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Left: A 10-metre high test tower directly over the IRP1600 machine for in-situ interferometry. Right: Polishing of the prototype segment using the IRP1600 machine

period of six years, the first segment will have to match the last, with no drift in the base radius. Glyndwr is using a custom-built pentaprism scanning-profilometer. This instrument mounts onto the IRP1600 polishing machine, measures the segment in situ, and determines the absolute base radius independently of the interferometry measurements. ‘The metrology has proved to be just as

difficult as achieving the edge control,’ says Walker. ‘However, we have excellent metrology, which has been certified by ESO. A benefit is that all the metrology is carried out with the segment on the polishing machine – we don’t have to take the segment off to measure it, meaning fewer inaccuracies caused by handling and less downtime.’ The secondary mirror (M2) creates a separate

problem in designing a telescope of this scale. It is four metres, convex, and strongly aspheric. This will be the first time that large convex mirrors are manufactured, because, in other telescopes, four metre mirrors have always been concave. Cayrel at ESO says: ‘We have been working with the industry to decide what would be the best way to test and manufacture such an optic.’

Other optical surfaces proved to be less

difficult to design, because they are comparable to existing technologies. The tertiary mirror (M3) is four metres in size, like the M2, but it is | @electrooptics

classical and similar to the optical design found in other large ESO telescopes, such as the New Technology Telescope (NTT) or the Very Large Telescope (VLT). The quaternary mirror (M4) is an adaptive

mirror about 2.5m in size, which is used to correct – in real time – the effects of the atmospheric turbulence. ‘It will be quite challenging to build, but because other

The segments have a

very long radius of curvature – 84m – ruling out a direct test to measure the centre of curvature

large adaptive mirrors have already been manufactured, such as the VLT secondary mirror, we can just expand that technology from 1m to 2.5m,’ explains Cayrel. The M5 is a tip-tilt mirror almost three metres in size. This optic needs to be both extremely stiff and extremely lightweight to allow for the tip-tilt corrections which stabilise the image. According to Cayrel: ‘We can benefit from developments that have been made in the past for astronomy, such as the manufacture of extremely lightweight, silicon-carbide optics,

and we would use the same type of technology for this.’

As with the primary mirror segments, the manufacture of the M2 mirror is at the limits of the technology of the optics industry, and manufacturing considerations influenced its design, as Cayrel explains: ‘The design of the whole telescope has been made so that the M2 is no larger than four metres, because this would create difficulties in finding potential vendors for polishing and for other optical processes. We focused on a design so that the M2 is feasible without being faced with major challenges from the industry.’

The issues of automation and mass production

in large lenses extend way beyond astronomy. The National Ignition Facility (NIF) in the USA is researching nuclear fusion using high-power lasers, and the UK’s HiPER laser-fusion project has been conducting its preparatory stage. This requires substantial quantities of large optics and, because the lasers damage the optics, there are markets in both their manufacture and their refurbishment. Walker believes the project may lead to power generation within the next 30 years or so. ‘What we, and other industries in the UK, are trying to do, is take our existing optical fabrication technologies and explore how they could be used to meet this need,’ he said. ‘Large optics is not just for astronomy, but also for solving the world’s energy crisis.’ l


David Walker

David Walker

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