13 May 2013 Approach to the measurement of astronomical mirrors with new procedures
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The manufacturing of optical components more often requires grinding and polishing of non rotational symmetric aspheres or freeform surfaces. Although there are measurement techniques available for small diameters of some 10th of mm the measuring of larger surfaces is not or only by extreme efforts feasible. Based on the specification for a large mirror in semi professional and scientific astronomy with up to 1.2 m diameter and a relative aperture of F# < 1.5 a final measurement approach for the measurement of radius and shape deviation (parameters of code number 3 of DIN ISO 10110) is described. The combination and evaluation of different sensors and measurement methods for the measurement of high-precision optical surfaces with concave radii of 3.000 mm to infinity is examined and presented. A reproducibility and absolute accuracy of better l/12 (PV) and l/40 (rms) is to be achieved. The absolute maximum radius difference should be smaller than 0,1 %. Thus, also the measurement of aspheric surfaces and free-form surfaces are investigated. For the measurement of large surfaces, up to 100 individual sub apertures with up to 100 Million Points are recorded by deflectometric or interferometric measurement techniques and composed algorithmically to a total surface area. A precision granite portal with multifunctional device carrier will be presented as precise movements are crucial for all tests. The realization of the required accuracy in the portal-measurement device is verified, documented and compared with a simulation. Results on specimens of 200 and 430 mm diameter are evaluated. The measurements were taken by deflectometry and interferometry on the described test equipment. The validation of the samples with various interferometric procedures was performed. The obtained results are presented, analyzed and discussed.
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E. Hofbauer, E. Hofbauer, R. Rascher, R. Rascher, Th. Stubenrauch, Th. Stubenrauch, J. Liebl, J. Liebl, R. Maurer, R. Maurer, A. Zimmermann, A. Zimmermann, O. Rösch, O. Rösch, J. Reitberger, J. Reitberger, "Approach to the measurement of astronomical mirrors with new procedures", Proc. SPIE 8788, Optical Measurement Systems for Industrial Inspection VIII, 87881D (13 May 2013); doi: 10.1117/12.2020414; https://doi.org/10.1117/12.2020414

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