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The 100 inch (2.5 m) Hooker reflecting telescope at Mount Wilson Observatory near Los Angeles, California.

A telescope is an instrument designed for the observation of remote objects and the collection of electromagnetic radiation. The first known practically functioning telescopes were invented in the Netherlands at the beginning of the 17th century. The name "telescope" was derived from the Greek tele = 'far' and skopein = 'to look or see', and was coined by the Greek mathematician Giovanni Demisiani for one of Galileo's instruments.^[1] "Telescopes" can refer to a whole range of instruments operating in most regions of the electromagnetic spectrum.

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Types of telescopes

The name "telescope" covers a wide range of instruments and is difficult to define. They all have the attribute of collecting electromagnetic radiation so it can be studied or analyzed in some manner. The most common type is the optical telescope; other types also exist and are listed below.

Optical telescopes

50 cm refracting telescope at Nice Observatory.

An optical telescope gathers and focuses light mainly from the visible part of the electromagnetic spectrum (although some work in the infrared and ultraviolet). Optical telescopes increase the apparent angular size of distant objects as well as their apparent brightness. In order for the image to be observed, photographed, studied, and sent to a computer, telescopes work by employing one or more curved optical elements—usually made from glass—lenses, or mirrors to gather light and other electromagnetic radiation to bring that light or radiation to a focal point. Optical telescopes are used for astronomy and in many non-astronomical instruments, including: theodolites (including transits), spotting scopes, monoculars, binoculars, camera lenses, and spyglasses. There are three main types:

The refracting telescope which uses lenses to form an image.
The reflecting telescope which uses an arrangement of mirrors to form an image.
The catadioptric telescope which uses mirrors combined with lenses—either in front of the mirror or somewhere within the optical path—to form an image.

Radio telescopes

The Very Large Array at Socorro, New Mexico, United States.

Radio telescopes are directional radio antennas that often have a parabolic shape. The dishes are sometimes constructed of a conductive wire mesh whose openings are smaller than the wavelength being observed. Multi-element Radio telescopes are constructed from pairs or larger groups of these dishes to synthesize large 'virtual' apertures that are similar in size to the separation between the telescopes; this process is known as aperture synthesis. As of 2005, the current record array size is many times the width of the Earth—utilizing space-based Very Long Baseline Interferometry (VLBI) telescopes such as the Japanese HALCA (Highly Advanced Laboratory for Communications and Astronomy) VSOP (VLBI Space Observatory Program) satellite. Aperture synthesis is now also being applied to optical telescopes using optical interferometers (arrays of optical telescopes) and aperture masking interferometry at single reflecting telescopes. Radio telescopes are also used to collect microwave radiation, which is used to collect radiation when any visible light is obstructed or faint, such as from quasars. Some radio telescopes are used by programs such as SETI and the Arecibo Observatory to search for exterrestrial life. One particularly exciting example is the Wow! signal, recorded in 1977.

X-ray and gamma-ray telescopes

X-ray and gamma-ray radiation go through most metals and glasses, but some X-ray telescopes use Wolter telescopes composed of ring-shaped 'glancing' mirrors made of heavy metals that are able to reflect the rays just a few degrees. The mirrors are usually a section of a rotated parabola and a hyperbola, or ellipse. Gamma-ray telescopes refrain from focusing completely and use coded aperture masks: the patterns of the shadow the mask creates can be reconstructed to form an image. These types of telescopes are usually on Earth-orbiting satellites or high-flying balloons since the Earth's atmosphere is opaque to this part of the electromagnetic spectrum.

Other types

Notable telescopes

Anglo-Australian Telescope
Arecibo Observatory
Atacama Large Millimeter Array
Chandra X-ray Observatory
CHARA (Center for High Angular Resolution Astronomy) array
Gemini Observatory
Giant Magellan Telescope (proposed)
Giant Metrewave Radio Telescope
Great Paris Exhibition Telescope of 1900
Hale telescope 1948, 200" reflector, Mount Palomar
Hexapod-Telescope
Hooker Telescope 1917, 100" reflector, Mount Wilson
Hubble Space Telescope
IceCube Neutrino Detector
Isaac Newton Telescope
James Clerk Maxwell Telescope
Keck telescope
Leviathan of Parsonstown 1849, 79" reflector, Birr, Ireland
Lick Observatory
LIGO
Lovell Telescope
McMath-Pierce Solar Telescope
McMath-Hulbert Observatory (Solar)
Magdalena Ridge Observatory
Multiple-Mirror telescope
Navy Prototype Optical Interferometer
Overwhelmingly Large Telescope (proposed)
Parkes Observatory
Southern African Large Telescope
Subaru Telescope
Submillimeter Array
Thirty Meter Telescope (proposed)
UK Schmidt Telescope
United Kingdom Infrared Telescope
Very Large Array
Very Large Telescope
Westerbork Synthesis Radio Telescope
William Herschel Telescope
XMM-Newton
Yerkes 1897, 40" largest optical refractor

Notes

^ Sobel (2000, p.43), Drake (1978, p.196). In the Starry Messenger Galileo had used the term "perspicillum".

