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71-553: Leviathan of Parsonstown , or Rosse six-foot telescope , is a historic reflecting telescope of 72 inches (1.83 m) aperture , which was the largest telescope in the world from 1845 until the construction of the 100-inch (2.5 m) Hooker Telescope in California in 1917. The Rosse six-foot telescope was built by William Parsons, 3rd Earl of Rosse on his estate, Birr Castle , at Parsonstown (now Birr ) in Ireland. Parsons improved

142-484: A Civil Engineer , but some states require licensure specifically for structural engineering, with experience specific and non-concurrent with experience claimed for another engineering profession. The qualifications for licensure typically include a specified minimum level of practicing experience, as well as the successful completion of a nationally-administered 16-hour exam, and possibly an additional state-specific exam. For instance, California requires that candidates pass

213-484: 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

284-476: 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

355-423: 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

426-455: 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

497-402: 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

568-412: 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

639-439: A master's degree, or two years after receiving a Ph.D. degree. Most US states do not have a separate structural engineering license. In 10 US states, including Alaska, California, Hawaii, Illinois, Nevada, Oregon, Utah, Washington, and others, there is an additional license or authority for Structural Engineering, obtained after the engineer has obtained a Civil Engineering license and practiced an additional amount of time with

710-515: 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

781-408: 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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852-486: A national exam, written by the National Council of Examiners for Engineering and Surveying (NCEES), as well as a state-specific exam which includes a seismic portion and a surveying portion. In most states, application for license exam is requires four years of work experience after the candidate graduated from an ABET-accredited university and passing the fundamentals of Engineering exam, three years after receiving

923-560: 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

994-450: 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

1065-495: 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

1136-400: 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,

1207-455: A spiral structure, suggesting "dynamical laws". The most notable spiral nebula observed by Parsons was Messier 51 , which he resolved into stars. After William Parsons (the 3rd Earl of Rosse) died in 1867, the 4th Earl ( Laurence Parsons ) continued to operate the six-foot telescope. From 1874 to 1878, J. L. E. Dreyer worked with the telescope and began the compilation of his New General Catalogue of Nebulae and Clusters of Stars . Although

1278-821: A structural engineer include buildings, towers, stadiums, and bridges. Other structures such as oil rigs, space satellites, aircraft, and ships may also be designed by a structural engineer. Most structural engineers are employed in the construction industry, however, there are also structural engineers in the aerospace, automobile, and shipbuilding industries. In the construction industry, they work closely with architects , civil engineers , mechanical engineers , electrical engineers , quantity surveyors , and construction managers . Structural engineers ensure that buildings and bridges are built to be strong enough and stable enough to resist all appropriate structural loads (e.g., gravity, wind, snow, rain, seismic ( earthquake ), earth pressure, temperature, and traffic) to prevent or reduce

1349-550: A total of 96 digital Receiver Units (RCU's). The Birr Castle station on its own is the Irish Low Frequency Array (I-LOFAR) I-Lofar telescope. In 2018, I-LOFAR observed for the first time a billion-year-old red-dwarf , flare star , namely CN Leo ( Wolf 359 ), 7.9 light years away. Reflecting 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

1420-406: Is a profession subject to licensure. Licensed engineers may receive the title of Professional Engineer, Chartered Engineer, Structural Engineer, or other title depending on the jurisdiction. The process to attain licensure to work as a structural engineer varies by location, but typically specifies university education, work experience, examination, and continuing education to maintain their mastery of

1491-671: Is made of aluminium, as a compromise between authenticity and utility in astronomical observation. In 2017, LOFAR radio-telescope station IE613, one of some 50 similar stations in Europe, was constructed in the grounds of the castle. This is the westernmost station in the LOFAR network. LOFAR makes observations in the 10 MHz to 240 MHz frequency range with two types of antennas: Low Band Antenna (LBA) and High Band Antenna (HBA), optimized for 10-80 MHz and 120-240 MHz respectively. The Birr Castle station consists of 96 LBA's and 96 HBA's and

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1562-413: 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

1633-578: 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

1704-467: Is supported at the mirror end by a "universal joint", a hinge with two axes, which allows the tube to be inclined through a large range of altitude and also to be turned through a limited range of azimuth . The azimuth range is limited to about one hour by the supporting walls that flank the tube on its eastern and western sides. The walls are 23 feet (7.0 m) apart, 40 feet (12 m) high, and 71 feet (21.6 m) long. A chain and counterweight keeps

