Abstract: A method for interferometrically determining error continuously in the radius of curvature of a planetary polishing surface and for providing compensating correction of the surface shape while the polishing machine is in operation. A conditioner glass body disposed on the polishing surface for reshaping the surface is itself a Fizeau cavity having upper and lower partially-reflective surfaces which are parallel or slightly non-parallel for plano polishing and spherical polishing. The conditioner is illuminated by a plano, monochromatic, coherent light wavefront substantially perpendicular to the polishing surface, preferably over the entire upper surface of the conditioner. A portion of the incident wavefront is reflected from the upper surface as a reference wavefront, and another portion of the incident wavefront is reflected from the lower surface as a measurement wavefront.