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Proceedings Paper

Research on fabrication of mirror segments for E-ELT
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Paper Abstract

The next generation ground-based giant telescope, the European Extremely Large Telescope (E-ELT), under development by the European Southern Observation (ESO) 1, will have nearly 1000 hexagonal segments of 1.45m across the flats. Fast processing of these segments with high form and edge specifications has proven to be a challenge. The Zeeko Precessions sub-aperture bonnet polishing plays an important role providing capability for polishing the surface and correcting the form to meet this target 2,3. BoXTM grinding has been adopted. This technology has the advantage of fast generating of aspheric surface with very low subsurface damage (SSD) 4. This will avoid the need of removing thick layer of stock at polishing stage to remove SSD. However the result grinding signatures has proven to be problematic for direct polishing with Zeeko’s standard bonnet technology. A novel ‘grolishing’ process which stands between ‘grinding’ and ‘polishing’ has been developed to deal with mid-spatial features left by BoXTM grinding. This tool is designed base on Zeeko’s R80 bonnet which will fits directly into the company’s IRP series machines. The process parameters have been optimised to have signatures less than 10 nm PV. The edge profile is 1μm upstand within 40 mm edge zone. The ‘grolished’ surface can be directly pre-polished together with all the form corrections. To meet the fabrication time target, R160 bonnet is used with 50 mm polishing spot, this will provide removal rate of 9.8 mm3/minute, which can be employed at pre-polishing stage and some form correction. Process parameters have been developed to leave slow upstand at edge zone without any form of sharp edge downturn. The following form correction stage, which employs smaller polishing spot of about 20 mm diameter, will continue to remove form errors of spatial frequency between 0.02 – 0.05 1/mm. Furthermore, the upstand edge will be, to a large part, removed at this stage. It is demonstrated that the form specs can be achieved after this process. The following smoothing process will improve surface textures and remove edge errors. Local edge rectification is normally necessary to bring the edge at same level. A final smoothing process will bring the bulk area and edge zone to meet all the specifications.

Paper Details

Date Published: 16 October 2012
PDF: 6 pages
Proc. SPIE 8416, 6th International Symposium on Advanced Optical Manufacturing and Testing Technologies: Advanced Optical Manufacturing Technologies, 841602 (16 October 2012); doi: 10.1117/12.2009290
Show Author Affiliations
Guoyu Yu, OpTIC Glyndwr Ltd. (United Kingdom)
David D. Walker, OpTIC Glyndwr Ltd. (United Kingdom)
Univ. College London (United Kingdom)
Zeeko Ltd. (United Kingdom)
Hongyu Li, Univ. College London (United Kingdom)

Published in SPIE Proceedings Vol. 8416:
6th International Symposium on Advanced Optical Manufacturing and Testing Technologies: Advanced Optical Manufacturing Technologies
Li Yang; Eric Ruch; Shengyi Li, Editor(s)

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