A glass of its own

A UK-designed observatory telescope, which is expected to be the world's largest and fastest for ground-based survey work, is nearing completion with the construction of a 4.1m diameter primary mirror.

The mirror is currently being installed on the

(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 prior to installing the main camera next month.

The mirror, which is 17cm thick and weighs 5.5 tonnes, is made of a special ultra-low expansion glass-ceramic called Zerodur, from

in Germany. It has been polished to a precise hyperboloid shape by optical glass manufacturer LZOS in Moscow, a process which took nearly two years.

According to the UK designers, a consortium of 18 UK universities, VISTA's primary mirror is the most strongly curved large mirror ever polished to such a precise surface accuracy.

The mirror was craned into the telescope dome at Paranal where it was washed and coated with a thin layer of silver in the facility's coating plant. Silver was deemed to be the best metal for the purpose since it reflects more than 98 per cent of near-infrared light, which is better than the more commonly used aluminium.