This is a photoheliograph, a type of telescope made for photographing the Sun. This example was originally commissioned in 1871 as one of five photoheliographs ordered from the maker John Henry Dallmeyer, for the British expeditions (organised by the Royal Observatory Greenwich) to view the Transit of Venus in 1874.

All five telescopes were erected at Greenwich in portable observatories in 1873, and a regular series of daily photographs of the sun was begun with one of them in July 1873. In 1874 all five photoheliographs were packed and shipped to take part in the Transit of Venus expeditions. In the mean time, a photoheliograph from the Kew Observatory was used to continue the regular Greenwich photographic series. In 1875 one of the Dallmeyer telescopes, known as Dallmeyer No. 3, returned to Greenwich and was used almost continuously there until 1910. It is likely that this instrument is Dallmeyer No. 2, which replaced No.3 as the principal photoheliograph in 1910 and was used continuously until the 1970s.

Dallmeyer No. 2, now principal photoheliograph at the Observatory, was mounted in 1911 in the Airy Altazimuth Dome (since demolished), situated above the Meridian Observatory. Observations continued during the Second World War and the last photograph of the Sun was taken by the Dallmeyer at Greenwich on 2 May 1949. On the same day the same instrument, now mounted on the Newbegin 6.25 inch refracting telescope (AST0929), was used to take another photograph of the sun at Herstmonceux in Sussex, site of the new Royal Greenwich Observatory. These two telescopes were in regular use until they were sold to the National Maritime Museum in 1980. They are now mounted together in the Christie Altazimuth Pavilion.

The telescope consists of a brass tube and a wooden projection box. It has an aperture of 4 inches (10cm) and a focal length of 5ft (152cm). The photoheliograph was used with camera plates and there are two camera plate holders. It has a projection lens and a right ascension hour circle with a microscope. It has polar axis adjustment and a driving wheel with 1080 teeth. Its driving mechanism (which is now in store) was replaced with a modern mechanism.

The Royal Observatory Greenwich collections include two of the other Dallmeyer photoheliographs (AST0967 and AST1187). Both were given to the National Maritime Museum by the Royal Greenwich Observatory in Herstmonceux in 1976 and 1988 respectively.

Shown with this record is a photograph of one of the photoheliographs in a portable hut, set up at Greenwhich in advance of the expedition. The inscription on the hut indicates that this one was sent to Station D, Christchurch, New Zealand.