Cryogenic Tests of a 0.1 K Dilution Cooler for Planck-HFI

Planck is the third medium-size scientific mission (M3) of the ESA HORIZON 2000 program and is to be launched in 2007. The bolometer-based Planck/HFI (High Frequency Instrument) is cooled down to 100 mK in order to measure the Cosmic Microwave Background (CMB) radiation with sensitivity close to the fundamental photon noise limit. We report the critical design drivers for the open-cycle dilution system and the solutions adopted.