Telescope

telescope,

traditionally, a system of lenses, mirrors, or both, used to gather light from a distant object and form an image of it. Traditional optical telescopes, which are the subject of this article, also are used to magnify objects on earth and in astronomy; other types of astronomical telescopes gather radio waves.

Types of Optical Telescopes

There are three major types of optical telescopes, classified according to the element that gathers and focuses the incoming light. In the refracting telescope, or refractor, light is bent, or refracted, as it passes through an objective lens. The objective lens is convex, i.e., thicker at the middle than the edges. Parallel light passing through the lens is refracted so that it converges to a point behind the lens, called the focus. The distance from the lens to the focus is called the focal length. In a reflecting telescope, or reflector, light is reflected by a concave mirror and brought to a focus in front of the mirror. If parallel light rays are to be reflected so that they converge to a single point, the mirror must be paraboloid in shape. Typically, a glass disk is ground to this shape and then coated with a thin layer of silver or aluminum to make it highly reflecting. The third type of telescope, the catadioptric system, focuses light by a combination of lenses and mirrors.

Images Produced by Optical Telescopes

The properties of the image produced by a telescope are similar, whether formed by lenses or mirrors. The real image produced is inverted; i.e., top and bottom are reversed, as are left and right. In a terrestrial refracting telescope used to view objects on the earth, an additional lens is used to invert the image a second time, so that objects appear as they do when viewed with the unaided eye; in an astronomical telescope, image inversion is unimportant and no lens is added to invert the image a second time. The angular size of an object as seen from the position of the telescope may be expressed in degrees or in radians (1 radian equals about 57°). The angle in radians determined by the object is given by the ratio of the object's diameter to its distance from the telescope. The size of the object's image is the product of this and the focal length of the image-forming lens or mirror. For example, the angular size of the moon's diameter is about 1-2°, or roughly 1-100 radian; a telescope with a focal length of 60 in. (152 cm) would produce an image of the moon 0.6 in. (1.52 cm) in diameter. The brightness of the image depends on the total light gathered and hence is proportional to the area of the objective or the square of the diameter of the telescope.

Resolving and Magnifying Power

The resolution of the telescope is a measure of how sharply defined the details of the image can be. The laws of diffraction make a certain amount of blurring unavoidable, because of the wave nature of light. If two stars are very close, a given telescope may not be able to separate them into two distinct points. The smallest angular separation that can be unambiguously distinguished is called the resolving power of the telescope and is proportional to the ratio of the wavelength of light being observed to the diameter of the telescope. Thus, the larger the diameter, the smaller the minimum angular separation and the higher the resolving power.

The magnification, or power, of the telescope is relevant only when an eyepiece, or ocular, is used to magnify the image for visual inspection. The angular size of the virtual image seen by the observer will be larger than the actual angular size of the object. The ratio of these two sizes is the magnifying power and is equal to the ratio of the focal lengths of the objective and ocular. Any desired magnification can be obtained with a given telescope by the use of an appropriate ocular, but beyond a point determined by the resolving power, higher magnification will reveal no further details. 

In addition to diffraction, other defects limit the performance of real optical systems. The most serious of these for lenses is chromatic aberration . Other defects include coma, astigmatism, distortion, and curvature of field. In general, it is easier to eliminate these faults in the reflector than in the refractor.

Arrangement of Mirrors in a Reflector

The prime focus of the reflector is inside the main tube of the telescope and thus the image cannot be observed there without blocking part of the incoming light. A variety of schemes are employed to divert the image to a more convenient location. The simplest of these, constituting the Newtonian reflector, is the placement of a flat secondary mirror in the path of the converging light just before the prime focus. The small secondary mirror, which blocks a negligible portion of the primary mirror, is tilted at an angle of 45° in order to reflect the convergent light at right angles and bring it to a focus outside the telescope tube. In the Cassegrain system, the secondary mirror is convex and reflects the convergent light directly back along the axis of the telescope through a hole in the center of the primary mirror. By causing light to traverse a longer path, the effective focal length is increased and a larger image is formed. The Gregorian system is similar to the Cassegrain, except that the secondary mirror is concave. The Coudé system uses both a convex secondary mirror and one or more diagonal flat mirrors to produce a focus outside the tube. The secondaries are arranged so that the position of the focus remains stationary as the telescope rotates, allowing the use of image-recording and analyzing devices that would be too heavy to mount directly on a moving telescope.

The Schmidt Telescope and Other Innovations

The Schmidt camera telescope, invented in 1930 by Bernard Schmidt, is a catadioptric system used for wide-angle photography of star fields. The primary mirror is spherical instead of paraboloidal, which requires that a special correcting lens be used on the front of the tube. The Maksutov telescope, invented by D. D. Maksutov in 1941, is similar in design and purpose to the Schmidt telescope but has a spherical meniscus in place of the correcting plate of the Schmidt.

Mounting the Telescope

Equal in importance to the mirrors and lenses constituting the optics of a telescope is the mounting of the telescope. The mounting must be massive, in order to minimize mechanical vibration that would blur the image, especially at high magnification or during long-exposure photography. At the same time, motion of the telescope must be precise and smooth. To allow the telescope to be pointed in any direction in the sky, the mounting must provide rotation about two perpendicular axes. In the altazimuth mounting, one axis points to the zenith and allows rotation along the horizon and the other allows changes in altitude, or distance above the horizon. This mounting is used for small terrestrial telescopes and, since the 1970s, most new astronomical telescopes use altazimuth mountings that are computer-driven in both axes. Before the 1970s, most astronomical telescopes used the equatorial mounting, in which one axis points at the celestial pole and hence is parallel to the earth's axis.

Evolution of Telescopes

Refracting Telescopes

The first practical telescopes were refracting telescopes produced at the beginning of the 17th cent. By 1610, Galileo had made extensive astronomical use of the simple refractor. The best telescopes of this period had very long focal lengths to minimize the chromatic aberration inherent in the single-element objective. The multielement objective, invented in 1733, allowed the construction of telescopes of large aperture. The art of building refracting telescopes reached a high point in the 19th cent. The largest refractor in existence, with an objective lens 40 in. (102 cm) in diameter, is located at the Yerkes Observatory in Williams Bay, Wis. A 36-in. (91-cm) refractor is located at the Lick Observatory in California and a 33-in. (84-cm) refractor is located at Meudon, France. These telescopes represent the practical limit on the size of a refractor.

Reflecting Telescopes

Because a lens can be supported only at its edge, the weight of the lens itself produces unavoidable distortion in the shape. Because a mirror can be supported from behind, it can be much more massive without incurring distortion, and mirrors many feet in diameter have been constructed. The first reflecting telescope, built by Isaac Newton in 1672, had a mirror made of a metal alloy. When techniques for depositing metal films on glass surfaces were developed, reflecting telescopes became comparable in precision to refractors. An important advantage of the reflecting telescope is the absence of chromatic aberration. Because only one surface must be ground to an exact shape, the reflector is also easier to manufacture. Although increasingly larger mirrors provide increasingly greater light-gathering ability, the cost increases even more rapidly. Several innovations were introduced toward the end of the 20th cent. to achieve the goal of increasing light gathering more economically.