18 June 2007 Absolute interferometric measurement of flatness: application of different methods to test a 600 mm diameter reference flat
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Abstract
LIL and LMJ are two french high power laser facilities dedicated to laser-plasma interaction experiments. In order to control the flatness requirements of their optics, the CEA has an 800 mm diameter Fizeau interferometer. We decided to qualify our cavity flats with absolute calibrations. To achieve three-flat methods, a third 600 mm diameter silica transmission flat with a custom design mount that enables an easy rotation around the optical axis with low distortion of the flat is being fabricated. Over the past two decades, many papers have been published explaining how the traditional three-flat method can be improved to obtain two-dimensional data of a reference flat with various precisions. After a quick overview of these different methods (mostly using translations and rotations of at least one of the flats), we shall present several two-flat methods to get absolute flatness measurements over a wide spatial period bandwidth that we will use during the fabrication of our flat. For low frequencies, translations of the flat provide absolute slopes that are then integrated. For high frequencies, averaging of a few measurements taken with specific positions of the flat under test enables to separate the contributions of the reference and the test optics. Simulation results show the efficiency of the flat reconstruction and put forward the loss of information due to the method.
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Franck Morin, Franck Morin, Stéphane Bouillet, Stéphane Bouillet, } "Absolute interferometric measurement of flatness: application of different methods to test a 600 mm diameter reference flat", Proc. SPIE 6616, Optical Measurement Systems for Industrial Inspection V, 66164G (18 June 2007); doi: 10.1117/12.725974; https://doi.org/10.1117/12.725974
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