telescope
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Optical instrument that magnifies images of faint and distant objects; any device for collecting and focusing light and other forms of electromagnetic radiation. A telescope with a large aperture, or opening, can distinguish finer detail and fainter objects than one with a small aperture. The refracting telescope uses lenses, and the reflecting telescope uses mirrors. A third type, the catadioptric telescope, is a combination of lenses and mirrors. See also radio telescope.
Refractor
In a refractor, light is collected by a lens called the object glass or objective, which focuses light down a tube, forming an image magnified by an eyepiece. Invention of the refractor is attributed to a Dutch optician, Hans Lippershey, in 1608. Hearing of the invention in 1609, Galileo quickly constructed one for himself and went on to produce a succession of such instruments which he used from 1610 onwards for astronomical observations. The largest refracting telescope in the world, at Yerkes Observatory, Williams Bay, Wisconsin, has an aperture of 102 cm/40 in.Reflector
In a reflector, light is collected and focused by a concave mirror. The first reflector was built about 1670 by Isaac Newton. Large mirrors are cheaper to make and easier to mount than large lenses, so all the largest telescopes are reflectors. The largest reflector with a single mirror, 6 m/236 in, is at Zelenchukskaya, Russia. Telescopes with larger apertures composed of numerous smaller segments have been built, such as the Keck Telescopes on Mauna Kea, Hawaii, and the Great Canary Telescope, on Las Palma in the Canary Islands, which is the world's largest optical telescope. A multiple-mirror telescope was installed on Mount Hopkins, Arizona, USA, in 1979. It originally consisted of six mirrors of 1.8 m/72 in aperture, which performed like a single 4.5-m/176-in mirror. The six mirrors were replaced in 1996 by a single 6.5-m/21.3-ft mirror. Schmidt telescopes are used for taking wide-field photographs of the sky. They have a main mirror plus a thin lens at the front of the tube to increase the field of view.In 1995 NASA completed a 3-m/9.8-ft liquid-mirror telescope at its Orbital Debris Observatory in New Mexico. The liquid-mirror telescope is a reflecting telescope constructed with a rotating mercury mirror.
Telescopes in space
Large telescopes can now be placed in orbit above the distorting effects of the Earth's atmosphere. Telescopes in space have been used to study infrared, ultraviolet, and X-ray radiation that does not penetrate the atmosphere but carries much information about the births, lives, and deaths of stars and galaxies. The 2.4-m/94-in Hubble Space Telescope, launched in 1990, can see the sky more clearly than can any telescope on Earth. The European Space Agencyseverals Herschel Space Observatory is scheduled for launch in July 2007. Stationed in the Earthseverals shadow, 1.5 million km/930,000 miles away, it will observe infrared (heat) radiation. Some time from 2013 onward, NASAseverals James Webb Space Telescope will be positioned in the same region of space, where instruments can be kept close to absolute zero in order to observe the faint heat radiation from space.One design for X-ray telescopes for satellites is based on the structure of a lobster's eye, which has thousands of square tubes reflecting light onto the retina. The Lobster-ISS mission is due to be flown on the International Space Station (ISS) in 2008–09, and features a telescope of lobster-eye design.
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