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11 Aug 2004
Lightweight carbon fiber composite mirrors that rival Zerodur and silicon carbide are being put through their paces by a UK team.
Carbon fiber composite (CFC) mirrors weighing ten times less than their glass ceramic counterparts are being developed in the UK. The hope is that the technology will make it easier to transport telescope mirrors to remote mountain tops and save valuable weight on satellites.
The team expects to receive its first active CFC mirror this month. With doubts over the long term stability of composite materials, the researchers are on a mission to win over industry sceptics.
Managed under the Smart Optics Faraday partnership the 2 year program involves UCL's Optical Science Laboratory, QinetiQ and Cobham Composites. The £185 000 project is supported under the MOD's joint grants scheme, PPARC's industrial support scheme and by its industrial partners who are keen to pioneer the new technology.
The group already has a 30 cm diameter CFC passive mirror which was manufactured at Cobham's site in Leicestershire, UK at the end of last year. Weighing just 497 g it is only 10% of the mass of an equivalent Zerodur mirror. The mirror's 150 micron thick Ni layer was ground and polished back at UCL to give a very smooth surface with a roughness of just 4 nm.
The team's molded passive mirror design features alternate layers of composite and aluminum honeycomb. Commonly found in space technology, the honeycomb geometry helps to stiffen the mirror and protect against launch forces of up to 5g.
With its strong interest in satellite systems, project partner QinetiQ is keen to exploit CFC's payload advantage. However, as UCL researcher Sarah Kendrew told Optics.org: "It's not just about space." The lightweight mirrors could also benefit unmanned airborne vehicles, helping to lighten their surveillance or environmental monitoring payload.
Being lightweight and robust, CFC mirrors have clear advantages over more established materials silicon carbide and Zerodur. But with issues such as moisture uptake raising questions as to CFC's long term stability, there is a real need for a technology demonstrator.
"The passive mirror was a test-bed for the manufacturing process," Kendrew explained. By looking at molding and coating techniques and examining effects such as print through, the team was in a position to kick start stability testing and roll out an active or 'adaptive' design.
Adaptive optics, which correct for optical aberrations induced by the atmosphere, are set to play a vital role in future extremely large telescopes such as Euro50 and OWL. The next step for the team is to replace the aluminum mirror in UCL's proven adaptive optics platform with a CFC design.
Author
James Tyrrell is reporter on Optics.org and Opto & Laser Europe magazine.
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