Mirror image

5 min read

UK optical technology expertise is at the heart of some of the world’s most exciting space and energy projects. Stuart Nathan reports

Tunnelled deep into the chalk of the South Downs, not far from Croydon, a set of workshops and testing suites turn out some of the most specialised mirrors in the world. Insulated by the surrounding rock, at a constant temperature and immune from vibration, a small team of craftsmen at

Optical Surfaces

specialises in oddly shaped mirror geometries that depart from the normal, symmetrical, flat or spherical sections, many of them destined for satellite telescopes to explore the far reaches of our galaxy and beyond.

Meanwhile, in north Wales, the Ultra Precision Structured Surfaces Laboratory (UPS2), armed with the latest techniques for grinding and polishing optical surfaces, is taking the first steps towards offering a commercial service: the supply of large components for the next generation of Earth-based super-telescopes and for an expected surge in research into laser-generated nuclear fusion.

Optical Surfaces and UPS2 occupy different areas of technology and scale, but both are looking optimistically towards the future, where their skills in making perfect reflective surfaces and lenses could give the UK a leading position in this most exacting of industries.

Optical Surfaces was founded in 1962 by optical technologists with strong links to astronomy. Jon Mathers, company director, explained that the early work of the company was split between astronomical telescope mirrors, flat mirrors for gearwheel projectors and specialised work making optics for university research laboratories. 'Then the company found that it had a particular set of talents suited to a type of optic called off-axis paraboloids and developing those led them into the niche we're in now,' he said. 'They found that they were the only people in the UK who were making them and there weren't many in Europe or the US either. We still don't have many competitors.'

Off-axis paraboloids are a particularly complex shape. Imagine taking a concave mirror that is a section of a sphere and deepening the curve into a paraboloid. 'The deeper the bowl of the mirror, the more it deviates from spherical, the more difficult it becomes,' Mathers added. If the shape of the mirror needed is a section of a paraboloid away from the axis of the shape, it becomes 'a hundred-times harder, because the mirror wants to curve in different directions in several planes and you're fighting nature all the time'.

Off-axis paraboloids are indispensable in many complex optical systems. For example, Optical Surfaces produced three for the TOPSAT Earth observation satellite: these help produce the satellite telescope's long focal length and large optical aperture. Another set of mirrors focuses the optics for an instrument monitoring global surface-temperatures and the El Nino effect for the Rutherford Appleton Laboratory: the mirrors' geometry reduces signal losses in important wavelength regions.

Making an aspherical mirror is highly skilled and Optical Surfaces takes a craftsman's approach. 'We use computer measuring and the polishing is done mechanically, but the actual setting of the machinery is done by hand,' Mathers said. 'We can set configurations of stroke length and other parameters and you have to balance the length of the polishing arm with its flexibility to get the result you're trying to achieve. If the polisher isn't flexible enough, it digs into the shape of the surface you're trying to create and if it's too flexible, it just flows over the surface.' These decisions are made by engineers that use iterations of testing to come up with the right combination of materials, construction techniques and tooling to produce the geometry they need. 'Towards the end, there is often some work done actually by hand; you're trying to get down to millions of an inch smoothness and a few minutes' work done wrong can wreck the mirror, so it's often more direct to go by hand.'

This makes finding and training craftsmen difficult. 'We had an increase in our order books recently and we tried to recruit some skilled lens polishers, but it proved to be impossible,' Mathers added. It takes about three years to find out whether a new recruit has the right talent; five years is the make-or-break point, then further training can take another five years.

Despite the difficulties, however, Optical Surfaces makes 500-1000 mirrors a year and has a full order book. It's this prospect that is powering UPS2, which is sited in a cluster of precision-optics specialists, the OpTIC Technium, near two of the UK's biggest firms in the sector, Qioptics (a manufacturer of optics, mainly for the defence industry) and Phoenix Optics, which specialises in optical fibre.

UPS2 is an Integrated Knowledge Centre, bringing together expertise in making large optics — bigger than 50cm — from Cranfield University, University College London and Cambridge University. Its director, Prof Peter Shore from Cranfield, explained this was a response to growth identified in the market.

'We did a study in 2003, with funding from the DTI, what's now Qioptics, and the Welsh Assembly,' he explained. 'And that was because we know the world capacity for large optics is going to be outstripped by demand and will be for quite some time.' There are two main reasons for this, he said. Telescopes are getting bigger, both in space and on the ground: in telescope technology, the size of the primary mirror is paramount.

'There's the 30m telescope project in the US and the 42m European Extremely Large Telescope; between them, those two will have £300m of optics, including 900 mirror segments in the E-ELT alone. Then there's NASA's planned International X-Ray Laboratory, which has another £300m of optics.'

The main suppliers for large optics are Sagem in France, which can routinely make 7m mirror segments, and L3 in the US. However, Shore said, the upcoming workload is so large that these companies won't be able to meet the demand within the project timescales. And, in terms of these types of optics, two of the UPS2 partners have significant experience and new technology to bring to bear.

'Historically, Cranfield has been very involved in making machines to make large optics,' Mathers said. The department's grinding machines have been used by NASA since the 1970s and it made the machines that Kodak used to make the primary mirror for the Keck Telescope on Mauna Kea in Hawaii. Cranfield also makes equipment to measure and verify the surface characteristics of mirrors: NASA is again a client, using Cranfield machines to verify the mirror on the Chandra space telescope, the next into orbit after Hubble.

Meanwhile UCL has been working on polishing technique. In the early 2000s a team under Dr David Walker, from the university's optical science laboratory, developed a new technique capable of freeform polishing: that is, polishing shapes up to a metre across with complex axes of symmetry or no symmetry at all. Spun out via a company called Zeeko, this technology uses an inflatable polishing tool, the internal pressure of which can be varied, and that can move in three rotating axes and two angles of inclination.

'We've also added an embryonic process that came out of a collaboration between Cranfield and the Lawrence Livermore Laboratory in California,' Shore said. This is called Reactive Atom Plasma Torch (RAPT) figuring, which uses a fluorine-containing plasma to remove material from a glass, metal or ceramic surface without applying any downward force. 'These three processes sit in a production line and go much faster than conventional techniques,' Shore added. The UPS2 IKC was set up with £4m funding to develop production facilities using these techniques. 'Nobody else has this technology. They should allow us to make a 1m optic in 10 hours,' Shore said. 'But as they're new machines there are a few problems.'

Towards the end of 2007, UPS2 made a bid to make prototype mirrors for the European Southern Observatory, where the E-ELT is to be based. 'The ESO placed contracts with both us and Sagem to each make seven mirrors for €5m (£4.45m),' Shore said. Such projects don't require the techniques used to make the mirrors to be validated, so the new technology isn't a barrier, Shore added: it's the quality of the product that counts, and the speed of the new techniques could make UPS2 cheaper than its competitors.

But there is another market prospect, which arises from the co-operation with Lawrence Livermore. 'They don't need large optics for telescopes; they need them for fusion energy at the National Ignition Facility (NIF),' Shore explained. 'The optics close to the focusing region for the lasers that will trigger fusion are the system's wear components; they have so much energy going through them that they degrade quite quickly and will have to be replaced and reconditioned regularly.' Once NIF is running, many other laser-fusion projects are likely to start: HiPER is already scheduled and is likely to spend £150m on optics. Several smaller projects in Europe, China, Japan and South Korea are also planned.