A detector system has been designed to meet the specifications of the focal plane detector for the double focusing mass spectrometer (DFMS) which is part of the ROSINA experiment to be flown on board the ESA Rosetta orbiter. This spacecraft will be launched in January 2003 and aims at a rendezvous with comet Wirtanen in 2011. In the first section, the specifications of the detector system are defined as a function of the desired measurements and instrument characteristics. The main component is an imaging detector, the design and rationale of which are described in the second section. The main characteristics are a very high resolution of 25μm per pixel and a large dynamical range of about 11 decades. In the last section, initial results from laboratory tests of a prototype are provided together with an overview of the actual performances. In conclusion, work anticipated in the future and possible improvements are summarized.