A glass of its own

UK-designed primary mirror is the centrepiece of what is expected to be the world’s largest and fastest ground-based telescope. Siobhan Wagner reports.

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 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 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 Schott Glass 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.

This 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 increased 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.’

The 64 megapixel infrared camera weighs 2.9 tonnes and is 3m long, which Sutherland said makes it the world’s biggest infrared camera.

Its focal-plane array consists of 16 infrared detectors, which is sufficient to image an area of the sky 10 times the area of the moon. The array also incorporates six CCD detectors that sense signals from the camera wavefronts and provide information on how to compensate for errors such as focus, alignment and aberrations.

‘Those signals go through a computer which decides what’s not right with the images,’ said Sutherland. ‘The computer then sends corrections to the mirrors to fix that.’

He added that the process happens once a minute all through the night.

VISTA will survey large areas of the southern sky at near infrared wavelengths two to four times that of visible light. This will enable it to study objects that are not easily seen in optical light, either because they are too cool or are surrounded by interstellar dust.

The telescope’s surveys will help to understand the nature, distribution and origin of known types of stars and galaxies, map the 3D structure of the galaxy and help determine the relation between the 3D structure of the universe and mysterious dark energy and dark matter.