Manufacturing processes for free-form optics

Off-axis and “freeform ” optical surfaces are in increasing demand. Such surfaces introduce additional degrees of freedom into the optical design optimisation, leading to simpler systems with fewer components and better performance. Key applications demand off-axis aspheres, the segmented astronomical telescope being one. The next generation of extremely large telescopes in the 30-40m aperture range will require many hundreds of hexagonal aspheric segments, each in the 1 to 2m size-range. Such ambitious applications, together with smaller parts for the defence and other industries, are driving the development of novel fabrication processes. This paper focuses on the Precessions CNC polishing and grolishing process for off-axis and freeform surfaces. It is hoped that the new techniques we describe will stimulate optical designers to be more ambitious in the designs they originate. We fist review the way surfaces are described and the relationship to metrology, and to the data-input for the polishing machines. 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. We also describe how the process has been moderated to remove tens of nanometres of stock material, rather then the more usual hundreds of nanometres and upwards 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. 1