope in 1789. In the Herschelian reflector the primary mirror is tilted so the observer's head does not block

2030-410: The incoming light. Although this introduces geometrical aberrations, Herschel employed this design to avoid the use of a Newtonian secondary mirror since the speculum metal mirrors of that time tarnished quickly and could only achieve 60% reflectivity. A variant of the Cassegrain, the Schiefspiegler telescope ("skewed" or "oblique reflector") uses tilted mirrors to avoid the secondary mirror casting

2088-423: The instrument on a rigid structure, rather than moving it with the telescope. Whilst transmission of the full field of view would require a standard coudé focus, spectroscopy typically involves the measurement of only a few discrete objects, such as stars or galaxies. It is therefore feasible to collect light from these objects with optical fibers at the telescope, placing the instrument at an arbitrary distance from

2146-575: The invention of the refracting telescope , Galileo , Giovanni Francesco Sagredo , and others, spurred on by their knowledge of the principles of curved mirrors, discussed the idea of building a telescope using a mirror as the image forming objective. There were reports that the Bolognese Cesare Caravaggi had constructed one around 1626 and the Italian professor Niccolò Zucchi , in a later work, wrote that he had experimented with

2204-501: The largest Earth based telescopes to reach higher resolution than the Hubble Space Telescope. Another advantage of Earth based telescopes is the comparatively low cost of upgrading and replacing instruments. This list is ordered by optical aperture, which has historically been a useful gauge of limiting resolution, optical area, physical size, and cost. Multiple mirror telescopes that are on the same mount and can form

2262-409: The largest in the world at the time of their construction, by the same aperture criterion as above. These telescopes are under construction and will meet the list inclusion criteria once completed: Selected large telescopes which are in detailed design or pre-construction phases: Reflecting telescope A reflecting telescope (also called a reflector ) is a telescope that uses a single or

2320-534: The largest telescope of the 19th century, the Leviathan of Parsonstown with a 6 feet (1.8 m) wide metal mirror. In the 19th century a new method using a block of glass coated with very thin layer of silver began to become more popular by the turn of the century. Common telescopes which led to the Crossley and Harvard reflecting telescopes, which helped establish a better reputation for reflecting telescopes as

2378-466: The light to the side of the telescope to allow for the mounting of heavy instruments. This is a very common design in large research telescopes. Adding further optics to a Nasmyth-style telescope to deliver the light (usually through the declination axis) to a fixed focus point that does not move as the telescope is reoriented gives a coudé focus (from the French word for elbow). The coudé focus gives

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2436-408: The liquid forms a paraboloidal surface of essentially unlimited size. This allows making very big telescope mirrors (over 6 metres), but they are limited to use by zenith telescopes . In a prime focus design no secondary optics are used, the image is accessed at the focal point of the primary mirror . At the focal point is some type of structure for holding a film plate or electronic detector. In

2494-534: The low-resolution spectrograph, the telescope has been used to identify Type Ia supernovae to measure the acceleration of the universe. The telescope has also been used to measure the rotation of individual galaxies. The telescope was upgraded for use in the Hobby–Eberly Telescope Dark Energy Experiment (HETDEX), which will provide the first observations to allow narrowing of the list of possible explanations for dark energy . Along with

2552-465: The magazine editor at the time. It uses a concave elliptical primary mirror and a convex spherical secondary. While this system is easier to grind than a classic Cassegrain or Ritchey–Chrétien system, it does not correct for off-axis coma. Field curvature is actually less than a classical Cassegrain. Because this is less noticeable at longer focal ratios , Dall–Kirkhams are seldom faster than f/15. There are several designs that try to avoid obstructing

2610-493: The major telescopes used in astronomy research are reflectors. Many variant forms are in use and some employ extra optical elements to improve image quality or place the image in a mechanically advantageous position. Since reflecting telescopes use mirrors , the design is sometimes referred to as a catoptric telescope . From the time of Newton to the 1800s, the mirror itself was made of metal – usually speculum metal . This type included Newton's first designs and

2668-647: The mass of the bulge of this spiral galaxy (14 percent of the total stellar mass of the galaxy). The Habitable Zone Planet Finder is a spectrograph for the Hobby–Eberly Telescope capable of detecting earth like planets. The design of the Hobby–Eberly was used as the basis for the Southern African Large Telescope . List of largest optical reflecting telescopes This list of the largest optical reflecting telescopes with objective diameters of 3.0 metres (120 in) or greater

