Editing Optical telescope (section)

==Astronomical research telescopes==
[[File:Two Unit Telescopes VLT.jpg|thumb|Two of the four Unit Telescopes that make up the [[ESO]]'s [[Very Large Telescope|VLT]], on a remote mountaintop, 2600 metres above sea level in the Chilean Atacama Desert.]]
Optical telescopes have been used in astronomical research since the time of their invention in the early 17th century. Many types have been constructed over the years depending on the optical technology, such as refracting and reflecting, the nature of the light or object being imaged, and even where they are placed, such as [[space telescope]]s. Some are classified by the task they perform such as [[solar telescope]]s.

===Large reflectors===
Nearly all large research-grade astronomical telescopes are reflectors. Some reasons are:
* In a lens the entire volume of material has to be free of imperfection and inhomogeneities, whereas in a mirror, only one surface has to be perfectly polished.
* Light of different colors travels through a medium other than vacuum at different speeds. This causes [[chromatic aberration]].
* Reflectors work in a wider [[spectrum]] of light since certain wavelengths are absorbed when passing through glass elements like those found in a refractor or catadioptric.
* There are technical difficulties involved in manufacturing and manipulating large-diameter lenses. One of them is that all real materials sag in gravity. A lens can only be held by its perimeter. A mirror, on the other hand, can be supported by the whole side opposite to its reflecting face.

[[File:Comparison optical telescope primary mirrors.svg|thumb|left|Comparison of nominal sizes of primary mirrors of some notable optical telescopes]]

Most large research reflectors operate at different focal planes, depending on the type and size of the instrument being used. These including the [[Reflecting telescope#Prime focus|prime focus]] of the main mirror, the [[Cassegrain telescope|cassegrain focus]] (light bounced back down behind the primary mirror), and even external to the telescope all together (such as the [[Reflecting telescope#Nasmyth and coudé focus|Nasmyth and coudé focus]]).<ref>{{cite book|author=Ian S. McLean|title=Electronic Imaging in Astronomy: Detectors and Instrumentation|url=https://books.google.com/books?id=FGHhZf-k8SkC&pg=PA91|year=2008|publisher=Springer Science & Business Media|isbn=978-3-540-76582-0|page=91}}</ref>

A new era of telescope making was inaugurated by the [[Multiple Mirror Telescope]] (MMT), with a mirror composed of six segments synthesizing a mirror of 4.5 [[metre|meter]]s diameter. This has now been replaced by a single 6.5 m mirror. Its example was followed by the [[Keck telescope]]s with 10 m segmented mirrors.

The largest current ground-based telescopes have a [[primary mirror]] of between 6 and 11 meters in diameter. In this generation of telescopes, the mirror is usually very thin, and is kept in an optimal shape by an array of actuators (see [[active optics]]). This technology has driven new designs for future telescopes with diameters of 30, 50 and even 100 meters.

[[Image:USA harlan j smith telescope TX.jpg|thumb|[[Harlan J. Smith Telescope]] reflecting telescope at [[McDonald Observatory]], Texas]]

Relatively cheap, mass-produced ~2 meter telescopes have recently been developed and have made a significant impact on astronomy research. These allow many astronomical targets to be monitored continuously, and for large areas of sky to be surveyed. Many are [[robotic telescope]]s, computer controlled over the internet (see ''e.g.'' the [[Liverpool Telescope]] and the [[Faulkes Telescope North]] and [[Faulkes Telescope South|South]]), allowing automated follow-up of astronomical events.

Initially the [[detector]] used in telescopes was the [[human eye]]. Later, the sensitized [[photographic plate]] took its place, and the [[spectrograph]] was introduced, allowing the gathering of spectral information. After the photographic plate, successive generations of electronic detectors, such as the [[charge-coupled device]] (CCDs), have been perfected, each with more sensitivity and resolution, and often with a wider wavelength coverage.

Current research telescopes have several instruments to choose from such as:
* imagers, of different spectral responses
* spectrographs, useful in different regions of the spectrum
* polarimeters, that detect light [[Polarization (waves)|polarization]].

The phenomenon of optical [[diffraction]] sets a limit to the resolution and image quality that a telescope can achieve, which is the effective area of the [[Airy disc]], which limits how close two such discs can be placed. This absolute limit is called the [[diffraction limit]] (and may be approximated by the [[Angular resolution#The_Rayleigh_criterion|Rayleigh criterion]], [[Dawes limit]] or [[Sparrow's resolution limit]]). This limit depends on the wavelength of the studied light (so that the limit for red light comes much earlier than the limit for blue light) and on the [[diameter]] of the telescope mirror. This means that a telescope with a certain mirror diameter can theoretically resolve up to a certain limit at a certain wavelength. For conventional telescopes on Earth, the diffraction limit is not relevant for telescopes bigger than about 10&nbsp;cm. Instead, the [[Astronomical seeing|seeing]], or blur caused by the atmosphere, sets the resolution limit. But in space, or if [[adaptive optics]] are used, then reaching the diffraction limit is sometimes possible. At this point, if greater resolution is needed at that wavelength, a wider mirror has to be built or aperture synthesis performed using an array of nearby telescopes.

In recent years, a number of technologies to overcome the distortions caused by [[Earth's atmosphere|atmosphere]] on ground-based telescopes have been developed, with good results. See [[adaptive optics]], [[speckle imaging]] and [[Optical interferometry#Astronomical optical interferometry|optical interferometry]].