Abstract: Convex aspheric surface testing is very difficult. In order to obtain better wavefront error, a method including back null compensation with auxiliary surface is proposed. This method, basing on third-order aberration theory, study the ability of auxiliary surface to compensate the normal aberration of convex asphere, in which . There are two functions of the auxiliary surface. One is to make the center of the asphere locate in the air. The second is using the combination of auxiliary surface and convex aspheric surface to compensate the normal aberration of convex asphere. When , the value of RMS, which is calculated by theory and Zemax, is better than . Furthermore , the surface figure accuracy can be practically reach by using above testing method. In conclution, this letter proved that back null compensation with auxiliary surface testing is able to achieve better wavefront error.

Key words: geometric optics；auxiliary surface compensation；convex asphere testing