Ward's World + McGraw Hill's AccessScience

37211_Ward's World+MGH Telescope

Issue link: https://wardsworld.wardsci.com/i/1448865

Contents of this Issue

Navigation

Page 2 of 5

ometry provide still finer resolution, because the diameter of the separation acts as a synthetic aperture, extending for some purposes the effective aperture to the diameter of the Earth or greater. Airglow in Earth's upper atmosphere, caused by the constant influx of charged particles from the Sun, called the solar wind, is another limit to seeing. Placing telescopes into space overcomes both of these limitations. Optical telescopes Optical telescopes collect large quantities of visible light and resolve fine details. The brightness of the image is proportional to the area of the light-gathering element, which is propor- tional to the square of that element's aperture. The brightness also depends on the area over which the image is spread. This area is inversely proportional to the square of the focal length of the lens. There are basically three types of optical systems in use in as- tronomical telescopes: refracting systems with main optical ele- ments that are lenses, which focus light by refraction; reflecting systems, with main imaging elements that are mirrors, which focus light by reflection; and catadioptric systems, with main elements that are a combination of a lens and a mirror. In each case, the main optical element, or objective, collects the light from a distant object and focuses it into an image that can then be examined. Refracting telescopes Small refracting telescopes are used in binoculars, cameras, gun sights, galvanometers, periscopes, surveying instruments, rangefinders, astronomical telescopes, and a great variety of other devices. In refracting telescopes, parallel or nearly parallel light from the distant object enters from the left, and the objec- tive lens forms an inverted image of it (Fig. 4). The inverted im- age is viewed with the aid of a second lens, called the eyepiece. The eyepiece is adjusted (focused) to form a parallel bundle of rays so that the image of the object may be viewed by the eye without strain. The objective lens is typically compound; that is, it is made up of two or more pieces of glass, of different types, designed to correct for aberrations such as chromatic aberra- tion. To construct a visual refractor, a lens is placed beyond the images formed by the objective and viewed with the eye. To construct a photographic refractor or simply a camera, an elec- tronic detector is placed at the position of the image (Fig. 5). A refractor lens must be relatively thin to avoid excessive absorption of light in the glass. Conversely, the lens can be supported only around its edge and thus is subject to sagging distortions that change as the telescope is pointed from the ho- rizon to the zenith; thus, its thickness must be great enough to give it mechanical rigidity. An effective compromise between these two demands is extremely difficult, making larger refrac- tors unfeasible. With the closing of the Yerkes Observatory in Wisconsin in October 2018, the largest refracting telescope presently operating is the Swedish 1 m (40 in.) Solar Telescope on La Palma in the Canary Islands. Reflecting telescopes The objective of a reflecting telescope is a mirror. The mir- ror forms an image of a celestial object (Fig. 6), which is then examined with an eyepiece, photographed, or studied in some other manner. By using a second mirror (and even a third one, in some tele- scopes), the optical path in a reflector can be folded back on itself (Fig. 7a), permitting a long focal length to be attained with an instrument housed in a short tube. A short tube can be held by a smaller mounting system and can be housed in a smaller dome than a long-tube refractor. Reflecting telescopes are often used in applications where great light-gathering capabilities are required. The telescopes Telescope (continued) Fig. 4 Simplified optical diagram of a refracting telescope. Fig. 5 Refracting optical system used to photograph a star field. Fig. 6 Viewing a star with a reflecting telescope. For a small telescope, an observer at the prime focus shown would block the mirror, so various configurations and secondary mirrors are shown to provide the image at other locations. For larger telescopes, detectors or even people can be at the prime focus without seriously blocking the light path. + ward ' s science

Articles in this issue

Links on this page

Archives of this issue

view archives of Ward's World + McGraw Hill's AccessScience - 37211_Ward's World+MGH Telescope