The partial null interferometric aspheric testing technique, based on the Twyman-Green interferometer system, is very
useful and of good versatility. In this technique, the under-test aspheric needs to be located precisely. Taking advantage of ray tracing and digital image processing technique, a new method to locate aspheric is proposed. Firstly, model and simulate the Twyman-Green interferometer system in the ray tracing software ZEMAX, find an optimal test position and generate an optimal referenced interferogram. Record the interferogram and make it a target for the experimental interferogram to achieve. At the same time, an experimental interferogram can be obtained by building the same testing system experimentally. Process the one-dimensional gray scale data in X-axis of the two interferograms, two curves, indicating the black and white change of the interference fringes, are obtained. By comparing the normalized X coordinates of the peaks of the two curves, we can determine whether the under-test aspheric is positioned well. In order to locate the aspheric precisely, the aspheric has to be moved repeatedly to get a perfect interferogram whose peaks of interference fringes match well with those of the target interferogram. An experiment for testing a paraboloid with diameter 100mm and asphericity 50μm is carried out. The result shows that this kind of locating method has an Accuracy of 3-5μm, which demonstrates that the method is practicable and high-precision.