Telescope

For other uses, see Telescope (disambiguation).

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 by the collection of electromagnetic radiation. The first known practically functioning telescopes were invented in the Netherlands at the beginning of the 17th century. "Telescopes" can refer to a whole range of instruments operating in most regions of the electromagnetic spectrum.

The name "Telescope" (from the Greek tele = 'far' and skopein = 'to look or see'; teleskopos = 'far-seeing') was coined in 1611 by the Greek mathematician Giovanni Demisiani for one of Galileo Galilei's instruments presented at a banquet at the Accademia dei Lincei. In the Starry Messenger Galileo had used the term "perspicillum".

History

Main article: History of the telescope

The earliest evidence of 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 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, more functional 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, and aluminized mirrors in 1932. The maximum physical size limit for refracting telescopes is about 1 meter (40 inches), dictating that the vast majority of large optical researching telescopes built since the turn of the 20th century have been reflectors. The largest reflecting telescopes currently have objectives larger then 10 m (33 feet).

The 20th century also saw the development of telescopes that worked in a wide range of wavelengths from radio to gamma-rays. The first purpose built 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

50 cm refracting telescope at Nice Observatory.

Main article: Optical telescope

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

Main article: Radio telescope

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

Main articles: X-ray astronomy and gamma-ray astronomy

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.

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

Other types

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

Notable telescopes

• Anglo-Australian Telescope
• Arecibo Observatory
• Atacama Large Millimeter Array

• Chandra X-ray Observatory
• CHARA (Center for High Angular Resolution Astronomy) array
• Giant Metrewave Radio Telescope
• Great Paris Exhibition Telescope of 1900
• Great Canary Telescope
• 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
• Keck telescope
• Leviathan of Parsonstown 1849, 79" reflector, Birr, Ireland
• Lick Observatory
• LIGO
• Lovell Telescope
• Magdalena Ridge Observatory
• McMath-Hulbert Observatory (Solar)
• McMath-Pierce Solar Telescope
• Multiple-Mirror telescope
• Navy Prototype Optical Interferometer
• Overwhelmingly Large Telescope (proposed)
• Parkes Observatory
• Southern African Large Telescope
• Subaru Telescope
• UK Schmidt Telescope
• Very Large Array
• Very Large Telescope
• Westerbork Synthesis Radio Telescope
• William Herschel Telescope
• XMM-Newton
• Yerkes 1897, 40" largest optical refractor

See also

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• Depth of field
• Dynameter
• f-number
• First light
• History of the telescope
• Keyhole problem
• Microscope
• Nimrud lens
• Remote Telescope Markup Language
• Robotic telescope
• Space observatory
• Timeline of telescope technology
• Timeline of telescopes, observatories, and observing technology

References

• Contemporary Astronomy - Second Edition, Jay M. Pasachoff, Saunders Colleges Publishing - 1981, ISBN 0-03-057861-2
• Template:Harvard reference
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• Sabra, A. I. & Hogendijk, J. P. (2003), The Enterprise of Science in Islam: New Perspectives, MIT Press, pp. 85-118, ISBN 0262194821

External links

• The Galileo Project - The Telescope by Al Van Helden
• "The First Telescopes". Part of an exhibit from Cosmic Journey: A History of Scientific Cosmology by the American Institute of Physics
• Timeline of telescopic technology