Sub-nm topography measurement using high-accuracy autocollimators

In the first part it is shown that high resolution electronic autocollimators can be calibrated using an angle comparator on base of the divided circle to an uncertainty of less than 0,01 arcsec (50 nrad). They are thus directly traceable to the SI unit radian of the plane angle. An application of highly accurate angle measurement is presented in the second part, namely, a scanning device for the ultra-precise measurement of the topography of near-plane optical surfaces. A reproducibility of 0,1 nm rms is achieved. The method is based on the measurement of differences of reflection angles and does not require an external form reference as it uses the propagation of light as a straightness reference, The topography is derived from measured quantities that are traceable to the SI units angle and length.