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The 100 inch (2.54 m) Hookerreflecting telescope at Mount WilsonObservatory near Los Angeles, USA.

TelescopeFrom Wikipedia, the free encyclopedia

For other uses, see Telescope (disambiguation).

A telescope is an instrument that aids in the observation of remoteobjects by collecting electromagnetic radiation (such as visible light).The first known practical telescopes were invented in the Netherlandsat the beginning of the 17th century, using glass lenses. They founduse in terrestrial applications and astronomy.

Within a few decades, the reflecting telescope was invented, whichused mirrors. In the 20th century many new types of telescopes wereinvented, including radio telescopes in the 1930s and infraredtelescopes in the 1960s. The word telescope now refers to a wide rangeof instruments detecting different regions of the electromagneticspectrum, and in some cases other types of detectors.

The word "telescope" (from the Greek τῆλε, tele "far" and σκοπεῖν,skopein "to look or see"; τηλεσκόπος, teleskopos "far­seeing") wascoined in 1611 by the Greek mathematician Giovanni Demisiani forone of Galileo Galilei's instruments presented at a banquet at theAccademia dei Lincei.[1][2][3] In the Starry Messenger, Galileo hadused the term "perspicillum".

Contents

1 History2 Types

2.1 Optical telescopes2.2 Radio telescopes2.3 X­ray telescopes2.4 Gamma­ray telescopes2.5 High­energy particle telescopes2.6 Other types of telescopes

3 Types of mount4 Atmospheric electromagnetic opacity5 Telescopic image from different telescope types6 By spectrum7 Lists of telescopes8 See also9 References10 Sources11 External links

History

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Modern telescopes typically useCCDs instead of film for recordingimages. This is the sensor array in theKepler spacecraft.

28­inch telescope and 40­foottelescope in Greenwich in 2015.

Main article: History of the telescope

The earliest recorded working telescopes were the refracting telescopesthat appeared in the Netherlands in 1608. Their development iscredited to three individuals: Hans Lippershey and Zacharias Janssen,who were spectacle makers in Middelburg, and Jacob Metius ofAlkmaar.[4] Galileo heard about the Dutch telescope in June 1609, builthis own within a month,[5] and improved upon the design in thefollowing year. In the same year, Galileo became the first person topoint a telescope skyward in order to make telescopic observations of acelestial object.[6]

The idea that the objective, or light­gathering element, could be amirror instead of a lens was being investigated soon after the inventionof the refracting telescope.[7] The potential advantages of usingparabolic mirrors—reduction of spherical aberration and no chromaticaberration—led to many proposed designs and several attempts tobuild reflecting telescopes.[8] In 1668, Isaac Newton built the firstpractical reflecting telescope, of a design which now bears his name,the Newtonian reflector.

The invention of the achromatic lens in 1733 partially corrected coloraberrations present in the simple lens and enabled the construction ofshorter, more functional refracting telescopes. Reflecting telescopes,though not limited by the color problems seen in refractors, werehampered by the use of fast tarnishing speculum metal mirrorsemployed during the 18th and early 19th century—a problemalleviated by the introduction of silver coated glass mirrors in 1857,[9]

and aluminized mirrors in 1932.[10] The maximum physical size limitfor refracting telescopes is about 1 meter (40 inches), dictating that thevast majority of large optical researching telescopes built since the turnof the 20th century have been reflectors. The largest reflectingtelescopes currently have objectives larger than 10 m (33 feet), andwork is underway on several 30­40m designs.

The 20th century also saw the development of telescopes that worked in a wide range of wavelengths fromradio to gamma­rays. The first purpose built radio telescope went into operation in 1937. Since then, atremendous variety of complex astronomical instruments have been developed.

Types

The name "telescope" covers a wide range of instruments. Most detect electromagnetic radiation, but there aremajor differences in how astronomers must go about collecting light (electromagnetic radiation) in differentfrequency bands.

