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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.
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