The Great Equatorial Telescope
Howard Grubb of Dublin (1844 to 1931) · 1893
Under the onion dome you come face to face with the largest refracting telescope in the United Kingdom, and one of the largest of its kind in the world. Its main lens is twenty eight inches across, about seventy one centimetres, and the tube stretches for more than eight metres, painted in a warm cream that catches the light from the dome's opening. The whole instrument was made in Dublin by Howard Grubb and installed here in eighteen ninety three, when Victorian astronomers were competing to build ever larger glass lenses before mirrors took over.
Look at the mount. The telescope is not fixed on a simple up and down pivot but tilted so that one axis points at the celestial pole. That equatorial mounting lets a single slow motion follow a star across the sky as the earth turns, which is essential for long observations and photography. The great weight is balanced by counterweights so that an astronomer could move it with one hand. The astronomers used it above all to measure double stars, pairs that orbit each other, and to weigh stars by watching how they moved. Visual observing continued here until nineteen sixty.
The dome you are standing in is not the original. The first onion dome of papier mache and iron was damaged by a flying bomb in nineteen forty four, and the telescope itself was removed to Herstmonceux in Sussex when the observatory left smoky London. It came back to Greenwich in nineteen seventy one, and a new fibreglass dome in the old shape was built over it in nineteen seventy five. Today it is still used on some evenings for public viewing of planets and the moon, so the great glass eye is not quite retired.
Did You Know? The onion dome shape was chosen to give the long telescope tube room to swing, and the original was made of papier mache on an iron frame.
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