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, only 4 Moliere radii wide, 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.