Telescope

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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 to aid the observation of remote objects by collecting some form of electromagnetic radiation (such as visible light). The first known practical telescopes were invented in the Netherlands at the beginning of the 17th century. The word telescope can refer to a wide range of instruments detecting different regions of the electromagnetic spectrum.

The word "telescope" (from the Greek τῆλε, tele "far" and σκοπεῖν, skopein "to look or see"; τηλεσκόπος, teleskopos "far-seeing") was coined in the 3rd century by the Greek philosopher Iamblichus who said, "... sight is made precise by the compass, rule, and telescope."[1] When telescopes were rediscovered in the renaissance, the word was again used in 1611 by the Greek mathematician Giovanni Demisiani for one of Galileo Galilei's instruments presented at a banquet at the Accademia dei Lincei.[2][3][4] 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.[5] Galileo greatly improved upon these designs the following year.

The idea that the objective, or light-gathering element, could be a mirror instead of a lens was being investigated soon after the invention of the refracting telescope.[6] The potential advantages of using parabolic mirrors—reduction of spherical aberration and no chromatic aberration—led to many proposed designs and several attempts to build reflecting telescopes.[7] In 1668, Isaac Newton built the first practical reflecting telescope, which 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,[8] and aluminized mirrors in 1932.[9] 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 than 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. One important classification is the type of radiation the telescope is detecting. Most telescopes detect electromagnetic radiation, but there are major differences in how astronomers must go about collecting light (electromagnetic radiation) in different frequency bands.

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

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).[10] 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, and/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:

Other optical telescopes:

  • Ultraviolet telescope, shorter wavelengths than visible light
  • X-ray telescope, shorter wavelengths than ultraviolet light

As wavelengths become longer, it becomes easier to use antenna technology to interact with electromagnetic radiation (although it is possible to make very tiny antenna). The near-infrared can be handled much like visible light, however in the far-infrared and submillimeter range, telescopes can operate more like a radio telescope. For example the James Clerk Maxwell Telescope observes from wavelengths from 3 μm (0.03 mm) to 2000 μm (2 mm), but uses a parabolic aluminum antenna.[11] On the other hand, the Spitzer Space Telescope, observing from about 3 μm (0.03 mm) to 180 μm (0.18 mm) uses a mirror (reflecting optics). Also using reflecting optics, the Hubble Space Telescope with Wide Field Camera 3 can observe from about 0.2 μm (.0002 mm) to 1.7 μm (.0017 mm) (from ultra-violet to infrared light).[12]

Radio telescopes

Main article: Radio telescope
The Very Large Array at Socorro, New Mexico, United States.

Radio telescopes are directional radio antennas used for radio astronomy. 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.

X-ray telescopes

Main article: X-ray telescope
Einstein Observatory was a space-based focusing optical X-ray telescope from 1978.[13]

X-ray telescopes can use X-ray optics, such as a 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. In 1952, Hans Wolter outlined 3 ways a telescope could be built using only this kind of mirror.[14][15] Examples of an observatory using this type of telescope are the Einstein Observatory, ROSAT, and the Chandra X-Ray Observatory. By 2010, Wolter focusing X-ray telescopes are possible up to 79 keV.[13]

Gamma-ray telescopes

Higher energy X-ray and 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.

X-ray and Gamma-ray 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. However, high energy x-rays and gamma-rays do not form an image in the same way as telescopes at visible wavelengths. An example of this type of telescope is the Fermi Gamma-ray Space Telescope.

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

High energy particle telescopes

High-energy astronomy requires specialized telescopes to make observations since most of these particles go through most metals and glasses.

In other types of high energy particle telescopes there is no image-forming optical system. Cosmic-ray telescopes usually consist of an array of different detector types spread out over a large area. A Neutrino telescope consists of a large mass of water or ice, surrounded by an array of sensitive light detectors known as photomultiplier tubes. Energetic neutral atom observatories like Interstellar Boundary Explorer detect particles traveling at certain energies.

Other types of telescopes

Atmospheric electromagnetic opacity

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.

Telescopic image from different telescope types

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

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 mm
Far Infrared - Far-infrared astronomy 30 µm - 450 µm
Infrared Infrared telescope Infrared astronomy 700 nm - 1 mm
Visible Visible spectrum telescopes Visible-light astronomy 400 nm - 700 nm
Ultraviolet Ultraviolet telescopes* Ultraviolet astronomy 10 nm - 400 nm
X-ray X-ray telescope X-ray astronomy 0.01 nm - 10 nm
Gamma-ray - Gamma-ray astronomy less than 0.01 nm

*Category

Lists of telescopes

See also

References

  1. ^ Iamblichus, Life of Pythagoras, 3rd century
  2. ^ archive.org "Galileo His Life And Work" BY J. J. FAHIE "Galileo usually called the telescope occhicde or cannocchiale ; and now he calls the microscope occhialino. The name telescope was first suggested by Demisiani in 1612"
  3. ^ Sobel (2000, p.43), Drake (1978, p.196)
  4. ^ Rosen, Edward, The Naming of the Telescope (1947)
  5. ^ 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 of Middelburg, Sacharias Janssen had a telescope at about the same time but was at the Frankfurt Fair where he tried to sell it"
  6. ^ Stargazer - By Fred Watson, Inc NetLibrary, Page 109
  7. ^ Attempts by Niccolò Zucchi and James Gregory and theoretical designs by Bonaventura Cavalieri, Marin Mersenne, and Gregory among others
  8. ^ madehow.com - Inventor Biographies - Jean-Bernard-Léon Foucault Biography (1819-1868)
  9. ^ 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."
  10. ^ Barrie William Jones, The search for life continued: planets around other stars, page 111
  11. ^ The James-Clerk-Maxwell Observatory: The largest submillimetre radio telescope in the world
  12. ^ ESA/Hubble - Hubble's Instruments: WFC3 - Wide Field Camera 3
  13. ^ a b NuStar: Instrumentation: Optics
  14. ^ Wolter, H. (1952), "Glancing Incidence Mirror Systems as Imaging Optics for X-rays", Ann. Physik, 10: 94.
  15. ^ Wolter, H. (1952), "A Generalized Schwarschild Mirror Systems For Use at Glancing Incidence for X-ray Imaging", Ann. Physik, 10: 286.

Further reading

  • Contemporary Astronomy - Second Edition, Jay M. Pasachoff, Saunders Colleges Publishing - 1981, ISBN 0-03-057861-2
  • Elliott, Robert S. (1966), Electromagnetics, McGraw-Hill
  • Rashed, Roshdi; Morelon, Régis (1996), Encyclopedia of the History of Arabic Science, vol. 1 & 3, Routledge, ISBN 0415124107
  • Wade, Nicholas J.; Finger, Stanley (2001), "The eye as an optical instrument: from camera obscura to Helmholtz's perspective", Perception, 30 (10): 1157–1177, doi:10.1068/p3210, PMID 11721819
  • Sabra, A. I. & Hogendijk, J. P. (2003), The Enterprise of Science in Islam: New Perspectives, MIT Press, pp. 85–118, ISBN 0-262-19482-1

External links

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