Recent advances in the control of form and texture on free-form surfaces (invited paper)

The recent upsurge in the demand for off-axis and complex "freeform" optical surfaces is driving the development of novel processes for their fabrication. This paper focuses on recent developments of the Precessions CNC polishing process for freeform surfaces, including off-axis as a special case. First, the surface-prescription and metrology-data, and their relation to the data-input for the polishing machines, are considered. The relevance of consistent coordinate frames is emphasised. An outline of how the process can 'polish' a ground freeform part (improve the texture), and then 'figure' the part (reduce the form errors) is given. Specific experimental case-studies are then presented, illustrating the versatility of the process on different materials and forms. Recent work is included in which the process-speed has been moderated in order to remove tens of nanometres of stock material, rather then the more usual hundreds of nanometres to tens of microns as in the standard Precessions process. The relevance of this to improving the ultimate surface-precision that should be achievable by this method is described. As a final illustration, the potential of the process to the rapid fabrication of the hundreds to thousands of 1-2 metre class mirror segments required for extremely large telescopes is considered.