1775-493: 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

1846-566: The construction industry showed that structural engineers in the UK earn an average wage of £35,009. The salary of structural engineers varies from sector to sector within the construction and built environment industry worldwide, depending on the project. For example, structural engineers working in public sector projects earn on average £37,083 per annum compared to the £43,947 average earned by those in commercial projects. Certain regions also represent higher average salaries, with structural engineers in

1917-460: 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

1988-538: The 4th Earl built a smaller 3 ft equatorial in 1876, the six-foot telescope remained in use until about 1890. After his death in 1908, the telescope was partly dismantled, and in 1914, one of the mirrors with its mirror box was transferred to the Science Museum in London. The walls remained. The tube, second mirror box, and universal joint survived. Following a TV programme, lecture, and book by Patrick Moore , there

2059-603: The Civil Engineering license. The scope of what structures must be designed by a Structural Engineer, not by a Civil Engineer without the S.E. license, is limited in Alaska, California, Nevada, Oregon, Utah, and Washington to some high importance structures such as stadiums, bridges, hospitals, and schools. The practice of structural engineering is reserved entirely to S.E. licensees in Hawaii and Illinois. The United Kingdom has one of

2130-535: 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

2201-914: The United States, there have been discussions in the structural engineering community about the knowledge base of structural engineering graduates. Some have called for a master's degree to be the minimum standard for professional licensing as a civil engineer. There are separate structural engineering undergraduate degrees at the University of California, San Diego and the University of Architecture, Civil Engineering, and Geodesy , Sofia, Bulgaria. Many students who later become structural engineers major in civil, mechanical, or aerospace engineering degree programs, with an emphasis on structural engineering. Architectural engineering programs do offer structural emphases and are often in combined academic departments with civil engineering. In many countries, structural engineering

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2272-518: 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

2343-481: The design lifetime. The education of structural engineers is usually through a civil engineering bachelor's degree, and often a master's degree specializing in structural engineering. The fundamental core subjects for structural engineering are strength of materials or solid mechanics , structural analysis (static and dynamic), material science and numerical analysis . Reinforced concrete , composite structure , timber, masonry and structural steel designs are

2414-402: 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

2485-569: The examination result. The candidate can register at the Engineering Council UK as a Chartered Structural Engineer once he or she has been elected as a Chartered Member. Legally it is not necessary to be a member of the IStructE when working on structures in the UK, however, industry practice, insurance, and liabilities dictate that an appropriately qualified engineer be responsible for such work. A 2010 survey of professionals occupying jobs in

2556-491: 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

2627-402: The eyepiece from a wooden gallery that spans the distance between the walls and can slide up and down guides to follow the telescope in altitude. A cage on the gallery moves sideways to reach the eyepiece at different azimuth. At high elevation, curved galleries on top of the western wall are used, which can be moved across the wall to follow the telescope in azimuth. The purpose of the telescope

2698-454: 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

2769-648: The general structural design courses that will be introduced in the next level of the education of structural engineering. The structural analysis courses which include structural mechanics , structural dynamics and structural failure analysis are designed to build up the fundamental analysis skills and theories for structural engineering students. At the senior year level or in graduate programs, prestressed concrete design, space frame design for building and aircraft, bridge engineering, civil and aerospace structure rehabilitation and other advanced structural engineering specializations are usually introduced. Recently in

2840-692: 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

2911-474: 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

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2982-410: 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

3053-423: 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

3124-559: 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

3195-524: 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

3266-462: 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

3337-408: 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

3408-474: The loss of life or injury. They also design structures to be stiff enough to not deflect or vibrate beyond acceptable limits. Human comfort is an issue that is regularly considered limited. Fatigue is also an important consideration for bridges and aircraft design or for other structures that experience many stress cycles over their lifetimes. Consideration is also given to the durability of materials against possible deterioration which may impair performance over

3479-465: 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

3550-513: 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,

3621-400: 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

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3692-582: The oldest professional institutions for structural engineers, the Institution of Structural Engineers . Founded as the Concrete Institute in 1908, it was renamed the Institution of Structural Engineers (IStructE) in 1922. It now has 22,000 members with branches in 32 countries. The IStructE is one of several UK professional bodies empowered to grant the title of Chartered Engineer ; its members are granted

