equatorial mounting

Equatorial mounting A form of telescope mounting in which the instrument can rotate about a polar axis, parallel to the Earth's rotation axis, and a declination axis, perpendicular to the polar axis. Rotation about the two axes enables the two equatorial coordinates to be set independently. Motion around the polar axis changes right ascension; motion about the other axis changes declination. The equatorial mounting has a particular advantage. In order to compensate for the apparent motion of the sky produced by the Earth's rotation, the telescope needs to be motor-driven around only one of the axes, the polar axis. Once set correctly, the telescope continues to point at the required declination without any adjustment. For this reason, the design of the mounting of all telescopes of any significant size was for many years exclusively of the equatorial type. However, advanced computer control has made it possible to point and drive very large telescopes on the simpler altazimuth mounting without difficulty. Equatorial mountings, though, remain popular and practical for many applications. Different forms of equatorial mounting have been developed to give adequate support and freedom of movement for telescopes of different sizes and types. The main classes are the German, English, yoke, horseshoe and fork (see illustration). Because the polar axis must be parallel to the Earth's axis (i.e. it must point to the north celestial pole), a particular equatorial mount is suitable only for use at the latitude for which it is designed.