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Light and other kinds of electromagnetic radiation coming from the universe outside the Earth must travel enormous distances through space and time to reach observers. Only the brightest and nearest stars can be seen with the unaided eye. To see farther and to clarify and measure what is seen, a telescope is needed. The word telescope is derived from the Greek words tele, "from afar, " and shops, "viewer. " Even a simple homemade telescope can clearly show Saturn's rings, Jupiter's bands and red spot, stars, nebulae, and nearby galaxies not visible to the unaided eye.
The ability to study the distant planets and other structures in the universe with these powerful yet remarkably simple instruments has revolutionized mankind's understanding of the natural world. All telescopes gather radiation from distant objects over a large area and focus it, thereby increasing the intensity of the radiation and allowing the objects to be magnified. Sophisticated telescopes are used to view radiation in all parts of the electromagnetic spectrum from long-wave radiation and radio waves to infrared radiation and light and much shorter wave radiation, including ultraviolet and X rays. This radiation travels through space at the speed of light in the form of waves of electric and magnetic fields. Because of its basic similarity, all such radiation can be focused by reflecting it off a curved surface or by refracting, or bending, it with glass lenses.
The devices that are used to do this, however, vary, depending on the wavelength or type of radiation being studied. Optical Telescopes The first telescope developed, and the one most widely used, is the optical telescope, which gathers visible light radiation. There are three basic types of optical telescopes: refractors that use lenses, reflectors that use mirrors, and cata dioptrics that use a combination of both lenses and mirrors. The refracting telescope has a closed tube.
At one end of the tube is the object glass, usually made of two or more lenses, that admits light emanating from the object observed. The light rays are refracted by the lenses to a point of focus at the lower end of the tube where the eyepiece is located. The eyepiece acts as a magnifying glass and enlarges the bright image. An observer can view objects through the eyepiece or attach a camera to the telescope to record images.
The reflecting telescope focuses light rays with a large curved concave mirror that is generally made of glass covered with a thin coating of aluminum. In the simplest reflector, called the Newtonian reflector after its inventor Sir Isaac Newton, light is collected by a primary curved mirror at the bottom of the tube and reflected forward to a secondary mirror. The secondary mirror is flat and mounted at a 45 -degree angle that deflects the converging light rays 90 degrees to the eyepiece. The light-gathering power of a telescope is determined by the diameter of its objective mirror or lens.
This light-gathering power determines how faint an object the telescope can observe. Telescope magnification is determined by the ratio of the objective focal length to that of the eyepiece. Focal length is the distance from objective to focal point. Thus, the longer the focal length the greater the magnification. In order to make long focal length telescopes more compact, the secondary mirror can also be made curved as well. In such reflectors -- which are called Cassegrain reflectors after N.
Cassegrain, the French optician who invented them -- the secondary mirror is a convex front surface mirror that reflects light collected by the concave primary mirror straight back down the tube through a hole in the center of the primary mirror. The combined action of the two mirrors dramatically increases the telescope's effective focal length over its actual length. All telescopes suffer from optical defects called aberrations. Aberrations are distortions in the image.
Refractors suffer from chromatic aberrations caused by the varying degree that light rays of different wavelengths are bent by the lens. By using compound lenses made of different types of glass, such chromatic aberrations can be eliminated. Reflectors also have various aberrations that occur when light from the side of the viewing region is not precisely focused. To correct both sets of aberrations, some telescopes use thin lenses called cata dioptrics. Catadioptrics are used for photographing wide areas of the sky with low distortion. Mountings and Size In general a telescope can be pointed in all directions if two mutually perpendicular axes of rotation are provided.
In both large and small visual telescopes these axes are often made vertical and horizontal in what is known as an altazimuth mount. For some telescopes, the equatorial mount is frequently used. In this mount one of the axes, known as the polar axis, is made accurately parallel to the axis of the Earth. The axis perpendicular to this is known as the declination axis. This type of mounting has the great advantage that any object can be followed from east to west by driving the polar axis at the uniform rate of one revolution in 24 hours.
A typical reflecting telescope used by an amateur astronomer may have a primary mirror measuring 4 to 8 inches in diameter. Reflecting telescopes used by professional astronomers usually have mirrors that measure more than 60 inches in diameter. One of the largest is a 236 -inch telescope in the Caucasus Mountains of Eastern Europe that began operating in 1976. From 1948 until 1976 the largest reflecting telescope in the world was the Hale instrument at the Palomar Observatory near Pasadena, Calif. , with its 200 -inch mirror. Other reflecting telescopes more than 150 inches in diameter are located at observatories near Tucson, Ariz. ; La Sean, Chile; and Siding Spring, Australia. The two largest refracting telescopes are the 36 -inch instrument at Lick Observatory of the University of California, located on Mount Hamilton, and the 40 -inch telescope of the Yerkes Observatory of the University of Chicago, located in Williams Bay, Wis.
The focal length of the Yerkes Observatory refractor is 63 feet. An obstacle to building ever larger telescopes is the distortion of large lenses and mirrors caused by gravity. In 1978 an innovative reflector called the Multiple Mirror Telescope (MMT) began operation at the Smithsonian Astrophysical Observatory in Arizona. Instead of one large mirror, the MMT features six mirrors arranged to focus together. The six-mirror combination acts like a single mirror 21 feet in diameter. Similarly, the Keck Telescope on Mauna Kea in Hawaii, completed in 1991, has a 33 -foot series of mirrors forming a mosaic of hexagons.
Astronomers operating the New Technology Telescope of the European Southern Observatory in La Silla, Chile, use a special computer system that frequently pushes and tugs on the mirror to keep it from sagging under its own weight. Other telescopes under construction in the early 1990 s that are based on innovative mirror designs include the Columbus Telescope on Mount Graham in Arizona and the Very Large Telescope in Chile. The Nordic Optical Telescope in the Canary Islands has the thinnest and lightest mirror of any comparably sized telescope in the world. A telescope's resolution is its ability to delineate distant objects that appear close in the sky -- increases proportionally to the diameter of the objective. A 6 -inch telescope theoretically can resolve stars 0. 6 second of arc apart. (A second of arc is a tiny unit of measure; for example, a penny must be 2. 5 miles away before it appears as small as 1 second of arc. ) This resolving power limits useful magnification to 60 power for every inch of the objective's diameter. Infrared, UV, and X-Ray Tele's...
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Research essay sample on Focal Length Distant Objects
