Optical surfacing process optimization using parametric smoothing model for mid-to-high spatial frequency error control

High performance optical systems aiming for very low background noise from scattering or a sharp point spread function with high encircled energy often specify their beam wavefront quality in terms of a structure function or power spectral density function, which requires a control of mid-to-high spatial frequency surface errors during the optics manufacturing process. Especially for fabrication of large aspheric optics, achieving the required surface figure irregularities over the mid-to-high spatial frequency range becomes a challenging task as the polishing lap needs to be compliant enough to conform to the varying local surface shapes under the lap. This compliance degrades the lap's smoothing capability, which relies on its rigidity. The smoothing effect corrects the mid-to-high spatial frequency errors as a polishing lap removes low spatial frequency (i.e. larger than the lap size) errors on the optical surface. Using a parametric smoothing model developed to quantitatively describe the smoothing effects during Computer Controlled Optical Surfacing (CCOS) processes, actual CCOS data from large aspheric optics fabrication projects have been analyzed and studied. The measured surface error maps were processed with the model to compare different polishing runs using various polishing parameters. The results showing the smoothing effects of mid-to-high spatial frequency surface irregularity will be presented to provide some insights for a CCOS process optimization in terms of smoothing efficiency.