This telescope was commissioned by the Astronomer Royal John Pond and built by Dollond. The telescope is sometimes referred to as Pond's Alpha Cygni telescope. This and an identical telescope (AST0996) were made for Pond by Dollond in response to a report made by Dr Brinkley, later Bishop of Cloyne about the Dublin mural circle. He claimed that the stars Alpha Cygni and Alpha Aquilae showed observable parallaxes of 2 to 3 inches. This was not confirmed by the Greenwich circle so Pond had these zenith sectors built, one pointing at each star in question, to investigate. Observations were made between 1816 and 1825 and these proved that no parallaxes of that magnitude existed.

Parallax is the aparent shift in position of a far away object in relation to its background caused by a change in position of the observer. If you close one eye and look at something far away, then open it and close the other, the object will appear to shift against its background. This is parallax. If you observe a star with a zenith sector and then observe the same star later in the year (so the observer has moved in relation to the star as a result of the Earth moving around the Sun) you should, if the star is not too far away see it move against the background stars. This is what astronomers were trying to observe when they built these telescopes.

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 in mind. This telescope for example was designed to allow observers to study the position of the star Alpha Cygni (the brightest star in the constellation Cygnus) very accurately over the course of the year. A major reason for doing this is to allow astronomers try to detect parallax. It also however allows them to account for the error introduced by atmospheric refraction. At any angle besides straight up the atmosphere with 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.

Telescope: This telescope is made up of a 3.05m (10ft) tube with a diameter (and object glass aperture) of 10cm (4 inches). The object glass is an achromatic lens.