The Common Muon and Proton Apparatus for Structure and Spectroscopy (COMPASS) is a new fixed target experiment at the CERN (Conseil Européen pour la Recherche Nucléaire) Super Proton Synchrotron (SPS) with the aim to study hadron structure with polarized muon beams and hadron spectroscopy with hadron beams.

The main physics objective of the muon beam physics program is the measurement of the gluon polarization in a longitudinally polarized nucleon. The hadronic program comprises a search for glue balls in the high mass region in exclusive diffractive pion proton scattering, a study of leptonic and semi leptonic decays of charmed hadrons with high statistics and precision and Primakoff scattering with various probes. A detailed investigation of charmed and doubly charmed baryons is performed in a second stage of the experiment.

The setup consists of two spectrometers, one for small angle and one for large angle particles, giving a wide angular acceptance for all measurements. Each spectrometer performs full particle identification using one Ring Imaging Cherenkov Counter (RICH), electromagnetic and hadronic calorimetry and muon detection. A high momentum resolution is obtained by using highly precise tracking with silicon detectors, gaseous strip detectors and drift tubes. The measurements are performed with high intensity beams allowing to collect the needed statistics.

The key-detector for particle identification is the RICH-1, which is a technological challenge in terms of photon detection and radiator gas supply. The COMPASS setup is presented with special focus on the radiator gas preparation and operational aspects of the gas system. Some first results on D meson analysis are presented.