The X-ray telescopes for the ESA XMM spacecraft

The high Throughput X-ray Spectroscopy Mission XMM is a "Cornerstone" Project in the ESA long term Programme for Space Science. This observatory, due to be launched in 1999, has at its heart three heavily nested Welter 1 grazing incidence X-ray telescopes which will provide a large collecting area (1420 cm(2) each at 1.5 keV and 600 cm(2) at 8 keV). This optical system has a spatial resolution of 16 arcsec and when coupled with reflection grating spectrometers and X-ray CCD cameras, it will provide a major advance in astrophysics by the end of the century. Beginning of this year, all the four Flight Model telescopes (3 for Flight and one spare) have been delivered, after two years of intense production of the X-ray mirrors and after a challenging machining of the X-ray baffle. The measurements carried out at the Centre Spatial de Liege (Belgium) and at the Max Planck Institute (Germany) indicate X-ray performance significantly better than the specifications. This paper first provides a concise description of the design of the telescope with the emphasis on the X-ray miners and the Xray baffles. We then analyze and compare the X-ray and optical test results achieved on the four Flight Models of the XMM telescope. In particular, we address two important issues: the determination of the focal length of the telescopes and the improvement of the scientific mission by the implementation of an X-ray baffle. Finally, based on the lessons learnt from XMM experience, the prospective on the next generation of ultra thin X-ray mirrors is also presented.