We report the operation of a new instrument which measures with + 5 nm precision the shape of steep aspheric surfaces. The instrument makes use of contacting probes, and its working is based on an optical differential technique. Special attention is paid to the mechanical construction, and a detailed error analysis is given.

A method of measuring the shape of high numerical aperture (NA <0.95), convex or concave, aspheric surfaces is described. The aspheric slope may be as large as 1200 waves/rad. The method is applied in two steps. First, a standard measurement is performed to obtain a reference surface. Second, the reproducibility of the fabrication of aspheric surfaces is tested by means of a holographic comparison method. The measuring error is smaller than 0.1 microm.