Airy's transit circle telescope, 1850 (photo)

Creator: Troughton & Simms; Ransomes & May This telescope was originally proposed as a more accurate, more modern version of the transit instrument introduced by the Astronomer Royal, John Pond [AST0982]. Constructing Airy's huge transit circle was a major undertaking. The engineering was carried out by agricultural machinery experts, Ransomes & May of Ipswich while prestigious London instrument makers, Troughton & Simms constructed both the optical parts (including the 206 mm object lens) and the main body. As a whole however the instrument was designed by then Astronomer Royal, Sir George Biddell Airy. As the transit circle was not wall mounted, repeated use often meant that the telescope fell out of true vertical alignment and needed to be realigned (or collimated) weekly. From 1854, a recording apparatus, or chronograph, was introduced to record its observations on a rolling drum of graph paper. The last observation was taken in 1954 just before the astronomers finally left the bright lights and polluted skies of Greenwich for their new site on the grounds of Herstmonceux Castle in Sussex. Since an international treaty was signed in 1884 this telescope has marked the internationally recognised Greenwich Meridian and so the point of 0 degrees longitude. Telescope: The object glass of this telescope has an aperture of 206mm (8.1 inches), a focal length of 3531mm (11 feet 7 inches) and a magnification of 195 (180 for the Sun). The eye-end had originally 7 vertical fixed wires & 1 horizontal wire movable by micrometer. 6 additional vertical wires were added in 1854 with the introduction of the chronograph to give 9 wires for galvanic registration & 7 for 'eye-and-ear'. In 1891 a 10-wire galvanic system was introduced to simplify the arithmetic and in 1915 a travelling wire micrometer fitted. The handle by window replaced was by conservation department. Mount: The east pier is made of granite and had previously supported the Troughton mural circle. The west pier is made of portland stone. This difference in materials has caused problems as each reacts subtly differently to seasonal variations causing the telescope to by shifted out of alignment. Accessories: This telescope in its long history has used various eyepieces many of which are now in the collection and labelled as part numbers to this object. The telescope also has a circle of microscopes on the west pier. These are for viewing the vertical circle scale so as to read off the angle at which the telescope is pointing when a star is viewed. There are 6 microscopes, evenly spaced so that an average can be taken making these readings more accurate. Another accessory now found in store and labelled as a part of this object is the mercury tray. This was used to check the accuracy of the telescope's alignment when pointing straight up.