Null lens design techniques.
There are a surprising number of discrete, well-corrected null lens designs with just two elements for testing a large, fast-speed parabolic mirror from its center of curvature. Some design techniques are discussed for generating these solutions, which may be applicable to the design of null lenses for other mirror shapes. Without examining these multiple solutions, which differ greatly in performance, there is no way to know if the optimum configuration has been found.