The GREGOR polarimetric calibration unit

The new Solar telescope GREGOR is designed to observe small-scale dynamic magnetic structures below a size of 70 km on the Sun with high spectral resolution and polarimetric accuracy. For this purpose, the polarimetric concept of GREGOR is based on a combination of post-focus polarimeters with pre-focus equipment for high precision calibration. The Leibniz-Institute for Astrophysics Potsdam developed the GREGOR calibration unit which is an integral part of the telescope. We give an overview of the function and design of the calibration unit and present the results of extensive testing series done in the Solar Observatory ``Einsteinturm'' and at GREGOR.