28-inch telescope

The 28-inch refracting telescope


Closures: Access to the 28-inch refracting telescope and the Telescope Dome may occasionally be closed. Please see Latest visitor information for details of all closures.


Onion Dome | 28-inch lens | telescope mount | telescope tube

Onion Dome, Royal Observatory, GreenwichTelescope dome open for observingThe 28-inch Greenwich refracting telescope is the largest of its kind in the UK and the seventh largest in the world. Completed in 1893, it was designed to keep the Royal Observatory at the forefront of contemporary astronomy.

Richard Dunn talks about the 28-inch telescope (On the Line podcast series)The telescope was used for research into double star systems until its retirement in the late 1960s. It is now a central part of educational programmes at the Royal Observatory. With the recent addition of a computer-aided guidance system and CCD camera, it continues to work as an excellent visual aid to observing the night sky.

Observing evenings

During the winter months the Observatory runs observing evenings when you can come and view the night sky through the 28-inch telescope.

(Please note: these events need to be booked in advance and often sell out early.)

The Onion Dome

Royal Observatory, GreenwichThe 'Onion Dome' and the ObservatoryThe dome which houses the 28-inch telescope has been likened variously to the Taj Mahal, a collapsed balloon, or an enormous Spanish onion!

The dome’s unique shape was actually an innovative solution to a design problem. The smaller predecessor to the 28-inch telescope had been housed in a flat-topped wooden ‘drum’ mounted on cannonballs which acted as ball-bearings. As the new 28-inch was over eight feet longer than the previous telescope, a new type of dome had to be designed to avoid major rebuilding of the supporting brick tower. The answer was the 'onion dome', which bulges to a maximum radius of about five feet wider than the supporting tower walls. The cannonballs were also replaced with a modern system.

The dome, showing war damage, after the removal of the 28-inch, c. 1950 (B7124) The dome, showing war damage, after the removal of the 28-inch, c. 1950 (B7124)The onion dome was originally constructed from a riveted iron frame covered with papier mache. It was damaged by an air raid in October 1940, and was stripped of its covering when a V1 flying bomb fell in Greenwich Park on 15 July 1944. The present dome is a fibreglass replica of the 1893 dome, installed after the 28-inch returned to Greenwich in 1971.

The 28-inch lens

The 28-inch lens, weighing 200lbs, was so difficult to make that there were only two glassmakers in the world capable of producing it, and from commission to completion took eight years. When installed it was described as '...the finest pair of discs of their size which have ever been made'.

The 28-inch lens was originally designed to be used for both visual and photographic observations, something which normally required two telescopes. However, this dual role was less successful in practice than on paper. Soon after the telescope was ready for use a new 26-inch refractor was commissioned purely for photographic work. From the mid-1890s, the 28-inch was used with great success in the programme of double star observations, a field of astrophysics that had been developing rapidly in the late 19th century.

The telescope mount

28-inch refracting telescope at the Royal ObservatoryThe 28-inch telescope and mountThe telescope mount was built for an earlier instrument and predates the telescope by over 30 years. The mount is an 'English equatorial' type, which means the telescope is mounted on an axis tilted parallel to the Earth's axis of rotation. By rotating the mount, the telescope can follow a star from east to west across the sky.

A clock drive moves the telescope at the same rate as the rotation of the Earth, keeping it fixed on the same celestial coordinates – very useful for long observations. The original drive was powered by falling water and often froze in winter. An electric drive was installed early in the 20th century.

The design of the mount allows the telescope to be moved to almost any position in the sky.

The telescope tube

The telescope tube is over 28 feet long and has the curious characteristic of being round at each end but rectangular in the middle. This is because the mount was built for a much smaller telescope, so the middle section of the tube had to be tapered in order to fit.

Attached to the outside of the tube are a variety of smaller telescopes, used as guides to target an object or to magnify the telescope’s scales.

The whole telescope is very finely balanced, using a series of counterweights. This is very important for such a large instrument that needs to be moved easily and accurately into a variety of positions.