Moli\`ere radius measurement using a compact prototype of LumiCal in a test set-up

The FCAL collaboration has performed a design study for luminometers at future electronpositron colliders. Compact sampling calorimeters with precisely positioned silicon sensors and a fast readout will reach the necessary performance even in the presence of background from beamstrahlung and two-photon processes. A prototype calorimeter has been built with special focus on ultra-thin fully instrumented sensor planes to ensure a very small effective Moli`ere radius. Preliminary results of measurements in a 5 GeV electron beam are presented.