Polishing of aspherical surfaces

In the fine-smoothing phase the grain size of the slurry becomes finer and finer down to 1 pm diameter and has to be absolutely uniform. The tool pressure and viscosity of the slurry have to be chosen so that aquaplaning is avoided. The wear rate is one to two magnitudes smaller than during lapping. 

The surface becomes shiny and is tested with a visual interferometer. A He-Ne laser with 632.8 nm wavelength is used. Then the fine structure of the surface becomes visible. In the worst case one has to return to the lapping process if ripple amplitudes are too large to be reduced by polishing. 

With a A is-interferometer testing becomes very sensitive so that vibrations and local seeing influence the fringe pattern. Then, the testing equi ment can be decoupled from vibrations up to 1 kHz by the use of real-time interferometry with the Direct 100 system and a 3D stabilization. Now interferometric tests are possible at any time with a resolution of 1 nm or even better. 

In case of concave surfaces the aspherical correction of the wavefront is done with a null system. If a convex surface has to be tested, such as the surface of a secondary mirror, testing is performed against a concave matrix glass. 

For large mirrors with fast focal ratio, the membrane tool technology has been developed. Polishing is done with a strip-like membrane tool to 

---