Sub-surface damage issues for effective fabrication of large optics

X. Tonnellier; P. Shore; P. Morantz; A. Baldwin; D. Walker; G. Yu; R. Evans

doi:10.1117/12.790639

23 July 2008 Sub-surface damage issues for effective fabrication of large optics

X. Tonnellier, P. Shore, P. Morantz, A. Baldwin, D. Walker, G. Yu, R. Evans

Proceedings Volume 7018, Advanced Optical and Mechanical Technologies in Telescopes and Instrumentation; 70180F (2008) https://doi.org/10.1117/12.790639
Event: SPIE Astronomical Telescopes + Instrumentation, 2008, Marseille, France

Abstract

A new ultra precision large optics grinding machine, BoX^® has been developed at Cranfield University. BoX^® is located at the UK's Ultra Precision Surfaces laboratory at the OpTIC Technium. This machine offers a rapid and economic solution for grinding large off-axis aspherical and free-form optical components. This paper presents an analysis of subsurface damage assessments of optical ground materials produced using diamond resin bonded grinding wheels. The specific materials used, Zerodur^® and ULE^® are currently under study for making extremely large telescope (ELT) segmented mirrors such as in the E-ELT project. The grinding experiments have been conducted on the BoX^® grinding machine using wheels with grits sizes of 76 μm, 46 μm and 25 μm. Grinding process data was collected using a Kistler dynamometer platform. The highest material removal rate (187.5 mm³/s) used ensures that a 1 metre diameter optic can be ground in less than 10 hours. The surface roughness and surface profile were measured using a Form Talysurf. The subsurface damage was revealed using a sub aperture polishing process in combination with an etching technique. These results are compared with the targeted form accuracy of 1 μm p-v over a 1 metre part, surface roughness of 50-150 nm RMS and subsurface damage in the range of 2-5 μm. This process stage was validated on a 400 mm ULE^® blank and a 1 metre hexagonal Zerodur^® part.

Citation Download Citation

X. Tonnellier, P. Shore, P. Morantz, A. Baldwin, D. Walker, G. Yu, and R. Evans "Sub-surface damage issues for effective fabrication of large optics", Proc. SPIE 7018, Advanced Optical and Mechanical Technologies in Telescopes and Instrumentation, 70180F (23 July 2008); https://doi.org/10.1117/12.790639

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