Reflecting telescope: Difference between revisions

A '''reflecting telescope''' (also called a '''reflector''') is a [[telescope]] that uses a single or a combination of [[curved mirror]]s 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 aberration]]s, it is a design that allows for very large diameter [[Objective (optics)|objectives]]. Almost all of the major telescopes used in [[astronomy]] research are reflectors. ReflectingMany telescopesvariant comeforms are in many design variationsuse and maysome employ extra optical elements to improve image quality or place the image in a mechanically advantageous position. Since reflecting telescopes use [[mirror]]s, the design is sometimes referred to as a '''[[catoptrics|catoptric]] telescope'''.

From the time of Newton to the 1800s, the mirror itself was made of metal{{snd}} usually [[speculum metal]]. This type included Newton's first designs and even the largest telescopestelescope of the 19th century, the [[Leviathan of Parsonstown]] with a 1.8 meter{{convert|6|ft|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 telescope|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 {{fract|2|3}} of the light and the metal would [[tarnish]]. After multiple polishings and tarnishings, the mirror could lose its precise figuring needed.

[[File:NewtonsTelescopeReplicaNewton telescope replica 1668.jpg|thumb|200px|A replica of Newton's second reflecting telescope thatwhich hewas presented to the [[Royal Society]] in 1672.<ref>{{cite book|url=https://books.google.com/books?id=KAWwzHlDVksC&q=history+of+the+telescope |title=The History of the Telescope |author=Henry C. King |page=74 |access-date=2013-08-01|isbn=978-0-486-43265-6 |year=1955 }}</ref>]]

[[Image:Great Telescope, Birr, Offaly 1.jpg|right|thumb|The great telescope of [[Birr Castle]], the [[Leviathan of Parsonstown]]. Modern day remnants of the mirror and support structure.]]

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]].<ref>{{cite book|author=Fred Watson|title=Stargazer: The Life and Times of the Telescope|url=https://books.google.com/books?id=2LZZginzib4C&pg=PA40|year=2007|publisher=Allen & Unwin|isbn=978-1-74176-392-8|page=108}}</ref> Soon after 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.<ref name="Fred Watson 2007 109">{{cite book|author=Fred Watson|title=Stargazer: The Life and Times of the Telescope|url=https://books.google.com/books?id=2LZZginzib4C&pg=PA40|year=2007|publisher=Allen & Unwin|isbn=978-1-74176-392-8|page=109}}</ref> There were reports that the [[Bologna|Bolognese]] [[Cesare Caravaggi]] had constructed one around 1626 and the Italian professor [[Niccolò Zucchi]], in a later work, wrote that he had experimented with a concave bronze mirror in 1616, but said it did not produce a satisfactory image.<ref name="Fred Watson 2007 109"/> The potential advantages of using [[parabolic reflector|parabolic mirrors]], primarily reduction of [[spherical aberration]] with no [[chromatic aberration]], led to many proposed designs for reflecting telescopes.<ref>theoretical designs by [[Bonaventura Cavalieri]], [[Marin Mersenne]], and [[James Gregory (astronomer and mathematician)|Gregory]] among others</ref> The most notable being [[James Gregory (astronomer and mathematician)|James Gregory]], who published an innovative design for a ‘reflecting’ telescope in 1663. It would be ten years (1673), before the experimental scientist [[Robert Hooke]] was able to build this type of telescope, which became known as the [[Gregorian telescope]].<ref>{{cite book|author=Fred Watson|title=Stargazer: The Life and Times of the Telescope|url=https://books.google.com/books?id=2LZZginzib4C&pg=PA62|year=2007|publisher=Allen & Unwin|isbn=978-1-74176-392-8|page=117}}</ref><ref>{{cite book|author=Henry C. King|title=The History of the Telescope|url=https://books.google.com/books?id=KAWwzHlDVksC&pg=PR1|year=2003|publisher=Courier Corporation|isbn=978-0-486-43265-6|page=71}}</ref><ref>{{Cite web|url=http://www.nms.ac.uk/explore/stories/science-and-technology/reflecting-telescope/telescopes/james-gregory/|title=Explore, National Museums Scotland|access-date=2016-11-15|archive-date=2017-01-17|archive-url=https://web.archive.org/web/20170117064000/http://www.nms.ac.uk/explore/stories/science-and-technology/reflecting-telescope/telescopes/james-gregory/|url-status=dead}}</ref>

Five years after Gregory designed his telescope and five years before Hooke built the first such Gregorian telescope, [[Isaac Newton]] hasin been generally credited1668 with building thebuilt [[Newton's reflector|firsthis own reflecting telescope]], inwhich 1668is generally acknowledged as the first reflecting telescope.<ref name="books.google.com">{{cite book|author=A. Rupert Hall|title=Isaac Newton: Adventurer in Thought|url=https://archive.org/details/isaacnewtonadven0000hall|url-access=registration|year=1996|publisher=Cambridge University Press|isbn=978-0-521-56669-8|page=[https://archive.org/details/isaacnewtonadven0000hall/page/67 67]}}</ref> It used a spherically ground metal [[primary mirror]] and a small diagonal mirror in an optical configuration that has come to be known as the [[Newtonian telescope]].

A curved [[primary mirror]] is the reflector telescope's basic optical element that creates an image at 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 '''[[Newtonian telescope]]''' was the first successful reflecting telescope, completed by [[Isaac Newton]] in 1668. It usually has a paraboloid primary mirror but at [[F-number|focal ratio]]s 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 a given size of primary, and is popular with [[amateur telescope making|amateur telescope makers]] as a home-build project.

The '''cassegrainCassegrain 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 the secondary mirror creates a telescope with a long focal length while having a short tube length.

A variant of the Cassegrain, the [[Schiefspiegler]] telescope ("skewed" or "oblique reflector") uses tilted mirrors to avoid the secondary mirror casting 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 observationobservations to theobjects Moonwhich andfit planetsin a moderate field of view. A 6" (150mm) f/15 telescope offers a maximum 0.75 degree field of view using 1.25" eyepieces.

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, the liquid forms a paraboloidal surface of essentially unlimited size. This allows making very big telescope mirrors (over 6 metres), but unfortunately they cannotare belimited steered,to as theyuse alwaysby point[[zenith verticallytelescope]]s.

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).<ref name="The Coude Focus">{{cite web|url=http://mthamilton.ucolick.org/public/tele_inst/3m/coude.html|title=The Coude Focus}}</ref> The coudé focus gives a narrower field of view than a Nasmyth focus<ref name="The Coude Focus"/> and is used with very heavy instruments that do not need a wide field of view. One such application is high-resolution [[spectrograph]]s 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 grating|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.