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A reflecting telescope (also called a reflector) is a telescope that uses a single or 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 the major telescopes used in astronomy research are reflectors. Reflecting telescopes come in many design variations and may 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 even the largest telescopes of the 19th century, the Leviathan of Parsonstown with a 1.8 meter 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. A major turning point in reflecting telescopes was the Paris Observatory 1.2 m of 1878, A.A. Common telescopes which led to the Crossley and Harvard reflecting telescopes, which helped establish a better reputation for reflecting telescopes as 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, the reflection telescope principle was applied to other wavelengths of light, and for example, X-Ray telescopes also use the reflection principle to make image forming optics.

Nasmyth/coudé light path.

Nasmyth[24]

Coudé

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).[25] The coudé focus gives a narrower field of view than a Nasmyth focus[25] 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 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 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).[25] The coudé focus gives a narrower field of view than a Nasmyth focus[25] 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 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 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.

Fibre-fed spectrographs