Research On Plane Absolute Inspection Technology Of Vertical Fizeau Interferometer

Interferometry is a common method for detecting the surface accuracy of optical flats.The measurement results inevitably carry the error of the reference flat.As the accuracy of optical surface manufacturing increases,the requirements for detection accuracy also increase.The absolute testing method is an effective way to improve the accuracy of the measurements by removing system errors.It is a key technology in interferometer calibration and high-precision interferometry.First of all,the absolute method for three-flat of large-aperture vertical interferometer is proposed.The method measures the change of gravity deformation when the flat is turned over,and calculates the absolute surface.Theoretical error derivation and numerical simulation of algorithm accuracy of different absolute methods are performed.N-position rotation method optimizes the rotation angle to 63°and increases the number of virtual rotation to 40 times to improve the accuracy of the algorithm.The relationship between the number of iterations and the accuracy of iterative method is analyzed.The edge interpolation error can be reduced by adding a rotation error elimination factor.The accuracy of method based on single rotation oddeven function is better than 0.3nm in numerical simulation,which meets the experimental accuracy requirements.Secondly,the clamping support and deformation scheme of the reference flat and test flat in the large-aperture vertical interferometer are designed.The number of support points at the bottom of the reference flat and the glue spot area,layer thickness and position of test flat are analyzed by using the finite element method.The PV values of clamping and gravity deformation are 15.4nm and 30.1nm respectively.The surface shape changes of the flat before and after the compensation are measured by the liquid reference method.The results show that the power term in the surface error is reduced from 0.017? to-0.014 ?,which proves the effectiveness of later adjustment.Finally,the absolute experiment of the vertical large-aperture interferometer is designed and completed.The peak-to-valley(PV)value is 41.0nm and the root mean square(RMS)value is5.6nm.Comparing the surface of reference flat to the liquid result,the deviation of the PV value is less than 9.8nm,and the deviation of the RMS value is less than 2.0nm on one-dimensional contour which verify the accuracy of method.The influence of rotation error,pixel misalignment error and optical homogeneity error of the test flat are analyzed.When the rotation angle error is less than 0.3°,the distance between the test flat to the reflective flat is less than 224 mm,and the optical homogeneity of the test flat is subtracted,the experiment accuracy meets the requirements.