Telescopes are becoming increasingly large and expensive. Reducing the cost has therefore become a priority for some astronomers. One example of this is the suggestion by a group of astronomers from Laval University in Quebec, Canada, that mirrors with a diameter of 8 metres could be made of metallic liquids rather than glass. The method works by placing the liquid mirror on a spinning platter. As the liquid spins, gravitational forces cause the liquid to deform into the shape of a perfect parabolic mirror (Astrophysical Journal 516 May 10 1999).
This is not the first time that astronomers have considered using rotating platters of liquid – usually mercury – for a central mirror. For example, a mercury-based telescope would cost one hundredth that of a glass mirror. But previous techniques had one particular flaw – the telescope could not be tilted without destroying the shape of the mirror. Ermanno Borra, Anna Ritcey and Etienne Artigau suggest that by using liquids based on glycerin – which has a high viscosity – the liquid would stay in a parabolic shape as the mirror is tilted. By coating the liquid with a thin layer of metal, the researchers hope to create a highly reflective surface on the mirror.
Borra, Ritcey and Artigau are confident that their design can cope with a telescope tilt of 10 degrees. They now hope to improve its performance to a tilt of 20 degrees – which would provide half the viewing angle of a normal telescope.
The new design also has one other major benefit – as the liquid mirror rotates, centrifugal forces cause a steady down-flow of air onto the mirror’s surface, bringing it into thermal equilibrium with its surroundings and reducing air turbulence inside the telescope.
