Photometric masking methods and predicted performances for the CoRoT exoplanetary mission

CoRoT mission for year 2006 is a small space telescope that will measure continuously for 6 months the light flux of 12000 star in a mission of 2.5 years . The aim is to detect small droops in the light curves revealing planets transitting in front of their star. For this, 12000 logical Regions Of Interest (ROI) are defined on the CCD to optimise each star Signal to Noise Ratio (s/n). Unfortunatly only less than 256 different shapes are permitted for all ROIs, forseeing a loss in global S/N. We found a method wich reduce the 12000 ROIs to a small set of 250 shapes in a lossless way. Overall perverformances are discussed.