IXO glass mirrors development in Europe

The mirrors of the International X-ray Observatory (IXO) were based on of a large number of high quality segments, aiming at achieving a global spatial resolution better than 5 arcsec (HEW). A study concerning the slumping of thin glass foils for the IXO mirrors is under development in Europe, funded by ESA and led by the Brera Observatory and is continuing even after that the programhas been descoped, in the perspective of using the technology under development for other future missions. After a preliminary trade-off study, we have focused our the effort on the "Direct" slumping approach, based on the use of convex moulds. In this case during the thermal cycle the optical surface of the glass is in direct contact with the mould surface. The thin plates are made of thin glass sheets (0.4 mm thick), with a reflecting area of 200 mm × 200 mm. The adopted integration process foresees the use of reinforcing ribs for bonding together the plates and forming in that way a rigid and stiff stack of segmented mirror shells; the stack is supported by a thick backplane. During the bonding process the plates are constrained to stay in close contact with the surface of the master (i.e. the same mould used for the hot slumping process) by the application of vacuum pump suction. In this way the spring-back deformations and low frequency errors still present on the foil profile after slumping can be corrected. In this paper we will give an overview and a status report of the project.