System For Phase Shifting Interferometry In The Presence Of Vibration

Very accurate and detailed testing of small and moderate size optical surfaces can be realized by phase shifting interferometry. However, for the very large primaries needed for the next generation of telescopes, it is difficult and expensive to keep vibrations low enough for this method to work. A modification to the usual phase shifting interferometry reduction algorithm permits measurements to be taken fast enough to essentially freeze out vibrations. Only two interferograms are needed with an exact phase relationship; and these can be recorded very rapidly on either side of the interline transfer of a standard CCD video camera, prior to charge transfer readout. The third required interferogram is a null. In the developed implementation, two frequencies, dv/v.10-8, are generated with orthogonal polarizations. A Pockels cell rapidly switches the frequency entering the interferometer, resulting in a phase shift over the long path difference of the interferometer. The two time critical interferograms are acquired with a lms separation resulting in a reduction in sensitivity to vibration of one to two orders of magnitude. Laboratory tests were performed to compare this "2 + 1" system with a commercial phase shifting package. Similar phase determination accuracies were found when vibrations were low. However, the "2 + 1" system also succeeded when vibrations were large enough to wash out video rate fringes.© (1989) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.