Telescope

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. [Sobel (2000, p.43), Drake (1978, p.196). In the "Starry Messenger" Galileo had used the term "perspicillum".] "Telescopes" can refer to a whole range of instruments operating in most regions of the electromagnetic spectrum.

History

In the 10th and 11th centuries, during the Islamic Golden Age, Ibn Sahl [http://unesdoc.unesco.org/images/0014/001412/141236E.pdf Designing the perfect lens] ] and Ibn al-Haytham [http://www.islamicspain.tv/Arts-and-Science/The-Culture-of-Al-Andalus/Physics-and-Optics.htm Physics and Optics] ] made advances in the physical and mathematical understanding of optics that were essential to the development of spectacle quality 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, [ [http://cnx.org/content/m11932/latest/ Galileo's Telescope - by: Albert Van Helden] ] Taqi al-Dincitation|first=Hüseyin Gazi|last=Topdemir|title=Takîyüddîn'in Optik Kitabi|publisher=Ministry of Culture Press, Ankara|year=1999] and Giambattista della Porta [Giambattista della Porta, (2005), "Natural Magick", page 339. NuVision Publications, LLC.] in the late 16th century. However, the earliest known working telescopes were the refracting telescopes 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 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 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, [ [http://www.madehow.com/inventorbios/39/Jean-Bernard-L-on-Foucault.html madehow.com - Inventor Biographies - Jean-Bernard-Léon Foucault Biography (1819-1868)] ] and aluminized mirrors in 1932. [ [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."] ] 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 to 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

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

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

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) [http://www.vsop.isas.ac.jp/ 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

* Binoculars
* Spotting scopes
* Monoculars
* Telephoto lens
* Solar telescope
* Theodolites

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

ee also

* Amateur telescope making
* Angular resolution
* Aperture synthesis
* ASCOM open standards for computer control of telescopes
* BOOTES
* Depth of field
* Dynameter
* Eyepiece
* First light
* f-number
* History of optics
* History of telescopes
* Keyhole problem
* List of largest optical reflecting telescopes
* List of largest optical refracting telescopes
* Microscope
* Nimrud lens
* Remote Telescope Markup Language
* Robotic telescope
* Space observatory
* Timeline of telescope technology
* Timeline of telescopes, observatories, and observing technology

Notes

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

*Harvard reference
last=Elliott
first=Robert S.
year=1966
title=Electromagnetics
publisher=McGraw-Hill
*Harvard reference
last1=Rashed
first1=Roshdi
last2=Morelon
first2=Régis
year=1996
title=Encyclopedia of the History of Arabic Science
volume=1 & 3
publisher=Routledge
isbn=0415124107
*Harvard reference
last=Wade
first=Nicholas J.
last2=Finger
first2=Stanley
year=2001
title=The eye as an optical instrument: from camera obscura to Helmholtz's perspective
journal=Perception
volume=30
issue=10
pages=1157-1177

External links

* [http://www.aip.org/history/cosmology/tools/tools-first-telescopes.htm "The First Telescopes". Part of an exhibit from Cosmic Journey: A History of Scientific Cosmology] by the American Institute of Physics
* [http://www.eso.org/projects/owl/ ESO 100 m telescope]
* [http://www.licha.de/astro_article_mtf_telescope_resolution.php The Resolution of a Telescope]
* [http://www.salt.ac.za/ Southern African Large Telescope (SALT)]
* [http://www.chocky.demon.co.uk/oas/diggeshistory.html The Digges telescope of the 1570s]
* [http://www.solarphysics.kva.se/ The Swedish Solar telescope]
* [http://www.reviewtelescopes.com/telescope-types/galileo-gets-credit-for-refracting-telescope-28/ History of Refracting Telescope]
* [http://online.physics.uiuc.edu/courses/phys199epp/fall06/Powers-NYTimes.pdf Best Idea; Eyes Wide Open]
* [http://www.timeline-help.com/telescope-timeline.html Timeline of telescopic technology]
* [http://brunelleschi.imss.fi.it/telescopiogalileo/etel.asp?c=50004 The Prehistory of the Invention of the Telescope]