Avoidance and elimination of errors in optical scanning

For the testing of flats, spheres or aspheres, the optical scanning technique has become a competitor to interferometric techniques. The set-up implemented at PTB for the ultra-precise optical scanning of optical flats will be presented and the underlying principles explained, with a special view to the avoidance of error influences in the optical set-up and to the elimination of the remaining errors. The basic principle of the scanning set-up is that an autocollimation telescope combined with a movable pentagon prism is used to register the surface slope angles. To avoid higher-order errors originating in the pentagon prism, its angular position is kept constant in space by active stabilization. Another advantageous principle is that difference measurements for slopes are performed. This has become possible thanks to a new error-free difference method recently developed by PTB. The combination of these principles eliminates, theoretically, the influences of all possible errors of the scanning facility. An experiment using a high-quality flat (lambda/60) 140 mm in diameter showed a reproducibility of 0,2 nm over one week.