Mirror image

A UK collaboration has produced machines to make superior, large-scale telescopic lenses in this country. Siobhan Wagner reports


Large-scale telescopic lenses that allow us to see into the depths of the universe have been manufactured abroad in recent years, but a new UK university and industry collaboration hopes to bring that production capability back home.

The group recently developed three new machine systems that can produce mirrors more than 1m in size for use on extra-large telescopes like those at the European Southern Observatory.

The UK always had the skills to produce such mirrors, said Paul Shore, head of the precision engineering centre at Cranfield University, but the manufacturing technology was always developed here and then sold to countries including the US or France. ‘You start to see that loads of these clever ideas came from the UK, but when you look and see who’s actually making money out of it, it’s the Americans or the French,’ he said.

Shore thought the best way to put a stop to that was to develop full large-optic production facilities in the UK. With £3.6m funding from the Joint Research Councils Basic Technology, the result after four years is three production machines with capabilities that he claims surpass anything else.

The first machine, called BoX, was developed by Cranfield University and Cranfield Precision for grinding the mirrors into their desired shape. The machine smoothly grinds away the mirror surface with stiffness and precision that is ensured with a laser driven metrology system. Shore claims the machine can grind a 1m sized mirror to an accuracy of 1 micron.

‘The machine is essentially the most accurate measuring machine in the UK, but it also has the ability to grind to surface,’ he said. ‘You can grind to surface and then measure the errors of the surface using its measuring system and then feed those errors back into the control system to correct the shape to make it more precise.’

Cranfield developed and sold a large-optic grinding machine to Kodak in the US in 1990, but Shore said this one is far superior. ‘The machine supplied to Kodak had a capability to grind 1m scale parts in 100 hours. What we’ve done with the BoX machine is put much more power into the system and much more temperature control, so we can grind the same part in about a tenth of the time.’

After a mirror goes through the BoX grinding process, its next stop on the production line is the Zeeko IRP1200, a polishing machine developed by University College London and Zeeko. The seven-axis CNC optical polishing machine produces nanometre smooth surfaces on optical materials and surface forms up to 1.2m. Shore said the machine uses a range of traditional abrasive pad and fluid polishing technologies that produce surfaces faster than other techniques.

A competing US technique uses an expensive and time-consuming magnetorheological (MR) finishing process, he claimed.

‘The Zeeko process uses an inflated rubber bonnet that carries polishing pads. It uses a time dwell polishing technique that essentially dwells on the high spots of an optic and polishes more quickly over the low regions, which allows it to converge the shape of the surface to the correct one very quickly,’ explained Shore.

Users of MR finishing technology might argue that it provides greater long-term stability, but Shore said this is not entirely true because MR fluid degrades with time.

After grinding and polishing, the optic journeys on for final figure correcting on the Helios 1200 developed by Cranfield University and RAPT Industries. The machine uses gases, contained within a shield of argon plasma, to chemically dissolve the optic material at nanometre level. The process can operate at atmospheric pressure which Shore said is much faster and cheaper than techniques such as ion beam figuring.

Shore and his colleagues are talking to UK companies interested in these machines for a full-scale large optic production facility. The plan is to manufacture mirrors not only for space projects, but also lenses for high power lasers within fusion energy systems.