The 13-inch (330mm) Astrographic Refractor, situated in Dome D is not open to the public during the day but is used on open evenings.
Built in 1890 by Grubb in Dublin, the telescope was used specifically to make use of the new technique of astronomical photography. It was initially acquired for the Carte du Ciel project, an ambitious international programme to map the entire sky in detail by photography. Initiated by Amédéé Mouchez, director of the Paris Observatory, the project involved 18 observatories world-wide each commissioned to photograph the sky at a specific declination zone.
Photography for the project began at Greenwich in 1892 and took 13 years. The precise positions of 179,000 stars were measured from the 1153 photographic plates exposed on the 13-inch refractor. The 16 cm glass plates covered a patch of sky four times the width of the full moon.
Later the telescope's main optical components were used to photograph total eclipses of the sun at various sites around the world. An important location was Brazil in 1919 when the lens and mounting were used for a crucial test of Einstein's new General Theory of Relativity. Einstein's prediction was that space becomes curved when there is matter in it. The Astronomer Royal at the time, Frank Dyson, predicted it would be possible to test the theory by seeing if starlight was bent when it passed near the sun during a total eclipse. Photographs were taken during the eclipse and compared with photographs taken of the same patch of sky when the sun was elsewhere. The stars in the photographs were found to have moved due to the effect of the sun's gravity. The movement was tiny - less than 1/100 of a millimetre - but agreed with Einstein's prediction. By proving that starlight was bent when it passed near the sun, Einstein was vindicated and became an overnight celebrity.
In 1958 the telescope was moved from Greenwich to Herstmonceux where it was used for measuring the ‘proper motion' of stars. This was achieved by comparing photographic plates with those it had taken many years earlier. It was also used for Solar System observations, principally of the brighter asteroids.
A 10-inch (254mm) refractor with a focal length of 11 feet (3.43m), the same as the main telescope, was designed for visual use and is mounted on top of the 13-inch telescope and acts as a guider. By looking through this telescope the observer can ensure that the main telescope is correctly aligned on the area to be photographed. A new German equatorial mount (Grubb, Parsons, Newcastle) replaced the original in 1969.
The 13-inch lens was designed to be a photographic refractor and as a result anything viewed visually through this part of the telescope is subject to colour problems caused by chromatic aberation. However, the lens in the 10-inch guider was designed as a visual refractor and is not subject to chromatic aberation to the same degree. As a result it is the 10-inch guider and not the 13-inch astrograph that is used for looking at celestial objects.
Using the lens of the 13-inch Astrographic refractor in the first verification of Einstein's General Theory of Relativity.