First experience with the ATLAS muon spectrometer

The ATLAS experiment at the Large Hadron Collider (LHC) at CERN is currently being assembled to be ready to take first data in fall 2007. Its muon spectrometer is designed to achieve a momentum resolution of better than 10% up to transverse muon momenta of 1 TeV. The spectrometer consists of one barrel and two endcap superconducting air-core toroid magnets instrumented with three layers of precision drift chambers as tracking detectors and with a dedicated trigger system. We report on our experience with the commissioning and installation of the precision and trigger chambers. First results of the cosmic ray test of the barrel muon spectrometer with magnetic field are presented, including results of the calibration of the drift chambers and of the spectrometer alignment.