Innovative nano-accuracy surface profiler for sub-50 nrad rms mirror test

Abstract

In order to promote the measurement accuracy to sub-50 nrad rms for both spherical and plane mirror tests and to resolve the difficulty in testing strongly curved mirror by use of the NOM, the Nano-accuracy Surface Profiler (NSP) was developed. The NSP system applies scanning sampling beam in fixed scanning distance combined with non-tilted reference beam for removing pitch error of scanning slide to ensure sub-50 nrad rms accuracy measurement. The NSP takes the advantage of the NOM in using precise autocollimator (Elcomat 3000) for both sampling and reference beams. In preliminary test, this NSP scheme verifies it has at least the same measurement accuracy as the NOM for plane mirror test. The NSP system scheme significantly reduces test error than the NOM for sphere test, and it also ensures the capability using single calibration curve to further correct systematic error in reaching sub-50 nrad rms accuracy for spherical mirror test.