A telescope (from the Greek tele = 'far' and skopein = 'to look or see'; teleskopos = 'far-seeing') is an astronomical tool which gathers and focuses electromagnetic radiation. Telescopes increase the apparent angular size of objects, as well as their apparent brightness. Galileo Galilei is credited with being the first to use a telescope for astronomical purposes in 1609, calling it at first a perspicillum, and then using the terms telescopium in Latin and telescopio in Italian (from which the English word derives). Later, Johannes Kepler described the optics of lenses (see his books Astronomiae Pars Optica and Dioptrice), including a new kind of astronomical telescope with two convex lenses (a principle often called Kepler telescope).
Telescopes used for non-astronomical purposes are often referred to as theodolites, transits, spotting scopes, monoculars, binoculars, camera lenses, microscopes or spyglasses.
The word "telescope" usually refers to optical telescopes, but there are telescopes for most of the spectrum of electromagnetic radiation.
Radio telescopes are focused radio antennas, usually shaped like large dishes. The dish is sometimes constructed of a conductive wire mesh whose openings are smaller than a wavelength. Radio telescopes are often operated in pairs, or larger groups to synthesize large "virtual" apertures that are similar in size to the separation between the telescopes: see aperture synthesis. The current record is many times the width of the Earth, utilizing space-based VLBI telescopes such as the Japanese VSOP satellite. Aperture synthesis is now also being applied to optical telescopes using optical interferometers.
X-ray and gamma-ray telescopes have a problem because these rays go through most metals and glasses. They use ring-shaped "glancing" mirrors, made of heavy metals, that reflect the rays just a few degrees. The mirrors are usually a section of a rotated parabola.
A simple telescope mounting is an altazimuth mount. It is similar to that of a surveying transit. A fork rotates in azimuth (in the horizontal plane), and bearings on the tips of the fork allow the telescope to vary in altitude (in a vertical plane).
The major problem with using an altazimuth for astronomy is that both axes must be continuously adjusted to compensate for the Earth's rotation. Even if this is done, by computer control, the image rotates at a rate that varies depending on the angle of the star from the celestial pole. The last effect especially makes an altazimuth mount impractical for long-exposure photography with small telescopes.
The preferred solution for astronomical telescopes is to tip the altazimuth mount so that the azimuth axis is parallel with the axis of the Earth's rotation; this is known as an equatorial mount.
Recently constructed large telescopes use a computer-controlled altazimuth mount, and for long exposures, they have (usually computer-controlled) variable-rate rotating erector prisms at the focus.
There are mountings even simpler than altazimuth, typically for specialised instruments. A few are: meridian transit (altitude only); fixed with movable plane mirror for solar observing; ball-and-socket (ancient and useless for astronomy).
Most large research telescopes can operate as either a cassegrain telescope (longer focal length, and a narrower field with higher magnification) or newtonian telescope (brighter field). They have a pierced primary, a newtonian focus, and a spider to mount a variety of replaceable secondaries.
A new era of telescope making was inaugurated by the MMT, a synthetic aperture composed of six segments synthesizing a mirror of 4.5 metres diameter. Its example was followed by the Keck telescopes, a synthetic-aperture 10 m telescope.
The current generation of telescopes being constructed have a primary mirror of between 6 and 8 meters in diameter (for ground-based telescopes). In this generation of telescopes, the mirror is usually very thin, and is kept in an optimal shape by an array of actuators (see active optics). This technology has driven new designs for future telescopes with diameters of 30, 50 and even 100 metres.
Initially the detector used in telescopes was the human eye. Later, the sensitized photographic plate took its place, and the spectrograph was introduced, allowing the gathering of spectral information. After the photographic plate, successive generations of electronic detectors , such as the charge-coupled device (CCDs), have been perfected, each with more sensitivity and resolution.
Current research telescopes have several instruments to choose from: imagers, of different spectral responses; spectrographs, useful in different regions of the spectrum; polarimeters, that detect light polarization, etc.
In recent years, some technologies to overcome the bad effect of atmosphere on ground-based telescopes were developed, with good results. See tip-tilt mirror and adaptive optics.
The phenomenon of optical diffraction sets a limit to the resolution and image quality that a telescope can achieve, which is the effective area of the Airy disc, which limits how close we may place two such discs. This absolute limit is called Sparrow's resolution limit . This limit depends on the wavelength of the studied light (so that the limit for red light comes much earlier than the limit for blue light) and on the diameter of the telescope mirror. This means that a telescope with a certain mirror diameter can resolve up to a certain limit at a certain wavelength, so if you want more resolution at that very wavelength, you have to build a wider mirror.
Famous optical telescopes
- The Hubble Space Telescope is in orbit outside of the Earth's atmosphere to allow for observations not distorted by refraction, in this way they can be diffraction limited, and used for coverage in the ultraviolet (UV) and infrared.
- The Very Large Telescope (VLT) is currently (2002) the record holder in size, with four telescopes each 8 metres in diameter. The four telescopes, belonging to ESO and located in the Atacama desert in Chile, can operate independently or together.
- There are many plans for even larger telescopes. One of them is the Overwhelmingly Large Telescope or OWL, which is intended to have a single aperture of 100 metres in diameter.
- The 200 inch (5.08 m) Hale telescope on Palomar Mountain is a conventional research telescope that was the largest for many years. It has a single borosilicate (Pyrex™) mirror that was famously difficult to construct. The mounting is also unique, an equatorial mount that is not a fork, yet permits the telescope to image near the north celestial pole.
- The 100 inch (2.54 m) Hooker Telescope in the Mount Wilson Observatory was used by Edwin Hubble to discover galaxies, and the redshift. The mirror was made of green glass by Saint-Gobain. It is now part of a synthetic aperture array with several other Mt. Wilson telescopes, and is still useful for advanced research.
- The 1.02 m Yerkes Telescope (in Wisconsin) is the largest aimable refractor in use.
- The 0.76 m Nice refractor (in France) that became operational in 1888 was at that time the world's largest telescope. This was the last time the most powerful operationnal telescope in the world was located in Europe. It was outperformed one year later by the 0.91 m refractor at the Lick Observatory.
- The largest refractor ever constructed was French. It was on display at the 1900 Paris Exposition. Its lens was stationary, prefigured so as to sag into the correct shape. The telescope was aimed by the aid of a Foucault sidérostat, which is a movable plane mirror with a 2 m diameter, mounted in a large cast-iron frame. The horizontal tube was 60 m long and the objective had 1.25 m in diameter. It was a failure.
- There is a constellation called Telescopium.
- Old-type telescopes were often made from tube sections that slid inside each other for easier stowage when not being used. As a result, anything that can lengthen and shorten in this way is said to be telescopic (adjective), and the process of shortening in this way is called telescoping (verb).