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The Observatory Science Centre
Herstmonceux
Hailsham
East Sussex
BN27 1RN
Tel: 01323 832731
Fax: 01323 832741

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Telescopes

The six green domes of The Observatory Science Centre, conveniently named Domes A - F, all house historic telescopes, three of which are open to the public during the day. On Open Evenings and Themed Evenings two of the telescopes closed to the public during the day are opened and weather permitting, will be available to look through (see below for further details). 

Each dome houses a different telescope:

Dome A - The 30-inch Thompson Reflector
Dome B - The 36-inch Yapp Reflector
Dome C - The Hewitt Camera
Dome D - The 13-inch Astrographic Refractor
Dome E - The 26-inch Thompson Refractor
Dome F - The Congo Scmidt and The Domes of Discovery Exhibition
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. Learn more about it on one of our telescope tours.

This telescope is one of the largest refracting telescopes 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.

Constructed in 1896 by Sir Howard Grubb of Dublin at a cost of £5,000, it was the gift of 1st Baronet Sir Henry Thompson, a London surgeon and amateur astronomer. 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.   
Yapp 36-inch Reflecting Telescope

The Yapp 36-inch reflecting telescope situated in Dome B is open to the public during the day. Learn more about it on one of our telescope tours.

Built in 1932 by Grubb Parsons of Newcastle-upon Tyne at a cost of about £15,000 (including the dome), the 36-inch reflector was a
gift from Mr William Yapp, a prominent industrialist. The telescope was moved to Herstmonceux in 1958 after 21 years of service in Greenwich, where it had been the largest working telescope.

There is a large concave mirror - the primary mirror - at the bottom of the telescope, facing upwards. Light from the stars falls onto this large mirror and is bounced back up the tube where a smaller mirror near the top reflects this light back through the a hole in the centre of the main mirror. The primary mirror aperture measures 91cm with a focal length of 4.6m and a cassegrain focus of f/15.

This telescope was used for astronomical research - mainly stellar spectroscopy and photometry - and later as a test bed for equipment developed for use elsewhere, particularly the Isaac Newton telescope, installed at Herstmonceux in 1967. Astronomers used this telescope mainly to find out about individual stars. 

The 13-inch Astrographic Refractor

The 13-inch Astrographic Refractor, situated in Dome D is not open to the public during the day but is used on open evenings.

The 13-inch (330mm) Astrographic refractor, built in 1890 by Grubb in Dublin, 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 abberation. However, the lens in the 10-inch guider was designed as a visual refractor and is not subject to chromatic abberation 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.  
The Thompson 30-inch Reflector

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 30-inch Reflecting telescope was constructed by Sir Howard Grubb of Dublin and presented to the Royal Greenwich Observatory by Sir Henry Thompson in 1896. 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.
The 34-inch Hewitt Camera 

The  34-inch Hewitt Camera, situated in Dome C is not open to the public but the dome may be accessed on some Open Evenings when the 16-inch Meade telescope is in use.

The Hewitt camera was designed by Joseph Hewitt in the 1950s for use in tracking ‘Blue Streak,' a British ballistic missile which was then under development.  When the ballistic missile programme was cancelled in 1960 it was decided to use the Hewitt design to track satellites. Two cameras were built and became operational in 1962, one in Evesham, Worcestershire (becoming known as the ‘Malvern' camera) and one in Lye Vallets, Herefordshire. The optics of the instruments is that of the ‘Schmidt Camera' which combines good light-gathering power with a wide field of view. The spherical focusing mirror is 864mm (34-inches) in diameter and has a focal length of 680mm (26.77-inches). The correcting plate with a diameter of 630mm (24.80-inches) was placed 1062mm (41.81-inches) from the front surface of the mirror, at or near its centre of curvature and gave an effective focal ratio of f/1. A field flattening lens, placed in front of the photographic plate allowed a flat photographic plate to be used instead of the curved ones usually required by the Schmidt design. Thus the camera was equipped to take high precision photographs of a significant part of the satellite's trajectory, together with enough bright background stars to give reliable positional references. In 1982 the ‘Malvern' camera came to Herstmonceux and proceeded to take 3300 spherical images which were used to record the tracks of artificial satellites passing overhead. Each image covered an area of sky approximately 20 times the width of the full Moon (10 degrees. Analysis of the satellites orbit gave information about the Earth's gravity field and the properties of the upper atmosphere.The second camera moved to Siding Springs in New South Wales, Australia and operated from 1982 to 1990, providing results to complement those obtained at Herstmonceux.
The 38-inch 'Congo Schmidt'

The 38-inch 'Congo Schmidt' telescope, situated in Dome F is open to the public during the day and forms an impressive centrepiece for a beautiful exhibition 'The Domes of Discovery' which tells the story of The Royal Greenwich Observatory.


This is the largest and newest of the telescopes at Herstmonceux but was never used for serious observing. It was built in 1960 for an observatory in the Belgian Congo in Africa, but civil war broke out there before it could be installed. It was eventually brought to Herstmonceux where tests showed that it was unsuitable for use for astronomical research here. As well as being used as a normal reflecting telescope, with its large mirror at the bottom facing up the tube, the telescope could also be converted into a Schmidt telescope. 
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