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  • New Mirror for Old ’Scope Revives Neglected Technology

Photonics Spectra
Mar 2000
Dan Drollette

The restoration of a 150-year-old telescope at an Irish castle may help bring back a largely forgotten way to see the stars. Consequently, tomorrow's professional observatories may use telescope mirrors made of metal instead of glass.

When William Parsons, third Earl of Rosse, built his 17-m Leviathan telescope at Birr Castle in 1845, he used a 72-in. mirror made from a polished bronze alloy. Although the mirror tarnished quickly in the Emerald Isle's humid climate, Parsons used it to make the first observations of the spiral nature of galaxies, as well as the first drawings of the Crab Nebula.

Then metal mirrors fell into disfavor. "The technology for producing glass mirrors was more understood," said David Brooks, manager of the Optical Science Laboratory at University College London. Glass is highly polishable and can be created with a zero expansion coefficient, preventing the distortions caused by extreme heat or cold. When silvering -- putting silver on the front of a sheet of optical glass to make it into a telescope mirror -- was improved in the late 1800s, metal mirrors were cast aside. The Leviathan's mirror was sent to a museum, and its support structure was melted down for scrap.

The Leviathan of Parsonstown, the first telescope with a 72-in.- diameter mirror, was designed to answer questions about the nature of nebulae. The telescope could move freely in a vertical direction, but horizontal movement was restricted to the extent that total viewing time on a particular object averaged one hour. © Birr Scientific and Heritage Foundation.

But the current push to make giant telescopes with mirrors the size of football fields may cause metal mirrors to make a comeback. When a restoration society asked for a new mirror for the Leviathan's historic telescope, Brooks and his colleagues used the opportunity as a test bed for casting aluminum mirrors.

"The current generation of gigantic telescopes, such as the 8-m Gemini, require extremely large and extremely thin mirrors that can be easily bowed by actuators to correct for atmospheric distortion," he said. Making such thin flexible mirrors is difficult with glass but relatively simple with metal. Metal mirrors are also cheaper; Brooks said the Leviathan's aluminum mirror cost only one-third the price of a glass mirror of the same size.

Nickel coating

To achieve the same optical finish as glass, they employed Nitec (Derbyshire) Ltd. to coat the soft aluminum with a thin outer layer of nickel. This had the added benefit of combining the light weight and flexibility of aluminum with the tough, hard finish of nickel. An amateur astronomical society plans to use the telescope once its drive train is completed.

Brooks said the project showed that nickel-coated aluminum could be used successfully for the mirrors of ambitious new observatories, such as the proposed 100-m OverWhelmingly Large telescope. Metal mirrors even offer a few advantages over glass, such as better emissivity, fewer problems with condensation and the ability to bolt instruments to the back.

Brooks sees the technology of telescope mirrors circling back to its roots. "It's quite amazing to find that this bloke was doing that work 150 years ago," he said.

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