Astrographic Telescope, Greenwich, London, 1904.
© Science Museum / Science & Society Picture Library
Photograph showing the Astrographic Telescope used at the Royal Observatory, Greenwich for the 'Carte du Ciel' photographic sky survey. The instrument consists of two refracting telescopes mounted together on an equatorial mounting. One was used to take the photograph while the other was for ensuring accurate tracking during the long exposures necessary for the poorly light sensitive films then available. Made by Howard Grubb & Sons in Dublin the design was used by observatories around the world. The international project to photograph the heaven in unprecedented detail was started in 1885. The mammoth undertaking was not completed until well into the 20th century. Today the catalogue is a valuable resource for comparison with more recent sky surveys using electronic detectors.