In order to certify the accuracy of a null corrector, a method using a single spherical lens is proposed in this paper. An inversed optical path of the infinite conjugated null corrector is introduced, and the aberrations are compensated by using the certifying lens with a reflective inner surface. Initial configurations of the certifying lens are deduced from the aberration characteristics of the null test. A F1.33 ellipsoidal mirror's null corrector is taken for an example. Based on the calculated parameters of the certifying lens, the contribution of the surface's spherical aberration is set as a merit function in the optimization. The root-mean-square wavefront error of the optimized design is 0.0016λ (λ = 632.8 nm). The method in this paper is simple and low-cost, compared with the existing methods.
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