Telescopes may be classified by the wavelengths of light they detect:

X­ray telescopes, using shorter wavelengths than ultraviolet light

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Light Comparison

Name Wavelength Frequency (Hz) Photon Energy (eV)Gamma ray less than 0.01 nm more than 10 EHZ 100 keV – 300+ GeV XX­Ray 0.01 to 10 nm 30 PHz – 30 EHZ 120 eV to 120 keV X

Ultraviolet 10 nm – 400 nm 30 EHZ – 790 THz 3 eV to 124 eVVisible 390 nm – 750 nm 790 THz – 405 THz 1.7 eV – 3.3 eV XInfrared 750 nm – 1 mm 405 THz – 300 GHz 1.24 meV – 1.7 eV X

Microwave 1 mm – 1 meter 300 GHz – 300 MHz 1.24 meV – 1.24 µeVRadio 1 mm – km 300 GHz – 3 Hz 1.24 meV – 12.4 feV X

50 cm refracting telescopeat Nice Observatory.

Ultraviolet telescopes, using shorter wavelengths than visible lightOptical telescopes, using visible lightInfrared telescopes, using longer wavelengths than visible lightSubmillimetre telescopes, using longer wavelengths than infrared lightFresnel Imager, an optical lens technologyX­ray optics, optics for certain X­ray wavelengths

As wavelengths become longer, it becomes easier touse antenna technology to interact withelectromagnetic radiation (although it is possible tomake very tiny antenna). The near­infrared can behandled much like visible light, however in the far­infrared and submillimetre range, telescopes canoperate more like a radio telescope. For example,the James Clerk Maxwell Telescope observes fromwavelengths from 3 μm (0.003 mm) to 2000 μm(2 mm), but uses a parabolic aluminum antenna.[11] On the other hand, the Spitzer Space Telescope, observingfrom about 3 μm (0.003 mm) to 180 μm (0.18 mm) uses a mirror (reflecting optics). Also using reflectingoptics, the Hubble Space Telescope with Wide Field Camera 3 can observe from about 0.2 μm (0.0002 mm) to1.7 μm (0.0017 mm) (from ultra­violet to infrared light).[12]

At the photon energy of shorter wavelengths and higher frequency, fully reflecting optics rather than glancing­incident optics are used. Telescopes such as TRACE and SOHO use special mirrors to reflect Extremeultraviolet, producing higher resolution and brighter images than otherwise possible. A larger aperture doesnot just mean that more light is collected, it also enables a finer angular resolution.

Telescopes may also be classified by location: ground telescope, space telescope, or flying telescope. Theymay also be classified by whether they are operated by professional astronomers or amateur astronomers. Avehicle or permanent campus containing one or more telescopes or other instruments is called an observatory.

Optical telescopes

Main article: Optical telescope

An optical telescope gathers and focuses light mainly from the visible part of theelectromagnetic spectrum (although some work in the infrared andultraviolet).[13] Optical telescopes increase the apparent angular size of distantobjects as well as their apparent brightness. In order for the image to beobserved, photographed, studied, and sent to a computer, telescopes work byemploying one or more curved optical elements, usually made from glass lensesand/or mirrors, to gather light and other electromagnetic radiation to bring thatlight or radiation to a focal point. Optical telescopes are used for astronomy andin many non­astronomical instruments, including: theodolites (includingtransits), spotting scopes, monoculars, binoculars, camera lenses, andspyglasses. There are three main optical types:

The refracting telescope which uses lenses to form an image.The reflecting telescope which uses an arrangement of mirrors to form animage.The catadioptric telescope which uses mirrors combined with lenses to

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

Einstein Observatory was a space­based focusing optical X­raytelescope from 1978.[14]

form an image.

Beyond these basic optical types there are many sub­types of varying optical design classified by the task theyperform such as astrographs, comet seekers, solar telescope, etc.

