Interference imaging for aspheric surface testing

Peak-valley accuracy of lambda/20 over a range of 2 lambda is not unusual i n an interferometric null test. For the larger dynamic ranges of a nonnull test, however, the fringe-imaging optics degrades the accuracy. We classify the errors introduced and analyze them in the context of both general and third-order aberration theory. Ne can predict the measurement error fi-om k nown interferometer parameters, and we illustrate this for a single mirror. The errors are tabulated for the specific case of a fourth-order asphere w ith 100 mu m of sag. Ne show that the third-order approximation is comparab le with exact ray-trace results for this case. (C) 2000 Optical Society of America.