References

Contemporary Astronomy - Second Edition, Jay M. Pasachoff, Saunders Colleges Publishing - 1981, ISBN 0-03-057861-2

Sabra, A. I. & Hogendijk, J. P. (2003), The Enterprise of Science in Islam: New Perspectives, MIT Press, pp. 85-118, ISBN 0262194821

External links

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[1] Sobel (2000, p.43), Drake (1978, p.196). In the Starry Messenger Galileo had used the term "perspicillum".

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 A '''telescope''' is an instrument designed for the observation of remote objects and the collection of [[electromagnetic radiation]]. The first known practically functioning telescopes were invented in the [[Netherlands]] at the beginning of the [[17th century]]. The name "telescope" was derived from the [[Greek language|Greek]] ''tele'' = 'far' and ''skopein'' = 'to look or see', and was coined by the Greek mathematician Giovanni Demisiani for one of Galileo's instruments.<ref>[[#Reference-Sobel-2000|Sobel (2000, p.43)]], [[#Reference-Drake-1978|Drake (1978, p.196)]]. In the ''Starry Messenger'' [[Galileo Galilei|Galileo]] had used the term "perspicillum".</ref> "Telescopes" can refer to a whole range of instruments operating in most regions of the [[electromagnetic spectrum]].
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-==History==
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-{{main|History of telescopes}}
-In the 10th and 11th centuries, during the [[Islamic Golden Age]], [[Ibn Sahl]]<ref name="design">[http://unesdoc.unesco.org/images/0014/001412/141236E.pdf Designing the perfect lens]</ref> and [[Ibn al-Haytham]]<ref name="design" /><ref name="optics">[http://www.islamicspain.tv/Arts-and-Science/The-Culture-of-Al-Andalus/Physics-and-Optics.htm Physics and Optics]</ref> made advances in the physical and mathematical understanding of [[optics]] that were essential to the development of spectacle quality [[Lens (optics)|lenses]] and the telescope. There is some documentary evidence, but no surviving designs or physical evidence, that the principles of telescopes were known to [[Leonard Digges (scientist)|Leonard Digges]],<ref>[http://cnx.org/content/m11932/latest/ '''Galileo's Telescope''' - by: Albert Van Helden]</ref> [[Taqi al-Din]]<ref name=Topdemir>{{citation|first=Hüseyin Gazi|last=Topdemir|title=Takîyüddîn'in Optik Kitabi|publisher=Ministry of Culture Press, [[Ankara]]|year=1999}}</ref> and [[Giambattista della Porta]]<ref>Giambattista della Porta, (2005), ''Natural Magick'', page 339. NuVision Publications, LLC.</ref> in the late 16th century. However, the earliest known working telescopes were the [[refracting telescope]]s that appeared in the [[Netherlands]] in 1608. Their development is credited to three individuals: [[Hans Lippershey]] and [[Zacharias Janssen]], who were spectacle makers in Middelburg, and [[Jacob Metius]] of [[Alkmaar]]. [[Galileo Galilei|Galileo]] greatly improved upon these designs the following year. [[Niccolò Zucchi]] is credited with constructing the first [[reflecting telescope]] in 1616. In 1668, [[Isaac Newton]] designed an improved a reflecting telescope that bears his name, the "[[Newtonian telescope|Newtonian reflector]].
-The invention of the [[achromatic lens]] in 1733 partially corrected color aberrations present in the simple lens and enabled the construction of shorter, higher functioning refracting telescopes. Reflecting telescopes, though not limited by the color problems seen in refractors, were hampered by the use of fast tarnishing [[speculum metal]] mirrors employed during the 18th and early 19th century—a problem alleviated by the introduction of silver coated glass mirrors in 1857,<ref>[http://www.madehow.com/inventorbios/39/Jean-Bernard-L-on-Foucault.html  madehow.com - Inventor Biographies - Jean-Bernard-Léon Foucault Biography (1819-1868)]</ref> and aluminized mirrors in 1932.<ref>[http://www.cambridge.org/uk/astronomy/features/amateur/files/p28-4.pdf Bakich sample pages Chapter 2, Page 3 ''"John Donavan Strong, a young physicist at the California Institute of Technology, was one of the first to coat a mirror with aluminum. He did it by thermal vacuum evaporation. The first mirror he aluminized, in 1932, is the earliest known example of a telescope mirror coated by this technique."'']</ref> The maximum physical size limit for refracting telescopes is about 1 meter (40 inches), dictating that the vast majority of large optical research telescopes built since the turn of the [[20th century]] have been reflectors.
-The 20th century also saw the development of telescopes that worked in a wide range of wavelengths from [[radio telescope|radio]] to [[gamma-ray telescope|gamma-rays]]. The first radio telescope went into operation in [[1937]]. Since then, a tremendous variety of complex astronomical instruments have been developed.
 ==Types of telescopes==