In previous work, wavefront coding technology has been applied on an off-axis three mirror anastigmatic optical system. The secondary mirror is selected as the wavefront coded element. After redesigned the surface of secondary mirror becomes an unusual unrotational-symmetric surface with cubic term, which can not be tested by traditional null testing with compensator. For preparing for manufacturing and testingthis kind of elements, a simple cubic surface whose equation is z = 3λ(x(3)+y(3)) (where x, y is normalized coordinate, λ=0.6328μm ) is polished. The final surface figure is 0.327λ (PV) and0.023λ (RMS). A non-null method is described for testing the experimental element in this paper. The deviation from a reference plane of the cubic surface is regarded as system error. In another words, the ideal cubic surface is set as the reference artificially. A special system error file for interferometer can be created so that the cubic term can be extracted during the testing process automatically. The residual error is just the departure from the ideal figure of the surface under machining by this way. The error and effective range is also presented. But the method may not be practical for the secondary mirror as wavefront coded element because the surface of that kind is convex asphere added cubic term. An improved non-null method is discussed for testing this kind of surface.
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