XMM-Newton in-orbit calibration

Two years after launch of the XMM-Newton Observatory saw a gathering of about 350 scientists at ESTEC for the Conference 'New Visions of the X-ray Universe'. This huge interest in the mission and the rapidly increasing number of scientific papers published as a result of XMM-Newton observations show the importance of ESA's latest observatory for astrophysics in the 21st century. A vital part of the scientific interpretation that enables this work is the accuracy and reliability of the instrument calibration. To highlight this feature, a session at the Conference was devoted to a Calibration Workshop, allowing the instrument teams to explain the details of the improving knowledge and remaining limitations. This article reflects some of the presentations made in that Workshop, and reviews the general in-orbit calibration activities, explaining some of the complexities involved.