Measuring apparatus for the star trail camera

Folding measuring apparatus for use with night sky camera. Eyepiece missing. Graduated glass slide. Mirror set into base.

This device was used to read the photographic plates taken by a camera installed on a telescope that was aimed towards Polaris, set for all-night exposure. Clear skies created long star trails around Polaris while cloudy skies resulted in partial or completely absent star trails.

The night sky camera came into use at the Royal Observatory, Greenwich, in January 1920. It was initially situated in the north courtyard on the collimation pier for the Bradley meridian. On 18 November 1940 it was moved to a new location south of the Altazimuth Pavilion, possibly for protection against bomb damage. It was then returned to its original location on 15 August 1946.

The night sky camera remained in use at Greenwich while the Observatory gradually moved to its new location in Herstmonceux, Sussex from 1948 onwards. On 6 July 1950 a new night sky camera, similar in design to the Greenwich version, was installed on the roof of the Solar building at Herstmonceux; it was subsequently joined by the Greenwich instrument in September 1952. A comparison of the data confirmed that Herstmonceux did indeed offer better observing conditions with the number of hours of clear sky 10% greater than Greenwich.

Another night-sky camera from the Meteorological Office, set to record on film rather than glass plates, was installed for comparison over the period April 1953 to February 1954 but it was decided to continue with the existing devices. The night sky cameras remained in use at Herstmonceux until the late 1950s; their current whereabouts are unknown.

There is another pole star trail telescope in the collection, ZBA1733, that was used for the selection of various observatory sites in the 1960s.