Improved Combined Diffractive Optical Elements for quasi-absolute testing of Aspherics

Computer-generated diffractive optical elements have widely been in use for testing of aspherics in different interferometric setups. We present a three position quasi-absolute test for aspherics using combined-diffractive optical elements (Combo-DOEs). The calibration procedure works similar to the sphericity test by using three positions, while the Combo-DOE carries the information for the aspheric surface as well as the best fit sphere with a slight linear offset. Further, we discuss the effects of DOE's substrate quality on the accuracy of the proposed calibration procedure and present an improved design of the Combo-DOE. The results obtained with the quasi-absolute three position procedure are compared to an independent procedure, an N-position test.