QMT Features: September 2007
Reaching for the stars
An ultra high accuracy CMM is set to speed up the search for new planets at Cranfield University

The Leitz pmm-f is facilitating the search for new planets in outer space
Hexagon Metrology is installing the UK's most accurate, large volume coordinate measuring machine in a deal designed to speed up the search for earth-like planets in outer space. The £400,000 machine - an ultra high accuracy Leitz PMM-F - has been supplied to Bedfordshire-based Cranfield University, one of Europe's top scientific research, development and design centres.

The CMM will be used by the academic institute's School of Applied Sciences to inspect optics destined for next generation telescopes, including the James Webb space telescope, the replacement for Hubble.

The Leitz machine will also be used in research to develop mirrors for ELTs - the extra large telescopes under development by the European Southern Observatory and other leading astronomical agencies.

The ELTs - made up of one to two-metre segments that interlock to create mirrors of up to 100 metres in diameter - are manufactured on a high-precision grinder known as the BoX (short for Big OptiX).

The machine - developed by the Cranfield University Precision engineering centre - is part of a four-year, £3.5 million grant-aided collaborative project designed to kick start a revival in UK ultra-precision surface manufacturing.

Mirrors for the new 50-100 metre super telescopes will be made up of between 600 and 2,000 segments each - in contrast to the 36 segments used in their predecessors - necessitating faster, more accurate, lower-cost production technologies.

"Earth-like planets, unlike stars, are extremely difficult to see, which is why a new breed of telescope is needed to detect them," said Paul Shore, Professor of Ultra Precision Technologies (UPT) at Cranfield." A significant stumbling block has been the time it takes to manufacture the mirror segments. Prior to the development of the Cranfield BoX machine, and large scale polishing systems developed by University College London, available manufacturing technologies for large ultra-precision mirror production were slow and costly.

"Processing would have taken hundreds of hours for each segment meaning the time taken to manufacture the mirrors for an ELT mirror would have been many years. But now the Cranfield and UCL-developed machines have removed that obstacle, reducing mirror processing from days to hours."

The Hexagon-supplied Leitz CMM is to be used to verify the performance of the BoX machine. It will also serve as a precision motion system to move laser-based curvature sensors around the surface of machined mirror surfaces to confirm their surface accuracy and relative position to edge features.

Cranfield's functional specification stipulated a pre-determined positioning capability for the CMM's measuring probe of less than one micron when inspecting one-metre parts. A further requirement was for the curvature sensor - a compact interferometer - to hold position with a stability of 100nm (0.1 of a micron) in order to permit stable measurements of the concave mirror surfaces.

"Our demanding specification meant there were only two or three credible manufacturers of CMMs in the world that could meet our specification" said Professor Shore. "And, of the CMMs we tested, only the Leitz PMM-F satisfied our needs.

"The Leitz engineering team was also extremely responsive to design changes we wanted to implement for our specific application. This gave us a high degree of confidence in the machine and the technical support behind it."

The CMM, which will be fitted with a Leitz 3D probe head and a special interferometer adaptor, will run Hexagon's PC-DMIS CAD++ measurement software and a Quindos metrology package.

It is being installed and calibrated by Hexagon's factory-trained engineers throughout July.

As part of the contract, Hexagon Metrology+, the company's aftermarket services division, will supply PC-DMIS and Quindos software training and an annual calibration service.

"We're delighted to be a part of this exciting project," said Hexagon Metrology's sales and marketing manager, Gavin Bell. "We worked long and hard with Professor Shore's UPT group both at Cranfield and over at our Leitz HQ in Wetzlar, Germany, to ensure the PMM system met all the group's requirements. "As a result of this collaboration the CMM in its 'Infinite' guise achieves a first term of better than 0.3µ (of a micron).”l
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The Cranfield Collaborative Project

Cranfield University's partners in the four year collaborative project to establish a world class UK facility for the manufacture of large optics include University College London and Technium OpTIC, the Welsh Assembly-backed opto-electronics technology and incubation centre based in St Asaph North Wales. The centre is now home to a new National Centre for Ultra-Precision Surfaces.
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