Recent developments of precessions polishing for larger components and free-form surfaces

Since the 2003 Annual Meeting, the Precessions process has become accepted as an efficient method
for polishing and figuring moderate-sized axially-symmetric aspheric parts in industry. In this paper,
we report on some very significant new advances beyond this capability. The first is the demonstration
of the process on substantially larger diameter parts than worked hitherto – in particular, a precisionground
500mm diameter deeply-concave aspheric mirror. We describe the consequences of polishing
large parts with the axis of the part vertical, in contrast to the horizontal axis of the smaller machines.
Issues include slurry puddling and settlement in concave forms, process-uniformity, adequate support
of the part and handling. We then report on recent work developing the Precessions process for non
axially-symmetric surfaces including free-form. The correct relationship of the process with metrology
has proved to be complex on several fronts, one example being differing descriptions of form either
along a surface or its projection. We present our experience using profilometry and interferometry on
precision-ground and polished surfaces, and in achieving absolute form with known base radius.
Finally, we remark on the potential power of a priori predictions of achievable surface quality when
optimizing optical system designs.