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zenith telescope[′zē·nəth ‚tel·ə‚skōp]
an astronomical-geodetic instrument designed to measure small differences in a star’s zenith distance. It is used for latitude determination by the Talcott method.
A zenith telescope consists of a vertically mounted refractor with an eyepiece micrometer placed in its focal plane. The central part of the telescope’s tube is attached to a horizontal axis, rotation around which helps establish the required zenith distance of the instrument’s sight line. There is a counterweight on the opposite side of the horizontal axis along with a divided circle for setting the tube at the zenith distance. The stability of the tube’s zenith distance during observation is monitored with two highly accurate levels, which are usually attached to the central part of the tube. The geographical latitude for a point of observation is computed from observations of the differences in stellar zenith distances with the necessary corrections included. The accuracy of a latitude determination (mean square error) from one observation with modern zenith telescopes is ±(0.10”-0.15”). During the late 1950’s a series of large zenith telescopes (type ZTL-180) were manufactured in the USSR with an objective measuring 18 cm in diameter and a focal length of 236 cm.
IU. I. PRODAN