The method to reduce the spinal error in the aspheric mirror testing with the CGH

Interferometric optical testing using computer-generated holograms(CGHs) has proven to supply a very good and accurate measurements method of the aspheric surfaces. However, the CGHs are diffractive optical elements which use diffraction to create wavefronts of light with desired amplitudes and phases. The different diffraction order of the light would be make some ghost image to the fringe pattern. It would introduce some spinal error to the measurement results. This error would not be avoided after the CGH designed and manufactured. In this work, we take two measurement steps to reduce the spinal error. The first step, the apheric mirror was tested with the CGHs. The second step, the aspheric mirror was tested with transmission sphere directly. Then the subaperture theory was used to obtain the final measurement results of the aspheric mirror surface. The experimental demonstrations were provided by testing an aspheric mirror. The results are shown that this method could reduce the spinal error.