The telescope is a device that intensifies and magnifies the image of distant objects. The telescope enables astronomers, scientists, and amateurs alike, to observe and study planets, stars, galaxies, and other features of the universe.
The observation of celestial objects is subjected to the limitations imposed on photons traveling over great distances, in that only a small quantity of photons will actually reach Earth. Consequently, the object appears to be faint, and because of the distance, it will also seem to be small. Therefore, the first function of the optical telescope is to intensify the light. This is done by using a lens or a mirror, or both. The larger the diameter of the objective lens, called the aperture, the more photons are captured. The world's largest optical telescope has an aperture of 236 in (6-m). Amateur astronomers will commonly use telescopes with a lens or mirror of 8 in (20-cm) in diameter, thus gathering 512 times more light than the human eye, which has a pupil aperture of less than 1 cm.
The second function of a telescope is to magnify the object. Magnification is achieved by the use of eyepieces of different angular resolution, resolution being defined as the ability to distinguish two close objects or two features on an object. However, magnification is limited by the effects of atmospheric turbulence and distortion, which are also inadvertently magnified. Obviously, visual observation using optical telescopes is only possible on clear nights. Two factors that influence the quality of the observation are transparency (the amount of light the atmosphere will transmit), and seeing, which describes the steadiness of the atmosphere. Wind and turbulence in the upper atmosphere distort images and reduce the sharpness of the objects. The geographical location of astronomical observatories is also of great importance in order to reduce the disturbances; thus they are ideally located on mountain tops and far from urban centers. The Hubble space telescope with its 96 in (2.4 m) aperture is free from such problems; it was placed in orbit far above any atmospheric interference.
Optical telescopes are characterized by the device used to collect the light. The invention of the first refracting telescope in 1609 is credited to Italian astronomer and physicist Galileo Galilei (1564-1642). In its simplest form, the light enters the telescope through the objective lens and passes through the eyepiece lens that produces the image. Galileo's largest instrument was about 48 in (120) cm) long and the width of its objective lens was 2 in (5 cm). Today, the world's largest refractor has an aperture of 40 in (100 cm), and was installed in 1897 at the Yerkes Observatory, Wisconsin.
The reflecting telescope developed by English physicist and mathematician Sir Isaac Newton (1642-1727) in 1668 relied on mirrors to collect the light. The Newtonian reflector captures light with its primary mirror. The captured light is then deflected onto a diagonal mirror near the front of the instrument which will reflect the image to the side through the eyepiece lens. In 1672, a French astronomer Jacques Cassegrain (1652-1712), developed a variation of the reflector. Cassegrain's telescope had a secondary mirror reflecting the image back toward the primary mirror, which had an opening in its center allowing the image to pass through to the eyepiece lens. The largest existing reflector telescope has a 236-in (6 m) reflector, and it is located at the Special Astrophysical Observatory in Zelenchukskaya, Russia. Current compound telescopic designs use both lenses, usually placed in the front, and mirrors. The most well-known of this type is the Schmidt-Cassegrain telescope developed by Estonian born German astronomer Bernhard Voldemar Schmidt 1879-1935). Its 8-in (20 cm) reflector version is widely used by amateur astronomers.
Until the 1930s, astronomical observations were limited to the visible wavelengths of the electromagnetic spectrum. Since then, technological advances have led to the development of radio, x-ray, infrared, and gamma-ray telescopes that allow the exploration of every region of the electromagnetic spectrum. Based upon the discovery of a source of static emanating from outside the solar system by American engineer Karl Janskey (1905-1945), in 1937, another American radio engineer, Grote Reber (1911-) constructed a primitive radio telescope using a parabolic reflector dish mounted in his own backyard. The first large radio telescope, which is still operational, was fitted with an 83-yard ( 76-m) antenna and built in 1957 at Jodrell Bank, England. The largest radio telescope, with its 109-yard (100-m) steerable antenna, is used at the Max Planck Institute for Radio Astronomy at Effelsberg, Germany. In contrast to optical telescopes, radio telescopes can operate at daytime and even in cloudy or rainy conditions. Radio telescopes capture the natural radio signals emitted by objects in space. Equipped with spectrometers and computers to collect, store, and analyze data, they provide information on temperature, radiation, and on the molecular and chemical composition of planets, stars, and gaseous envelopes around stars.
This is the complete article, containing 806 words
(approx. 3 pages at 300 words per page).
