The Thompson 26-inch refracting telescope

The Thompson 26-inch Refracting Telescope, situated in Dome E is open to the public during the day and is used on Open Evenings. You can learn more about it during our telescope tours.

This telescope is the twelfth largest refracting telescope in the world and the second largest in Great Britain. Between 1897 and 1988 it was used to take 60,000 photographs of the night sky.

It was constructed in 1896 by Sir Howard Grubb of Dublin. It was a gift of the 1st Baronet Sir Henry Thompson, a London surgeon and amateur astronomer and cost £5,000. This was originally for the refractor alone but the then Astronomer Royal William Christie, decided to add a 30-inch refractor to the mount. Henry Thompson approved and agreed to pay 'out of pocket expenses, not exceeding £200.' 

The objective lens, a doublet has an aperture of 66cm (26 inches) and is fixed inside the top of a large grey tube. The focal length is 6.82m. The telescope works as a powerful camera and the "film", which is actually a six-inch square glass plate coated with photographic emulsion, is loaded at the bottom of the telescope.

The tube is so long that the telescope has to be mounted high up in the dome in order for it to move freely. It can be pointed vertically to look at celestial objects overhead and be moved through all declinations to the horizontal. This presents a problem to the observer in terms of gaining access safely to the lower end of the telescope. This is overcome with an unusual feature of the dome it is housed in - a mechanism to raise the whole floor up and down like a lift. 

A smaller guiding telescope rides piggyback on the 26-inch tube. By looking through this guider the observer can ensure that the main telescope is trained on exactly the right spot in the sky. This guiding telescope has an even longer history than the 26-inch refractor. The main lens, by Merz, has an aperture of 33cm (12.8-inches) and a focal length of 5.44m. It was originally used in a telescope known as the ‘Great Equatorial' which was acquired by Greenwich in 1859 by the Astronomer Royal at the time, Sir George Airy. The acquisition was in response to criticism concerning the discovery of Neptune by a German and not a British astronomer using a telescope that was substantially larger than any at Greenwich. The Great Equatorial was the first of the large equatorially mounted telescopes at Greenwich. The mount for the telescope was so strong and stable that it was subsequently used for the much larger 28-inch refractor. When mounted on the 26-inch telescope as a guider, the 12.8-inch lens was given a new tube.    

Although the moving parts of the whole telescope weigh 12 tonnes it is extremely finely balanced with a large counterweight. 

As mentioned previously thousands of large-scale photographs were taken with the 26-inch refractor and the faintest stars photographed were approximately 19th magnitude (100,000 times fainter than the faintest ones we can see with the naked eye). Large scale photographs are useful for ‘positional astronomy' - measuring the exact positions of stars in the sky to within 1/50th the width of a human hair. From this it is possible to work out how fast the stars are moving and how far away they are. 

In the period 1930-1931 the asteroid Eros passed close to Earth. This provided a rare opportunity for measuring the exact scale of the solar system more precisely than ever before. The method to be used was triangulation, familiar to all surveyors. The Astronomer Royal at the time, Harold Spencer Jones, organised an international programme to track the asteroid, in which Greenwich was joined by 43 other observatories. The result, after ten years of calculation gave the Earth-Sun distance as 149,650,000 km (93,005,000 miles). Both the 13-inch and the 26-inch refractors took part in this project.

As well as positional astronomy the telescope was used to investigate quasars. In the 1960s nobody knew what quasars were so the 26-inch was pressed into service to monitor the changing brightness of one of them. The surprising result showed that the quasars brightness could vary by as much as twenty times over a period of a few months. This meant that although the quasar might be as bright as an entire galaxy, its size must be millions of times smaller than that of a galaxy. A 60-minute exposure from the telescope revealed quasar 0957+56 which is one of the most distant objects ever detected from Herstmonceux, lying a third of the way out to the edge of the observable universe. 

In 1969 Donald Lynden-Bell, one of the Royal Greenwich Observatory's astronomers suggested that a black hole's intense gravity could power a brilliant quasar. His prediction that a black hole could lie at the centre of most galaxies, including our own, has been proved right. The 26-inch telescope monitored the galaxy NGC4151 for several years before a team of RGO astronomers using a telescope on the Ultraviolet Explorer satellite were able to work out that a supermassive black hole millions of times heavier than the sun may lie at the galaxy's centre.