Null lenses, for use with rotationally symmetric but nonspherical surfaces, are extremely useful for testing during production. In general, each different system to be tested requires a different null lens. A variable null lens is proposed which consists of two aspheric plates, each of which introduces approximately fourth- or sixth-order spherical aberration. When the plates are inserted in a diverging beam, the effect on the wavefront may be changed by varying the plate spacing. The system may be used to test parabolas and other nonspherical mirrors at center of curvature, aspheric plates in conjunction with spherical mirrors, and various other systems. Once the two plates have been produced and measured, the spacing of the plates used may be determined for each test. This would normally be done by computer, using any of the optimization techniques for lens design. Examples are shown, and an estimate of obtainable accuracy is given.
R. S. Hilbert and M. P. Rimmer, "A Variable Refractive Null Lens," Appl. Opt. 9, 849-852 (1970)