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.
The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2012, Columbia University Press. All rights reserved.