Subaperture stitching interferometry has been proven to be a promising method for precision metrology of aspheric surfaces. However, due to the aspheric departure, selection of F/number of the transmission sphere for aspheres is more complicated than spheres or flats. This paper proposed an optimization method to determine the maximum subaperture width based on the slope resolution of the standard interferometer. With it, the transmission sphere is selected with an optimal F/number. On the basis of the transmission sphere and overlap rate, we not only can ensure the fringes of off-axis subapertures are resolvable by the interferometer, but also can minimize the number of subapertures required to cover the full aperture of the surface. Finally, some numerical examples are given to illustrate the procedure, and also to verify the validity of our proposed method.
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