Heritage Buildings: The Birr Telescope

When it was completed in 1845 the great Birr Telescope was the largest in the world.

Birr Telescope image by Tpower.

The Birr Telescope was constructed by William Parsons, the Earl of Rosse, on the grounds of his estate at Birr Castle Demesne, Co. Offaly. Parsons, who was President of the Royal Society and Chancellor of Trinity College Dublin, was a noted astronomer with a research interest in star nebulae. At the heart of the Birr Telescope is a 3 ton 1.8m mirror which was designed by Parsons. It was cast of speculum metal, ground to a mirror surface and installed in a 16.5m wooden tube. The huge telescope was supported by a universal joint and was set within a hollow flanked by Gothic style limestone walls with arches and battlements at east and west. A chain and counterweight kept the telescope in balance and another chain and winch controlled the altitude.

When it was completed in 1845 the Birr Telescope was the largest in the world and it remained the largest until the completion of the 2.5m Hooker Telescope at Mount Wilson Observatory in Los Angeles in 1917. With the telescope Parsons was the first to observe the spiral nature of nebulae. The telescope went out of use about 1890 and was later dismantled. It was partly reconstructed in 1996-7 and a new mirror was installed in 1999. The Birr Scientific and Heritage Foundation and the Office of Public Works are now working to fully restore the telescope to working order.

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