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 , tele "far" and , skopein "to look or see"; τηλεσκόπος, 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

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.

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. 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. 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, 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 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 EM 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

.]] 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, 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:

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 to form an image.

Other optical telescopes:

Infrared telescopes

Submillimetre telescopes

Ultraviolet telescopes

Fresnel Imager

Radio telescopes

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.

High energy particle telescopes

, an X-ray telescope originally named the HEAO B (High Energy Astrophysical Observatory B)]] High-energy astronomy requires specialized telescopes to make observations since most of these particles go through most metals and glasses.

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. In 1952, Hans Wolter outlined 3 ways a telescope could be built using only this kind of mirror.

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.

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.

Other types of telescopes

Gravitational wave telescopes

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

Lists of telescopes

List of optical telescopes

List of largest optical reflecting telescopes

List of largest optical refracting telescopes

List of largest optical telescopes historically

List of radio telescopes

List of space telescopes

List of solar telescopes

List of telescope types

:Category:Telescopes

:Category:Cosmic-ray telescopes

:Category:Gamma-ray telescopes

:Category:Gravitational wave telescopes

:Category:High energy particle telescopes

:Category:Infrared telescopes

:Category:Submillimetre telescopes

:Category:Ultraviolet telescopes

:Category:X-ray telescopes

See also

Amateur telescope making

Angular resolution

Aperture synthesis

ASCOM open standards for computer control of telescopes

Dynameter

f-number

First light

Bahtinov mask

Carey mask

Hartmann mask

Keyhole problem

Microscope

Remote Telescope Markup Language

Robotic telescope

Timeline of telescope technology

Timeline of telescopes, observatories, and observing technology

References

Further reading

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 0-262-19482-1

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

Outside the Optical: Other Kinds of Telescopes

Category:Astronomical imaging Category:Navigation Category:Astronomical instruments Category:Dutch inventions