Lapping of aspherical surfaces

To reach a large wear rate, a coarse grain size and large tools are used. The last point is of essential importance to avoid ripple structures because they become visible in only the final polishing phase. The tool size and stroke amplitude define the final surface smoothness with respect to short-wave errors. 

A fast metrology is needed to control the lapping process and continuously to obtain the exact topography of the optical surface. With that information the tool shape can be adapted to the next figuring step, as can the function of the tool wear rate. Lapping and testing become an iterative process. 

The null system can either be made as a reflecting system or as a lens design. Typical IR materials for lenses are germanium, barium fluoride, or zinc selenide. 

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