Testing of optical systems by the use of two wires situated in defocused positions gives qualitative and quantitative data on spherical aberration. The test device allows two knife edge tests to be made in addition to the double wire test. These three correlated tests minimize errors in interpretation.
A. S. DeVany, "Spherical Aberration Analysis by Double Wire Testing," Appl. Opt. 6, 1073-1075 (1967)