Accurate localization method for subaperture stitching interferometry in aspherical optics metrology

Aspherical optics have superior performance in many applications. However, precision metrology for an aspherical surface is a key stage in its manufacture. Subaperture stitching interferometry (SSI) is one of the mainstream testing methods for aspherical metrology. In many cases, Annular Stitching Subaperture Interferometry (ASSI) is more suitable for components having rotational symmetry because, compared to SSI, the number of motion directions that need to be adjusted is less. Although motion only along the optical axis is of concern in ASSI, inaccurate localization for subapertures may also occur due to the machining error at the optical center of the optics under test and errors in Cat's eye location, thereby leading to measurement errors. To overcome these problems, an accurate localization method for subaperture stitching is proposed. It is based on the maximum asphericity information between the reference spherical wavefront and the test aspherical surface. First, by using the test aspherical formula and theoretical spherical wavefront, we can calculate the minimum peak-to-valley (PV) value of maximum asphericity. The theoretical location of an optimal reference sphere, which corresponds to this value, can be obtained. Then, we perform a practical test, which starts at the initial zero position, and find an actual minimum PV value near its theoretical location. The difference between the theoretical location and the actual one is the compensation quantity. Finally, we execute ASSI measurement to aspherical optics. The location coordinate of each subaperture is compensated with the acquired quantity. Through the experiments, it can be concluded that the proposed method can improve the measurement accuracy of ASSI in terms of error elimination. The results produced by the new method are more desirable than those of the conventional one.