Radio telescopes

Main article: Radio telescope

Radio telescopes are directional radio antennas used for radio astronomy.The dishes are sometimes constructed of a conductive wire mesh whoseopenings are smaller than the wavelength being observed. Multi­elementRadio telescopes are constructed from pairs or larger groups of thesedishes to synthesize large 'virtual' apertures that are similar in size to theseparation between the telescopes; this process is known as aperturesynthesis. As of 2005, the current record array size is many times thewidth of the Earth—utilizing space­based Very Long BaselineInterferometry (VLBI) telescopes such as the Japanese HALCA (HighlyAdvanced Laboratory for Communications and Astronomy) VSOP (VLBISpace Observatory Program) satellite (http://www.vsop.isas.ac.jp/).Aperture synthesis is now also being applied to optical telescopes using optical interferometers (arrays ofoptical telescopes) and aperture masking interferometry at single reflecting telescopes. Radio telescopes arealso used to collect microwave radiation, which is used to collect radiation when any visible light is obstructedor faint, such as from quasars. Some radio telescopes are used by programs such as SETI and the AreciboObservatory to search for extraterrestrial life.

X­ray telescopes

Main article: X­ray telescope

X­ray telescopes can use X­ray optics, such as a Wolter telescopescomposed of ring­shaped 'glancing' mirrors made of heavy metals that areable to reflect the rays just a few degrees. The mirrors are usually asection of a rotated parabola and a hyperbola, or ellipse. In 1952, HansWolter outlined 3 ways a telescope could be built using only this kind ofmirror.[15][16] Examples of an observatory using this type of telescope arethe Einstein Observatory, ROSAT, and the Chandra X­Ray Observatory.By 2010, Wolter focusing X­ray telescopes are possible up to 79 keV.[14]

Gamma­ray telescopes

Higher energy X­ray and Gamma­ray telescopes refrain from focusing completely and use coded aperturemasks: the patterns of the shadow the mask creates can be reconstructed to form an image.

X­ray and Gamma­ray telescopes are usually on Earth­orbiting satellites or high­flying balloons since theEarth's atmosphere is opaque to this part of the electromagnetic spectrum. However, high energy X­rays andgamma­rays do not form an image in the same way as telescopes at visible wavelengths. An example of thistype of telescope is the Fermi Gamma­ray Space Telescope.

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Equatorial­mounted Kepleriantelescope

The detection of very high energy gamma rays, with shorter wavelength and higher frequency than regulargamma rays, requires further specialization. An example of this type of observatory is VERITAS. Very highenergy gamma­rays are still photons, like visible light, whereas cosmic rays includes particles like electrons,protons, and heavier nuclei.

A discovery in 2012 may allow focusing gamma­ray telescopes.[17] At photon energies greater than 700 keV,the index of refraction starts to increase again.[17]

High­energy particle telescopes

High­energy astronomy requires specialized telescopes to make observations since most of these particles gothrough most metals and glasses.

In other types of high energy particle telescopes there is no image­forming optical system. Cosmic­raytelescopes usually consist of an array of different detector types spread out over a large area. A Neutrinotelescope consists of a large mass of water or ice, surrounded by an array of sensitive light detectors known asphotomultiplier tubes. Energetic neutral atom observatories like Interstellar Boundary Explorer detect particlestraveling at certain energies.

Other types of telescopes

Astronomy is not limited to using electromagnetic radiation.Additional information can be obtained using other media. Thedetectors used to observe the Universe are analogous to telescopes,these are:

Gravitational­wave detector, the equivalent of a gravitationalwave telescope, used for gravitational­wave astronomy.Neutrino detector, the equivalent of a neutrino telescope, usedfor neutrino astronomy.

Types of mount

Main article: Telescope mount

A telescope mount is a mechanical structure which supports atelescope. Telescope mounts are designed to support the mass of thetelescope and allow for accurate pointing of the instrument. Many sorts of mounts have been developed overthe years, with the majority of effort being put into systems that can track the motion of the stars as the Earthrotates. The two main types of tracking mount are:

Altazimuth mountEquatorial mount

Atmospheric electromagnetic opacity

See also: Airmass

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Since the atmosphere is opaque for most of the electromagnetic spectrum, only a few bands can be observedfrom the Earth's surface. These bands are visible – near­infrared and a portion of the radio­wave part of thespectrum. For this reason there are no X­ray or far­infrared ground­based telescopes as these have to beobserved from orbit. Even if a wavelength is observable from the ground, it might still be advantageous toplace a telescope on a satellite due to astronomical seeing.

