A technique is described for obtaining interferometric data on an optical system without modification of the system, and using the detector intrinsic to the system. A pair of coherent point sources is imaged by the system and examined in an out-of-focus position. The period of the resulting fringe pattern is used to determine the focus position unambiguously. The structure of the fringe pattern indicates other system errors. An experiment to verify the practicality of the technique is described.
R. R. Shannon and C. Rodriguez-Torres, "Paired Source Testing of Optical Systems," Appl. Opt. 10, 264-269 (1971)