A 4.1m diameter primary mirror designed in the UK is the centrepiece of what is expected to be the world“s largest and fastest ground-based telescope.
The mirror is currently being installed on the …
A 4.1m diameter primary mirror designed in the UK is the centrepiece of what is expected to be the world“s largest and fastest ground-based telescope. The mirror is currently being installed on the Visible and Infrared Survey Telescope for Astronomy (VISTA) at the mountain-top Paranal Observatory in Chile. The primary mirror will be coupled with a small infrared camera on the telescope for initial testing before installing the main camera in June 2008. The mirror, which is 17cm thick and weighs 5.5 tonnes, is made of a special ultra-low expansion glass-ceramic called Zerodur, from Schott Glass in Germany. It has been polished over a period of almost two years to a precise hyperboloid shape by optical glass manufacturer LZOS in Moscow. According to the UK designers, a consortium of 18 of the country“s universities, VISTA“s primary mirror is the most strongly curved large mirror ever polished to such a precise surface accuracy. The mirror was lifted by crane into the telescope dome at Paranal where it was washed and coated with a thin layer of silver in the coating plant. Silver was considered to be the best metal for the purpose since it reflects more than 98% of near-infrared light, better than the more commonly used aluminium. It is only the second time a silver coating has been used on an observatory telescope. Will Sutherland of Queen Mary, University of London and the project scientist on VISTA, said the mirror“s strong curvature was needed to give the telescope shorter focal length, which provides it with greater optical power. “Traditionally if you want a short focal length the easy way to do it is to attach a camera to the telescope at the prime focus, so the camera is looking down on to the mirror”, he said. “But our camera is much too big to do that. It has to go on the back”.