The usefulness of a refractive null corrector is limited by how closely its aberrated wavefront can match the aspheric surface that requires testing. Guidance in both the design of the asphere to insure its nullability and the design of the null corrector is presented. A functional and constructional compendium of null corrector designs is proffered.
Paul L. Ruben, "Refractive null correctors for aspheric surfaces," Appl. Opt. 15, 3080-3083 (1976)