Corrective machining of monolithic multiple freeform mirrors based on holographic null test

Abstract

Monolithic multiple freeform mirrors integrate two or more surfaces on a single substrate. Alignment of these surfaces is no longer required because they are frozen there as they are machined. Null test is then critical as a feedback to corrective machining which is indispensable for high accuracy optics. This paper presents a methodology for corrective machining of monolithic multiple freeform mirrors based on holographic null test capable of measuring multiple surfaces simultaneously. The surfaces are firstly related and modeled by coordinate transformation. A single computer generated hologram is then proposed with different regions to compensate aberrations of different surfaces. A combined tilt and power carrier is designed to separate the disturbance orders of diffraction. Mapping distortion in the null test is corrected through a pure ray tracing procedure by introducing a virtual reference sphere. The surface error including the orientation error is then superposed on the nominal surface to generate the tool path for corrective machining with fast axis servo. The tool radius and decenter is recognized by pre-cutting a spherical surface. The remounted workpiece is aligned with the machine tool by using an on-machine probe. Finally the proposed method is experimentally verified by corrective machining monolithic two mirrors with surface error effectively reduced and null fringes observed for both surfaces.