The X-ray telescope of the CAST experiment

The CERN Axion Solar Telescope (CAST) searches for solar axions employing a 9 tesla superconducting dipole magnet equipped with 3 independent detection systems for X-rays from axion-photon conversions inside the 10 m long magnetic field. Results of the first 6 months of data taking in 2003 imply a 95% CL upper limit on the axion-photon coupling constant of gagamma < 1.16times10-10 GeV-1 for axion masses ma < 0.02 eV. The most sensitive detector of CAST is an X-ray telescope consisting of a Wolter I type mirror system and a fully depleted pn-CCD as focal plane detector. Exploiting the full potential of background suppression by focussing X-rays emerging from the magnet bore, the axion sensitivity obtained with telescope data taken in 2004, for the first time in a controlled laboratory experiment, will supersede axion constraints derived from stellar energy loss arguments