GridPix as a candidate for the future of CAST

A central part of the CAST upgrade program is the further reduction of the background rates and the extension of the experiment's sensitivity to lower energies. Both aims could be reached with a highly pixelized readout of the detector as, for example, with an InGrid, the combination of a Timepix ASIC and a Micromegas grid. The high granularity and efficiency of such a device make it possible to detect and to distinguish single primary electrons. Therefore, we have studied the energy resolution by electron counting and the separation of photon events from tracks with an event shape analysis.