Absolute testing of spherical optics

Interferometric testing of tight tolerance optics commonly requires correction for the errors introduced into the system by the reference surface (commonly referred to as a 'transmission sphere' in commercially available Fizeau phase measuring interferometers (PMIs)). The process of subtracting the reference surface errors from the measured wavefront is commonly, if somewhat misleadingly, referred to as 'absolute testing'. Absolute testing of flats was discussed at the 1992 Optical fabrication and Testing workshop. Here we consider spherical optics.