Mid-spatial frequency errors of mass-produced aspheres

For the (CNC) polishing of aspheres, generally a compliant, sub-aperture tool is applied, which may cause mid- spatial frequency errors on the surface of the workpiece. The tolerance on surface figure is commonly given in peak-to-valley (PV) or root-mean-square (RMS). Even if a surface is fabricated within specified tolerances according to one of the mentioned metrics, the optical performance may be inadequate for the desired application. For the specification of the tolerance on mid-spatial frequency errors, several other characteristics have been proposed, e.g. power spectral density (PSD) or surface slope error. This paper presents an investigation into the mid-spatial frequency form error of mass-produced aspheres, discusses the results and draws relevant conclusions.