The Only Way Is Up!
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 they are usually designed with the observation of a particular star in mind.
This 2.87m (9.5ft) zenith tube (centre of image) was commissioned by the Astronomer Royal John Pond and built by Edward Troughton in 1812.
It was built to work alongside the mural circle (right of image) 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 very successful because of a fault in the way the illumination of the plumb-line and micrometer worked.
It was replaced by a 2.44m (8ft) achromatic zenith telescope in 1816. This telescope was designed to allow observers to study the position of the star Gamma Draconis (the third brightest star in the constellation Draco) very accurately over the course of the year.
A major reason for doing this is to allow astronomers to account for the error introduced by atmospheric refraction. At any angle besides straight up, the atmosphere will refract the light coming in from a far away star making it appear in a slightly different location to its actual position. The amount the atmosphere refracts the light will vary depending on the weather. If observations are compared with those made with the zenith sector however, this error can be accounted for.