Telescope Mounting

the part of a telescope on which the optical tube rests. The mounting allows the telescope to be aimed at the region of the sky to be observed, ensures its stability in position, and provides for the convenient performance of various types of observations. The main parts of a telescope mounting are the base (or column), two mutually perpendicular axes for rotating the telescope tube, driving gears, and a system for reading off the angles of rotation. A distinction is made between equatorial (or parallactic) mountings and azimuthal (or horizontal) mountings (see Figure 1).

In an equatorial mounting, the first axis is directed toward the celestial pole and is called the polar or hour axis, and the second axis lies in the plane of the equator and is called the declination axis; the telescope tube is attached to the second axis. When the telescope rotates about the polar axis, the polar (or hour) angle changes and the declination remains constant; when it rotates about the declination axis, the declination changes and the polar angle remains constant. In observations made with an equatorially mounted telescope, a celestial body that moves because of the apparent diurnal rotation of the sky is tracked by rotating the telescope at constant speed about the polar axis. In an azimuthal mounting, the first axis is vertical, and the second axis, which

Figure 1. Types of telescope mountings: (a) symmetric English mounting (in yoke), (b) asymmetric English mounting, (c) German mounting on straight column, (d) German mounting on bent column, (e) supported German mounting, (f) American (fork) mounting, (g) yoke horseshoe mounting, (h) azimuthal mounting of Soviet 6-meter BTA telescope; (1) base (or column), (2) polar axis, (3) declination axis, (4) telescope tube, (5) worm-gear axis drive, (6) polar drive mechanism, (7) circle for measuring angles of rotation of axes, (8) hydraulic axis bearings, (9) counterweight, (10) vertical axis of azimuthal mounting, (11) horizontal axis of azimuthal mounting

supports the tube, lies in the horizontal plane. The first axis is for rotation of the telescope about the azimuth, and the second is for altitude (zenith distance). Continuous and precise rotations about both axes simultaneously, with speeds that change in a complex fashion, are necessary for observation of stars with an azimuthally mounted telescope.

More complex mounting designs are used in astronomical instruments designed for certain special operations. In instruments designed for photographic observation of artificial earth satellites (satellite cameras) a three-axis or four-axis mounting is used; this makes possible the tracking of a rapidly moving satellite by rotating the camera about only one “orbital” axis (the positions of the remaining axes are set in advance and are only corrected during the observations).