3763-426: 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

3834-432: 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

3905-454: 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

3976-461: The remains of the telescope, incidental comments in observing logs, and contemporary photographs taken by Mary Rosse , wife of the 3rd Earl. Reconstruction work lasted from early 1996 to early 1997. It had been planned to include a working mirror, but due to budget constraints this had to be left for a separate project. The new mirror was installed in 1999. Unlike the speculum original, and unlike modern aluminium- or silver-coated glass mirrors, this

4047-445: The safety and comfort of users or occupants. Their work takes account mainly of safety, technical, economic, and environmental concerns, but they may also consider aesthetic and social factors. Structural engineering is usually considered a specialty discipline within civil engineering , but it can also be studied in its own right. In the United States, most practicing structural engineers are currently licensed as civil engineers , but

4118-574: 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

4189-469: The situation varies from state to state. Some states have a separate license for structural engineers who are required to design special or high-risk structures such as schools, hospitals, or skyscrapers. In the United Kingdom, most structural engineers in the building industry are members of the Institution of Structural Engineers or the Institution of Civil Engineers . Typical structures designed by

4260-413: The subject. Professional Engineers bear legal responsibility for their work to ensure the safety and performance of their structures and only practice within the scope of their expertise. In the United States, persons practicing structural engineering must be licensed in each state in which they practice. Licensure to practice as a structural engineer usually be obtained by the same qualifications as for

4331-550: The techniques of casting, grinding and polishing large telescope mirrors from speculum metal , and constructed steam-powered grinding machines for parabolic mirrors. His 3 foot (91 cm) mirror of 1839 was cast in smaller pieces and then fitted together before grinding and polishing; its 1840 successor was cast in a single piece. In 1842, Parsons cast his first 6 foot (1.83 m) mirror, but it took another five casts before he had two ground and polished mirrors. Speculum mirrors tarnished rapidly; with two mirrors, one could be used in

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4402-418: 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

4473-401: The telescope in balance, another chain with a winch controls the altitude. A rack and pinion beam underneath the tube controls the azimuth. This beam is connected to the eastern supporting wall, where it can move on a circular iron arc to allow the telescope to change altitude. The tube is of the Newtonian design with the eyepiece on its western side. At low altitude, the observer accesses

4544-420: The telescope while the other was being re-polished. The telescope tube and supporting structure were completed in 1845. The mirror was 5 inches (13 cm) thick and weighed almost 3 tons. This required a mirror cell to support and to prevent the mirror deforming under its own weight. The length of the tube and mirror box is about 54 feet (16.5 m); including the mirror it weighed about 12 tons. The tube

4615-791: The telescope. Examples of fiber-fed spectrographs include the planet-hunting spectrographs HARPS or ESPRESSO . Additionally, the flexibility of optical fibers allow light to be collected from any focal plane; for example, the HARPS spectrograph utilises the Cassegrain focus of the ESO 3.6 m Telescope , whilst the Prime Focus Spectrograph is connected to the prime focus of the Subaru telescope . Structural engineer Structural engineers analyze, design, plan, and research structural components and structural systems to achieve design goals and ensure

4686-495: The title of Chartered Structural Engineer . The overall process to become chartered begins after graduation from a UK MEng degree, or a BEng with an MSc degree. To qualify as a chartered structural engineer, a graduate needs to go through four years of Initial Professional Development followed by a professional review interview. After passing the interview, the candidate sits an eight-hour professional review examination. The election to chartered membership (MIStructE) depends on

4757-582: 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,

4828-481: Was renewed interest in the six-foot telescope in the 1970s. Gradually, the telescope became a visitor attraction. But it was not until the 1990s that plans to actually rebuild the telescope came to fruition. In 1994 the retired structural engineer and amateur astronomer Michael Tubridy was called in to research and re-design the Rosse six-foot telescope. The original plans were lost, and so it took detective work to review

4899-419: 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

4970-409: 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

5041-403: Was to re-visit the nebulae in the catalogues of Charles Messier and John Herschel . These catalogues list star clusters as well as nebulae, and the question was whether the latter were merely unresolved star clusters or genuinely nebulous regions of space. It resolved into stars unclear areas which might be the first galaxies to be identified as such. Parsons discovered that several nebulae had

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