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Large-aperture optical elements are widely employed in high-power laser system,astronomy,and outer-space technology.Sub-aperture stitching has proven an effective technique for overcoming the lateral size constraints of conventional interferometers.Sub-aperture stitching is also an effective way to extend the lateral and vertical dynamic range of a conventional interferometer In 1982,the C.J.Kim in Arizona optical center of United States proposed the concept of sub-aperture testing,who achieved a parabolic mirror of self-alignment test by using small-caliber plane array instead of large-caliber plane.2003,U.S.QED technology company successfully developed the SSI automatically stitching interferometer.In addition to increased aperture capability(both clear and numerical aperture),the SSI provides automatic in-line calibration of systematic errors-reference wave and distortion in particular.Most of the commercial available sub-aperture stitching interferometers measure the surface with a standard lens that produces a reference wavefront and the precision of the interferometer are generally limited by the standard lens.To overcome the accuracy limitation due to the aberration of reference wavefront in the interferometer testing one of body of so-called absolute tests can be used which can test the systematic errors,including the reference surface,of the instrument to be used to improve the test accuracy.Flats are compared in pairs in the traditional absolute testing method.Recently there are some methods to measure the flatness of the entire surfaceIn the traditional sub-aperture stitching method testing result contain the errors of reference lens.Removing the errors caused by reference surface can improve the test accuracy of the sub-aperture stitching interferometer.In our paper we use the different subapertures as the different flats to get the profile of the reference lens.According to the N position tests method and the special equipment we get the profile of the reference lens and the testing lens.Only two lens in the testing process which is fewer than the traditional 3-flat method.In the testing equipment,we add a reflective lens and a lens which can transparent and reflect to get the nonrationally symmetric errors of the testing flat.The arithmetic is present in this paper which uses the absolute testing method to improve the testing accuracy of the sub-aperture stitching interferometers by removing the errors caused by reference surface.