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In the quest for ever more lightweight, adaptable and cost-effective optics, UK scientists and engineers are developing large mirrors using a carbon fibre composite (CFC) material.
The team, a collaboration of University College London (UCL), QinetiQ and Cobham Composites, aim to develop for the first time a deformable or active CFC mirror for optical wavelengths. With a projected weight of a mere tenth of an equivalent glass ceramic mirror, the applications for this technology are virtually endless.
Because of their composite fibre-matrix structure, CFCs are very versatile materials, commonly used in the aerospace industry; at the same time lightweight, stiff and robust, they are also ideal for spacecraft and satellite structures, where cost scales directly with the weight of the craft and payload. Using CFC for ultra-lightweight space optics therefore has a double advantage: weight and cost are reduced, and using similar materials will keep the stresses at bay as the craft journeys through varying temperature and gravity environments.
The idea of using CFCs for optical components is by no means new, but making a carbon fibre mirror for visible light presents a whole new problem – how to make a dull surface reflective. Conventional polishing methods cannot make carbon fibre material smooth enough to reflect visible light, and the team have addressed this by coating the mirror with a sliver of nickel during the manufacturing process. This technique was first tested by Cobham Composites with a non-deformable, or passive, CFC mirror in late 2003 with considerable success: repeated grinding and polishing of the nickel surface produced a final roughness of less than 5 nanometres.
Using a combination of practical experience and modelling results, the team have now proceeded to design and develop a 30 centimetre deformable mirror, which is expected to be delivered in the weeks to come.
“The passive mirror was really just meant as a testbed for manufacturing, coating and polishing techniques,” comments Dr. Peter Doel from UCL, the project’s Principal Investigator. “The active mirror is our ultimate goal.”
As its name suggests, an active mirror is supported by an array of electrically or magnetically driven actuators that nudge the mirror into the desired shape. This means that any small deviations from its ideal shape can be corrected, and the initial tolerances are thus much reduced. If the mirror sags or deforms over time, the actuators can again compensate, making it an altogether more stable system.
The use of actively controlled mirrors is already widespread in large astronomical telescopes, where form accuracy is of the utmost importance.
A tougher application for deformable mirrors lies in adaptive optics, a relatively new but fast growing technology in astronomy. In the newest generation of telescopes, astronomers rely on real-time correction systems to compensate for the degrading effect of the turbulent air in the atmosphere on incoming starlight. The key component in these adaptive optical systems is the deformable mirror, whose shape is adjusted in real time to match the shape of the deformed light to provide a crisp image at the telescope focus.
With telescopes’ sizes set to increase dramatically in the next decade, technology for making these deformable mirrors bigger and better will become very valuable.
Here too, carbon fibre offers a viable alternative to the traditional active mirror materials, such as Zerodur and silicon carbide. To keep these mirrors deformable they need to be very thin. Manufacturing, handling and transporting a glassy plate of several metres in size and a few millimetres thick becomes very hazardous, whereas CFC is a very sturdy material.
The mould-based manufacturing method also makes the process entirely repeatable.
“The original driver for this project definitely lay in space technology,” affirms Doel, “but there really is a growing interest from the astronomical community. I think astronomers realise that there’s a limit to what can be done with the existing methods and that carbon fibre could offer a competitive solution.”
The team are planning to continue and expand the work in the future as much remains to be investigated.
“This project has really been about exploring possibilities,” says Doel, “but we need to look into many more aspects of the technology. We’re very excited about the work and we’ve really benefited from having collaborators from such different industries who can all contribute their own perspectives. It’s allowed us to progress more rapidly than if it had just been an academic project”.
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