New technologies for polishing and testing large optics.

Abstract

Two new technologies, for polishing and testing large optics, are presented in this dissertation. The first is a new approach to generating and polishing aspheric surfaces which utilizes a full size stressed lap. The lap specifications are chosen to automatically generate a specific amount of spherical aberration. The required comatic distortion of the lap is induced by a system of levers and springs which are stretched and relaxed as the lap moves. A lap was constructed to grind and polish a 20cm glass blank. The resultant polished surface, in agreement with the predicted asphericity, has 28 microns of spherical aberration at its edge, appropriate for a F/2.0 convex paraboloid. The average radial profile has a residual peak-to-valley error of 200nm and an rms error of 60nm. This experiment serves as a first successful test of the stressed lap concept and as a demonstration of a new method for generating aspheric secondary mirrors. The second new technology is concerned with testing large optics where vibrations can be a serious problem. 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. An analysis of potential phase errors was performed for this "2 + 1" algorithm. In the developed implementation, two frequencies, dν/ν≈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 1ms 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.