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Absolute flatness measurement with 10 nm resolution over a 400 mm field

Project MegaJoule is a large facility devoted to laser fusion, for the assessment of nuclear physics simulations. It is now under construction by CEA/DAM and is planned to be operative around 2008. In this facility, 240 laser beams of square cross-section (400 mm x 400 mm) will deliver 1.8 MJ on a deuterium-tritium target within a 2.5 ns pulse. The requirements on the optical elements in the laser path are very stringent: the controls of flat elements need to detect the best fit sphere with a resolution equivalent to a deviation to flatness of 10 nm at the center of the 400 mm x 400 mm area. Also, the deviation of slopes should be evaluated with a resolution of 0.1 mrad and a spatial resolution of 10 mm. The components are to be tested in vertical position, and the measurements should be absolute (i.e. no reference plane of ideal flatness is available). We propose a non interferometric technique that should be able to meet this requirement. The idea is to control the magnifying effect of the component using deflectometry and spatial phase-shifting. We present here the basic theory and first measurements showing the feasability of the system.