Telescope Mounting
An optical telescope, in order to follow an object as the earth's rotation carries it across the sky, must be free to move. To track stars accurately and to permit a telescope to be conveniently pointed in any direction, the equatorial mounting system is used for most telescopes. This system has two axes of rotation: The telescope can rotate in an east-west direction, called hour-angle, around its polar axis, which is aligned with the earth's axis of rotation; another allows the telescope to swing in a north-south direction about the declination axis, which is perpendicular to the polar axis.Large telescopes are usually positioned by a computer from an operating console and guided to the exact location with hand controls. Once a large telescope is properly set, the computer operating a clock drive slowly turns it westward around its polar axis at the same rate as the earth turns eastward, keeping the stellar images locked in position in the field of view. The great simplicity in equatorial mounting is that tracking requires continuous motion about only one of its two axes. The disadvantage, which obtains in the largest telescopes now in operation and planned for the future, lies in the stresses on the polar axis due to gravity. The polar axis is inclined in the earth's gravitational field and must rotate on one edge of its end. For a very large telescope that is a difficult engineering problem.
One means of removing some of the stress from the primary axis is to align it with gravity. Such a mounting is known as altazimuth mounting; with it a telescope rotates about a vertical axis and about a horizontal axis. This mounting's disadvantage is that, to track a star, it must turn continuously about both axes at the same time. When the telescope approaches the area of the sky directly overhead, continuous tracking becomes virtually impossible. Even with this disadvantage the altazimuth mounting will be the primary mounting for very large telescopes to be constructed in the future.
