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Proceedings Paper

A swing arm profilometer for large telescope mirror element metrology
Author(s): M. J. Callender; A. Efstathiou; C. W. King; D. D. Walker; A. E. Gee; A. J. Lewis; S. Oldfield; R. M. Steel
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Paper Abstract

The next generation of ground-based extremely large telescopes of 30 m to 100 m aperture calls for the manufacture of several hundred sub-aperture segments of 1 m to 2 m diameter. Each annulus of the overall aperture is formed from separate elements of the appropriate off-axis conic section (usually a paraboloid). Manufacture of these segments requires a systematic approach to in- and post-process metrology for all stages of manufacture, including the grinding stage, despite the fact that the resulting ground surface is generally not amenable to optically reflective measurement techniques. To address the need for measurements on such 1 m to 2 m telescope segments, a swing arm profilometer has been constructed as part of a collaborative project between University College London (UCL) and the UK National Physical Laboratory (NPL). The current swing-arm profilometer is intended as a proof-of-concept device and has the capability to measure concave and convex surfaces of up to 1 m in diameter with a minimum radius of curvature of 1.75 m for concave and 1.25 m for convex surfaces. Results will be traceable to national length standards. Principles of the swing-arm instrument will be described together with the mechanics of the arm design, its bearing and adjustment arrangements and surface probe options. We assess the performance requirements of 20 nm RMS form measurement accuracy in the context of the tolerances of the selected profilometer components, the error budget, and preliminary system measurements. Initial results are presented with a Solartron linear encoder. We also plan to mount optical sensors on the end of the arm as an alternative to traditional contact probes. Initially these will include an Arden AWS-50 wavefront curvature sensor and a Fisba μ-phase interferometer. The method of attachment of the Arden AWS-50 is outlined. The swing arm profilometer is to be located at a specialised facility, the OPtiC Technium, Denbigh, North Wales, where it will form part of a tool-kit of metrology and polishing devices for researching the production of large aspheric surfaces.

Paper Details

Date Published: 6 July 2006
PDF: 12 pages
Proc. SPIE 6273, Optomechanical Technologies for Astronomy, 62732R (6 July 2006); doi: 10.1117/12.671304
Show Author Affiliations
M. J. Callender, Univ. College London (United Kingdom)
A. Efstathiou, Univ. College London (United Kingdom)
C. W. King, Univ. College London (United Kingdom)
D. D. Walker, Univ. College London (United Kingdom)
Zeeko Ltd. (United Kingdom)
A. E. Gee, Univ. College London (United Kingdom)
A. J. Lewis, National Physical Lab. (United Kingdom)
S. Oldfield, National Physical Lab. (United Kingdom)
R. M. Steel, National Physical Lab. (United Kingdom)

Published in SPIE Proceedings Vol. 6273:
Optomechanical Technologies for Astronomy
Eli Atad-Ettedgui; Joseph Antebi; Dietrich Lemke, Editor(s)

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