Subaperture stitching is often adopted when measuring large size optic with interferometry method. To reduce the error of defocus in the process of stitching, an improved subaperture stitching method which can eliminate the high-order defocus error is presented based on the experimental study of defocused wavefront. Through detecting the wavefront difference with variable defocus, the actual defocus wavefront is proposed to replace the traditional paraboloid defocus basement so as to realize eliminating high-order defocus. A reflector sample is tested, the roughness parameter Ra is 1.252nm and 0.403nm respectively using the paraboloid and the actual decocus wavefront as the basement. The amendatory defocus basis is taken into the subaperture stitching process, for the two subaperture wavefront, different area of the actual defocus wavefront is used. A SiC super-smooth surface is measured with the improved subaperture stitching method, the Ra is 0.236nm after eliminating high-order defocus. Results show that it is a useful way to erase high-order defocus error without changing the low frequency information of test wavefront.
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