New Polishing Techniques for Optical Components

Over the past years a number of new techniques for polishing high-performance optical surfaces has been established. A characteristic feature of these is the use of small area tools, allowing highly aspheric or freeform surfaces to be polished, in contrast to the classic techniques for polishing large areas that are usually applied to spherical surfaces.

But the use of small area tools in general also brings about an increased susceptibility to so-called mid-spatial frequency (MSF) errors.

In the past, MSF errors typically have not been addressed by specific requirements but a new activity with TDE and Media Lario, Italy has assessed the performance of these recently established polishing techniques when used to manufacture light-weigh non-spherical mirrors for high performance optical instruments, such as telescopes or spectrometers. They were assessed in terms of optical performance and polishing speed, with an emphasis on the control of MSF errors. Two circular aspheric mirrors (diameter 250 mm) of different materials (aluminium (AlSi40 with NiP coating), Zerodur) have been manufactured and polished. The mirrors are light-weighted with identical mechanical geometry. The polishing technique applied was bonnet polishing.

The activity was able to improve our understanding of the performance of bonnet polishing as a single polishing technique applied to aspheric, light-weight, very low surface error mirrors made of Ni-plated aluminium and Zerodur, representing typical components of high-performance optical space instruments.

T116-402MM closed in 2021