A flexible angle compensation method for freeform surface testing based on tip/tilt mirror

Freeform surfaces with large asphericity are generally difficult to test. We propose a novel interferometric method based on tip/tilt mirror (TTM) suitable for such surfaces. The incident ray directions are dynamically compensated in several dimensions by the TTM, to control the incident angles of the rays at locations where the local surface profile is steep. Therefore, the tilt angle depend on the local gradients of the tested surface. When the incident angles on all sub-zones are controlled, we can obtain a series of lower density interferograms based on which the surface profile can be determined. The compensating range in our method is analyzed, and the largest measurable slope is 2.8°based on 20-mrad tilt angle and 3.3 F-number. Besides, the influence of the compensating precision of the TTM is simulated, and the compensation error can be ignored when its precision is under 40 μrad. The precision and efficiency of this method were proved by a series of tests.