A diagram of the electromagnetic spectrum with the Earth's atmospheric transmittance (or opacity) and the types oftelescopes used to image parts of the spectrum.

Telescopic image from different telescope types

Different types of telescope, operating in different wavelength bands, provide different information about thesame object. Together they provide a more comprehensive understanding.

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A 6′ wide view of the Crab nebula supernova remnant, viewed at different wavelengths of light byvarious telescopes

By spectrum

Telescopes that operate in the electromagnetic spectrum:

Name Telescope Astronomy Wavelength

Radio Radio telescope Radio astronomy(Radar astronomy) more than 1 mm

Submillimetre Submillimetre telescopes* Submillimetre astronomy 0.1 mm – 1 mmFar Infrared – Far­infrared astronomy 30 µm – 450 µmInfrared Infrared telescope Infrared astronomy 700 nm – 1 mmVisible Visible spectrum telescopes Visible­light astronomy 400 nm – 700 nm

Ultraviolet Ultraviolet telescopes* Ultraviolet astronomy 10 nm – 400 nmX­ray X­ray telescope X­ray astronomy 0.01 nm – 10 nm

Gamma­ray – Gamma­ray astronomy less than 0.01 nm

*Links to categories.

Lists of telescopes

List of optical telescopesList of largest optical reflecting telescopes

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List of largest optical refracting telescopesList of largest optical telescopes historicallyList of radio telescopesList of solar telescopesList of space observatoriesList of telescope parts and constructionList of telescope typesCategory:TelescopesCategory:Cosmic­ray telescopesCategory:Gamma­ray telescopesCategory:Gravitational wave telescopesCategory:High energy particle telescopesCategory:Infrared telescopesCategory:Submillimetre telescopesCategory:Ultraviolet telescopesCategory:X­ray telescopes

See also

AirmassAmateur telescope makingAngular resolutionASCOM open standards for computer control of telescopesBahtinov maskBioptic telescopeCarey maskDynameterf­numberFirst lightGoTo telescopeHartmann maskKeyhole problemMicroscopeRemote Telescope Markup LanguageRobotic telescopeTimeline of telescope technologyTimeline of telescopes, observatories, and observing technology

References1. archive.org "Galileo His Life And Work" BY James La Rosa "Galileo usually called the telescope occhicde orcannocchiale ; and now he calls the microscope occhialino. The name telescope was first suggested by Demisiani in1612" (http://www.archive.org/stream/galileohislifean011377mbp/galileohislifean011377mbp_djvu.txt)

2. Sobel (2000, p.43), Drake (1978, p.196)3. Rosen, Edward, The Naming of the Telescope (1947)4. galileo.rice.edu The Galileo Project > Science > The Telescope by Al Van Helden: "The Hague discussed the patent

applications first of Hans Lipperhey of Middelburg, and then of Jacob Metius of Alkmaar... another citizen ofMiddelburg, Sacharias Janssen had a telescope at about the same time but was at the Frankfurt Fair where he tried tosell it" (http://galileo.rice.edu/sci/instruments/telescope.html)

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Wikiquote has quotationsrelated to: Telescope

Wikimedia Commons hasmedia related to Telescope.

5. Aleck Loker, Profiles in Colonial History, page 15 (http://books.google.com/books?id=Lq1rd1ecFCYC&pg=PA15&lpg=PA15&dq=Galileo++Dutch+telescope+in+June+1609&source=bl&ots=BXPFWi9EYE&sig=QKgRU4Fuj5lSSrY3OHNtlOk983M&hl=en&sa=X&ei=F7hbUMDmHqmP0QGLooDoAg&ved=0CDcQ6AEwAg#v=onepage&q=Galileo%20%20Dutch%20telescope%20in%20June%201609&f=false)

6. Telescope history (http://www.nasa.gov/audience/forstudents/9­12/features/telescope_feature_912.html)7. Stargazer – By Fred Watson, Inc. NetLibrary, Page 109 (http://books.google.com/books?

