The FoCal prototype - an extremely fine-grained electromagnetic calorimeter using CMOS pixel sensors

A prototype of a Si-W EM calorimeter was built with Monolithic Active Pixel Sensors as the active elements. With a pixelsize of 30 $\mu$m it allows digital calorimetry, i.e. the particles' energy is determined by counting pixels, not by measuring the energy deposited. Although of modest size, with a width of only four Moliere radii, it has 39 million pixels. We describe the construction and tuning of the prototype and present results from beam tests and compare them with predictions of GEANT-based Monte Carlo simulations. We show the shape of showers caused by electrons in unprecedented detail. Results for energy and position resolution will also be given.