2726-449: The metal mirror designs were noted for their drawbacks. Chiefly the metal mirrors only reflected about 2 ⁄ 3 of the light and the metal would tarnish . After multiple polishings and tarnishings, the mirror could lose its precise figuring needed. Reflecting telescopes became extraordinarily popular for astronomy and many famous telescopes, such as the Hubble Space Telescope , and popular amateur models use this design. In addition,

2784-520: The new Visible Integral-Field Replicable Unit Spectrograph (VIRUS) to enable observations for the HETDEX project with 192 spectrographs. The Hobby–Eberly Telescope is operated by The University of Texas McDonald Observatory for a consortium of institutions which includes The University of Texas at Austin , Pennsylvania State University , Ludwig Maximilian University of Munich , and Georg August University of Göttingen . The physical main reflector mirror

2842-400: The obstruction as well as diffraction spikes caused by most secondary support structures. The use of mirrors avoids chromatic aberration but they produce other types of aberrations . A simple spherical mirror cannot bring light from a distant object to a common focus since the reflection of light rays striking the mirror near its edge do not converge with those that reflect from nearer

2900-426: The past, in very large telescopes, an observer would sit inside the telescope in an "observing cage" to directly view the image or operate a camera. Nowadays CCD cameras allow for remote operation of the telescope from almost anywhere in the world. The space available at prime focus is severely limited by the need to avoid obstructing the incoming light. Radio telescopes often have a prime focus design. The mirror

2958-432: The primary mirror focuses light to a common point in front of its own reflecting surface almost all reflecting telescope designs have a secondary mirror , film holder, or detector near that focal point partially obstructing the light from reaching the primary mirror. Not only does this cause some reduction in the amount of light the system collects, it also causes a loss in contrast in the image due to diffraction effects of

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3016-458: The reflection telescope principle was applied to other electromagnetic wavelengths, and for example, X-ray telescopes also use the reflection principle to make image-forming optics . The idea that curved mirrors behave like lenses dates back at least to Alhazen 's 11th century treatise on optics, works that had been widely disseminated in Latin translations in early modern Europe . Soon after

3074-574: The secondary mirror creates a telescope with a long focal length while having a short tube length. The Ritchey–Chrétien telescope, invented by George Willis Ritchey and Henri Chrétien in the early 1910s, 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 nearly flat focal plane if the primary and secondary curvature are properly figured , making it well suited for wide field and photographic observations. Almost every professional reflector telescope in

3132-418: The telescope as it slews; this places additional requirements on the strength of the instrument support structure, and potentially limits the movement of the telescope in order to avoid collision with obstacles such as walls or equipment inside the observatory. The Nasmyth design is similar to the Cassegrain except the light is not directed through a hole in the primary mirror; instead, a third mirror reflects

3190-591: The world is of the Ritchey–Chrétien design. Including a third curved mirror allows correction of the remaining distortion, astigmatism, from the Ritchey–Chrétien design. This allows much larger fields of view. The Dall–Kirkham Cassegrain telescope's 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,

3248-694: Was cast by Corning and polished at Contraves Brashear Systems in Pennsylvania, USA. This table does not include all the largest mirrors manufactured. The Steward Observatory Mirror Lab produced the 6.5 metre f/1.25 collimator used in the Large Optical Test and Integration Site of Lockheed Martin , used for vacuum optical testing of other telescopes. Segmented mirrors are also referred to as mosaic mirrors. Single mirrors are also referred to monolithic mirrors, and can be sub-categorized in types, such as solid or honeycomb. These telescopes were

3306-419: Was the convex secondary, and its own radius of curvature distant from the secondary. Because the tertiary mirror receives parallel light from the secondary, it forms an image at its focus. The focal plane lies within the system of mirrors, but is accessible to the eye with the inclusion of a flat diagonal. The Stevick-Paul configuration results in all optical aberrations totaling zero to the third-order, except for

3364-409: Was the first successful reflecting telescope, completed by Isaac Newton in 1668. It usually has a paraboloid primary mirror but at focal ratios of about f/10 or longer a spherical primary mirror can be sufficient for high visual resolution. A flat secondary mirror reflects the light to a focal plane at the side of the top of the telescope tube. It is one of the simplest and least expensive designs for

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