id=2LZZginzib4C&pg=PA40&dq=intitle:Stargazer+digges+coins&lr=&as_brr=0&ei=BIwrSc6pB4OClQT4zfyxBg#PPA108,M1)

8. Attempts by Niccolò Zucchi and James Gregory and theoretical designs by Bonaventura Cavalieri, Marin Mersenne,and Gregory among others

9. madehow.com – Inventor Biographies – Jean­Bernard­Léon Foucault Biography (1819–1868)(http://www.madehow.com/inventorbios/39/Jean­Bernard­L­on­Foucault.html)

10. Bakich sample pages chapter 2, page 3: "John Donavan Strong, a young physicist at the California Institute ofTechnology, was one of the first to coat a mirror with aluminum. He did it by thermal vacuum evaporation. The firstmirror he aluminized, in 1932, is the earliest known example of a telescope mirror coated by this technique."(http://www.cambridge.org/uk/astronomy/features/amateur/files/p28­4.pdf)

11. The James­Clerk­Maxwell Observatory: The largest submillimetre radio telescope in the world (http://astro­canada.ca/_en/a2111.html)

12. ESA/Hubble – Hubble's Instruments: WFC3 – Wide Field Camera 3(http://www.spacetelescope.org/about/general/instruments/wfc3/)

13. Barrie William Jones, The search for life continued: planets around other stars, page 111(http://books.google.com/books?id=5wX9aHqfBS0C&pg=PA111&dq=%22optical+telescope+is%22&lr=&cd=55#v=onepage&q=%22optical%20telescope%20is%22&f=false)

14. NuStar: Instrumentation: Optics (http://www.nustar.caltech.edu/about­nustar/instrumentation/optics)15. Wolter, H. (1952), "Glancing Incidence Mirror Systems as Imaging Optics for X­rays", Ann. Physik 10: 94,

Bibcode:1952AnP...445...94W, doi:10.1002/andp.19524450108.16. Wolter, H. (1952), "A Generalized Schwarschild Mirror Systems For Use at Glancing Incidence for X­ray Imaging",

Ann. Physik 10 (4–5): 286, Bibcode:1952AnP...445..286W, doi:10.1002/andp.19524450410.17. Tim Wogan – Silicon 'prism' bends gamma rays (May 2012) – PhysicsWorld.com

(http://physicsworld.com/cws/article/news/2012/may/09/silicon­prism­bends­gamma­rays)

Sources

Contemporary Astronomy – Second Edition, Jay M. Pasachoff, Saunders Colleges Publishing – 1981,ISBN 0­03­057861­2Elliott, Robert S. (1966), Electromagnetics, McGraw­HillRashed, Roshdi; Morelon, Régis (1996), Encyclopedia of the History of Arabic Science, 1 & 3,Routledge, ISBN 0­415­12410­7Wade, Nicholas J.; Finger, Stanley (2001), "The eye as an optical instrument: from camera obscura toHelmholtz's perspective", Perception 30 (10): 1157–1177, doi:10.1068/p3210, PMID 11721819Sabra, A. I.; Hogendijk, J. P. (2003). The Enterprise of Science in Islam: New Perspectives. MIT Press.pp. 85–118. ISBN 0­262­19482­1.

External links

Galileo to Gamma Cephei – The History of the Telescope(http://telescopes.stardate.org/)The Galileo Project – The Telescope by Al Van Helden(http://galileo.rice.edu/sci/instruments/telescope.html)"The First Telescopes". Part of an exhibit from Cosmic Journey:A History of Scientific Cosmology

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(http://www.aip.org/history/cosmology/tools/tools­first­telescopes.htm) by the American Institute ofPhysicsOutside the Optical: Other Kinds of Telescopes(http://spiff.rit.edu/classes/phys230/lectures/nonoptical/nonoptical.html)Gray, Meghan; Merrifield, Michael (2009). "Telescope Diameter". Sixty Symbols. Brady Haran for theUniversity of Nottingham.

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Categories: Telescopes Astronomical imaging Astronomical instruments Dutch inventions

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