Editing Reflecting telescope (section)

{{Short description|Telescopes which utilize curved mirrors to form an image}}
[[File:SOFIA 2.5M Primary Mirror.jpg|thumb|[[Stratospheric Observatory for Infrared Astronomy]] ]]
[[File:Franklin reflector 24.jpg|thumb|24-inch convertible Newtonian/Cassegrain reflecting telescope on display at the [[Franklin Institute]]]]

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. 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 [[mirror]]s, the design is sometimes referred to as a '''[[catoptrics|catoptric]] telescope'''.<ref name="European Space Agency">{{cite web |title=Freeform catoptric telescope with pointing and nodding capabilities |url=https://www.esa.int/Enabling_Support/Space_Engineering_Technology/Freeform_catoptric_telescope_with_pointing_and_nodding_capabilities |website=European Space Agency |publisher=European Space Agency |access-date=23 December 2024}}</ref>

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 the largest telescope of the 19th century, the [[Leviathan of Parsonstown]] with a {{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.

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 electromagnetic wavelengths, and for example, [[X-ray telescope]]s also use the reflection principle to make [[image-forming optical system|image-forming optics]].