Etymology

The word telescope (from the Ancient Greek τῆλε, tele "far" and σκοπεῖν, 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.^[2]^[3] In the Starry Messenger, Galileo had used the term perspicillum.

History

The earliest existing record of a telescope was a 1608 patent submitted to the government in the Netherlands by Middelburg spectacle maker Hans Lippershey for a refracting telescope.^[4] The actual inventor is unknown but word of it spread through Europe. Galileo heard about it and, in 1609, built his own version, and made his telescopic observations of celestial objects.^[5]^[6]

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.^[7] 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.^[8] In 1668, Isaac Newton built the first practical reflecting telescope, of a design which now 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,^[9] and aluminized mirrors in 1932.^[10] 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), and work is underway on several 30-40m designs.

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 large variety of complex astronomical instruments have been developed.

Types

The name "telescope" covers a wide range of instruments.

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

X-ray telescopes, using shorter wavelengths than ultraviolet light
Ultraviolet telescopes, using shorter wavelengths than visible light
Optical telescopes, using visible light
Infrared telescopes, using longer wavelengths than visible light
Submillimetre telescopes, using microwave wavelengths that are longer than those of infrared light
Radio telescopess that use even longer wavelengths

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).

With photons of the shorter wavelengths, with the higher frequencies, glancing-incident optics, rather than fully reflecting optics are used.

Telescopes may also be classified by location: ground telescope, space telescope, or flying telescope. They may also be classified by 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.

An optical telescope gathers and focuses light mainly from the visible part of the electromagnetic spectrum (although some work in the infrared and ultraviolet).^[13] 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 optical 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.

A Fresnel Imager is a proposed ultra-lightweight design for a space telescope that uses a Fresnel lens to focus light.

Beyond these basic optical types there are many sub-types of varying optical design classified by the task they perform such as astrographs, comet seekers and solar telescopes.

Radio telescopes are directional radio antennas that typically employ a large dish to collect radio waves. The dishes are sometimes constructed of a conductive wire mesh whose openings are smaller than the wavelength being observed.

Unlike an optical telescope, which produces a magnified image of the patch of sky being observed, a traditional radio telescope dish contains a single receiver and records a single time-varying signal characteristic of the observed region; this signal may be sampled at various frequencies.

By collecting and correlating signals simultaneously received by several dishes, high-resolution images can be computed.

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 has the advantage of being able to pass through the atmosphere and interstellar gas and dust clouds.

Some radio telescopes are used by programs such as SETI and the Arecibo Observatory to search for extraterrestrial life.

X-rays are much harder to collect and focus than electromagnetic radiation of longer wavelengths. X-ray telescopes can use X-ray optics, such as 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.^[15]^[16] Examples of observatories 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 photon energies of 79 keV.^[14]

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 installed on Earth-orbiting satellites or high-flying balloons since the Earth's atmosphere is opaque to this part of the electromagnetic spectrum. 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.

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

Astronomy is not limited to using electromagnetic radiation.

Cosmic-ray telescopes detect cosmic rays and usually consist of an array of different detector types spread out over a large area.
Energetic neutral atom instruments study the magnetosphere of various bodies by detecting fast moving electrically neutral atoms created by the solar wind.
Neutrino detectors, the equivalent of neutrino telescopes, used for neutrino astronomy. They consist of a large mass of water and ice, surrounded by an array of sensitive light detectors known as photomultiplier tubes. Originating direction of the neutrinos is determined by reconstructing the path of secondary particles scattered by neutrino impacts, from their interaction with multiple detectors.
Gravitational-wave detectors, the equivalent of gravitational wave telescopes, are used for gravitational-wave astronomy. Gravitational waves, caused by violent collisions in space, are detected by extremely precise measurements of the change in length of large earth-bound structures.

Types of mount

A telescope mount is a mechanical structure which supports a telescope.

Atmospheric electromagnetic opacity

Since the atmosphere is opaque for most of the electromagnetic spectrum, only a few bands can be observed from the Earth's surface.

Telescopic image from different telescope types

Different types of telescope, operating in different wavelength bands, provide different information about the same object.

By spectrum

Telescopes that operate in the electromagnetic spectrum:

*Links to categories.