X-Ray scattering from grazing incidence telescopes

For almost 30 years grazing incidence telescopes for astronomical observations have been developed and matured to low scattering performance. A culmination point was reached with the launch of the X-ray astronomy satellite ROSAT, which is in operation since 2 years. ROSAT carries two grazing incidence telescopes covering the XUV (~50 - 750Å) and the soft X-ray (~5 - 120Å) spectral regions, separately. The Wolter type I X-ray telescope consists of 4 nested mirror pairs with a maximum aperture of 835mm and 2.4m focal length, characterized by a half energy width of <4 arcsec and extremely low scattering wings due to the superb mirror surface microroughness of <2.8Å. The point spread function, which has been measured on ground in a 130m long beam test facility prior to launch, will be compared with the performance obtained in orbit. Future grazing incidence X-ray telescopes aim at improved angular resolution, larger collecting area and broader spectral coverage. This is pursued in various projects including SAX, BBXRT, ASTRO-D, Spectrum-X, SOHO-CDS, AXAF and XMM, requiring new fabrication techniques and metrology.