Zenith sector telescope

This reflecting zenith telescope, or zenith micrometer, was commissioned by the sixth Astronomer Royal, John Pond, from Edward Troughton in 1812. It was designed to determine the zenith (highest) point of the mural circle (AST0973) by comparing observations of the star gamma Draconis. It was installed after the circle was criticised by members of the Royal Society for its lack of built-in vertical reference such as a plumb-line. However, this telescope was never used successfully because the system for illuminating the plumb-line, so that it could be seen at the same time as the micrometer and the star being observed, was defective. Only four years later it was replaced by an 8ft achromatic zenith telescope (AST0997).

A zenith sector is a telescope that points straight up, to the zenith. This means that it can only see the stars directly above and are usually designed with the observation of a particular star such as gamma Draconis (the third brightest star in the constellation Draco). One reason for doing this is to allow astronomers to account for the error introduced by atmospheric refraction, which is least for observations at the zenith. They can thus be used to check the accuracy of other instruments.
Edward Troughton