Distance-dependent autocollimator errors: Experimental characterisation and ray trace modelling

Autocollimators are versatile optical devices for the contactless measurement of the inclination angles of reflecting surfaces. The distance between the autocollimator and the reflecting surface influences its angle response substantially. This is especially the case for autocollimators which are used in deflectometric profilometers for the precision form measurement of extended optical surfaces, such as beam-shaping synchrotron optics and FEL optics. At PTB, measurements have been performed to investigate the distance-dependent angle errors of an autocollimator experimentally. Additionally, a ray-tracing model of the autocollimator has been created in ZEMAX. In this contribution, we introduce the measurement set-up and the ray tracing model. We present the measurement results for various distances between 300 mm and 1800 mm and compare them to the results of the ray-tracing.