Leviathan will have sight restored to become watcher of the skies again

Sight will soon be restored to what was in its time the most powerful telescope in the world, the Birr Castle Leviathan. The 3rd Earl of Ross who built the original instrument 150 years ago wasn't put off by Ireland's unco-operative weather, nor has the current 7th Earl shied away from the challenge of rebuilding it and having a 1.8-metre, 1.4-tonne metal mirror prepared to allow it to be used once again for astronomical observations.

It was made of metal because the technology wasn't available to make one of polished glass, explained Mr John Joyce, chief guide at Birr Castle Demesne. The original was the largest white bronze cast metal mirror ever made and was presented to London's science museum in 1914.

The wooden telescope tube fell into disrepair and metal works that held the heavy instrument were dismantled. Some of the related optics also disappeared and so things stood until 1979 when the present earl decided to redevelop the telescope as a monument, a way to raise funds to protect the future of the spectacular gardens at the demesne, Mr Joyce explained.

Over time the project evolved from a static heritage site to a complete science museum with the telescope as its centrepiece. The telescope restoration cost more than £1.35 million (€1.65 million) and the preparation of the science centre and its galleries is ongoing with the current phase worth £1.7 million.

The telescope was fully rebuilt and remounted in its huge swivel mechanism but was without its "eyes", the mirrors and eye pieces that would allow it to be used. The decision was taken to make new primary and secondary mirrors after contributions from Dr Tony O'Reilly and this £200,000 project is now almost complete.

It was decided to go to University College, London, to have a new metal mirror built, Mr Joyce said. "Very few organisations were capable of making a metal mirror of this size for the telescope."

The work is being done by PhD candidate Mr David Brooks of UCL's Optical Science Laboratory within the Department of Physics and Astronomy. The contract includes both main mirrors, all of the eye pieces and the support and trolley systems for the mirror, Mr Brooks said. "They wanted their optics remade but couldn't afford the glass," he said. Birr wanted a less expensive mirror and UCL wanted to develop methods for building and testing the quality of large metal mirrors.

The work, he said, began two years ago and the challenge was "doing one of this size". UCL has the largest grinding and polishing machine in Britain but the mirror itself is enormous. It is a disc of aluminium 1.8 metres across and 20 centimetres thick.

The disc was cast and went through early machining to get it very close to its final concave shape. A precise "tool" is builtwhich is the same size as the mirror but which has a perfect reverse cross section of the mirror surface that is being sought.

The two are brought together with the mirror rotating underneath the tool. Grinding and polishing slurries are placed between the two surfaces and the slow process of cutting down the mirror takes place. The aluminium disc was then shipped to Nitec (Darbyshire) Ltd, a specialist metal plating company. "We specialise in electroless nickel plating," explained company director, Mr Shaun Burns. "It was made of aluminium because it is easier to work and cheaper than glass."

Metal plating is usually done by applying an electrical potential that causes metal in solution to deposit on a surface, as with galvanising. This did not offer the precision plating that was required to produce a telescope mirror, Mr Burns said, hence Nitec's involvement. The company specialises in chemical deposition using active solutions. This gave very good control over plating thicknesses, he said.

Plating took about a day. Test pieces were placed in the large vat used to hold the mirror and these were taken out periodically and tested with X-ray systems to gauge the nickel metal thickness. Nitec applied a coating of pure nickel just 100 microns thick, about one four-thousandth of an inch. The plated mirror was returned to UCL and to Mr Brooks' care where it underwent final, very precise polishing to bring the metal surface up to a bright finish. He is now in the process of placing the mirror in the mount which will hold it in place on the Leviathan.

When fully assembled the telescope at Birr will be the second largest instrument in Europe but will be the largest using a metal mirror, stated Lady Ross. "We are hoping for it to come in some time during the summer," she said.

Decisions have yet to be made about access to the telescope as a working instrument. It will be the focal point of the science centre at Birr, but this is not open at night when observations could be made. She is considering videoing observations made using the telescope so that they can be put on display to the public.

She also expects demand from scientists who may seek to use the Leviathan.