The Thompson 30-inch Reflector, situated in Dome A is not open to the public during the day but is used on Open Evenings.
The telescope was constructed by Sir Howard Grubb of Dublin and presented to the Royal Greenwich Observatory by Sir Henry Thompson in 1896. Sir Henry Thompson was a London surgeon and amateur astronomer. Whilst at Greenwich, it was originally mounted on the same support as the Thompson 26-inch refractor, where they counterbalanced each other. It remained the largest telescope at Greenwich until the 1930s and was used to photograph comets, asteroids and the satellites of outer planets. On 28th January 1908 Philibert Jacques Melotte, a British astronomer, discovered the 8th Moon of Jupiter using the Thompson reflector. The satellite was simply known as Jupiter VIII until 1975 when it was named Pasiphaë after the wife of the Greek King Minos. In Greek legend Pasiphaë was the mother of the minotaur - half man with a bull's head.
When the telescope moved to Herstmonceux in the 1950s, it was given its own mount, an equatorial fork and was used for researching the nature of stars. Its function was to collect light from individual stars and beam it into a high-resolution spectrograph which was installed on a lower level of the dome.
Light is reflected off the concave primary mirror (1) to the smaller secondary concave mirror (2) and then onto a third flat mirror (3) that is mounted on a rotating arm. This mirror then reflects the light onto a fourth mirror (4) located at the top of a gantry, outside the telescope. A flat mirror (5) can be slid into place so the light is reflected at a 90 degree angle into the eyepiece or the light continues on into the spectrograph on the floor below where it is split into its component colours and recorded either on a photographic plate or using an electronic detector. As the telescope moves, the orientation of the mirror at position 3 is automatically adjusted to ensure the emerging beam is always directed towards the mirror at position 4. This system whereby light is delivered to a fixed point even when the telescope is moved is called Coudé focus.
Originally the telescope was designed to take photographs at either prime focus or at the Cassegrain focus. However, in 1960 tests were carried out to check the stability of a steel scaffolding which was to be fitted with a permanent Coudé spectrograph. By 1964 the spectrograph had been installed.
The time-lapse video below by John Fox, shows the telescope being used on one of our Open Evenings.