Ballistische Messkammer with 10k CCD's (BMK10k) central goal is supporting the upcoming PLAnetary Transits and Oscillation of stars (PLATO) mission by reducing its number of false positives (planet-compatible transit candidates but not planets at the end). For Kepler, the number of false positives had been estimated to still be about 10%. Understanding the PLATO false alarm rate is not as straightforward, and preparatory data from the ground will dramatically improve the target selection and minimize the number of false positives. We aim to identify the target contamination for every PLATO target and initially provide well-sampled light curves of all detectable contaminants in the long-duration southern PLATO field. Thereby, it will be possible to identify background eclipsing binaries and rotating spotted stars with periods between 1 hr and 100 days down to 17-18th mag. This will be achieved with a dedicated robotic telescope with one of the world's largest CCDs with the smallest pixels, enabling a field of view of 52.5 square degrees with a sampling of 2.5″/pix. Auxiliary science is also foreseen and will focus on the period-mass relation of Ptolemy's cluster (M7). In this paper, we describe the entire telescope system and show initial commissioning data.