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Perfect surfaces

02 April 2012

The National Facility for Ultra-Precision Surfaces in the OpTIC Glyndwr centre, St Asaph, North Wales, has emerged as one of the world's leading centres for the preparation of large surfaces of the order of one square metre to nanometre accuracy. An important aspect of this work has been the integration of both polishing and measurement within a single facility.

Close-up of the Zeeko 7-axis CNC form correction and polishing process operating at OpTIC
Close-up of the Zeeko 7-axis CNC form correction and polishing process operating at OpTIC

One of OpTIC's first successes is the recent completion of a 1.5 metre master reference mirror as the first stage of a £5m contract to produce seven prototype segments for the European Extremely Large Telescope (E-ELT). The master has been corrected and polished to a superb quality, with overall form error less than 17nm rms, 5nm smoothness and l.4nm surface texture.

The first 1.4 metre hexagonal prototype segment is now being processed on a 1.6m capacity Zeeko seven-axis CNC robotic polishing machine installed at the OpTIC facility. The 40m E-ELT main mirror will require 931 such segments at a capital value of some Euro100-150m, and the commercial partners of the Welsh centre will be able to quote for producing them, along with other companies from around the world.

Zeeko, the Coalville based polishing and metrology equipment specialist, has established a research unit within the OpTIC building. Zeeko co-founder and director of research, Professor David Walker, a professor at Glyndwr University and also at UCL, is leading Zeeko's E-ELT research in the UK.

"Worldwide, there is a yawning gulf between today's pilot production plants for metre-class mirror segments and tomorrow's mass production factories," says Professor Walker. "The sector is in a race to close this gap and the OpTIC Glyndwr-led consortium, having created a world-leading R&D facility, is well placed to compete for a large part of the business and create a significant number of jobs.

"Part of the reason is the Zeeko method of form control and polishing, which can be considerably faster than competing processes, with significant strides being made towards solving the most challenging problem - direct polishing of the all-important edges of the mirror segments.

"We have an engineer dedicated to researching and modelling the polishing of edges - the 10mm of surface nearest the periphery - to avoid the natural roll-over due to the overlap of a polishing tool. We are achieving 100 - 200nm of edge-roll - a world beating result by direct polishing."

The Zeeko optic fabrication centre, integrating both polishing and measurement in a unified facility, won the Queen's Awards for Innovation in 2011. OpTIC, meanwhile, has installed a 10m high optical test tower above the IRP 1600 corrective polishing machine platform and a demountable scanning profilometer for in-situ measurement.

Zeeko, in turn, has provided the first of a new generation of in-situ texture measurement systems. The benefit of combining polishing and metrology on the machine is that the fragile and precise optical component remains undisturbed throughout the production cycle. This avoids the risk of damaging the optic by repeatedly transferring it between the polisher and a separate metrology station, and helps to preserve the critical co-ordinate relationships between the optical surface, the machine tool-path, and the measurement data.

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