Grazing incidence x-ray optics: ROSAT and future telescopes

Abstract

On 1 June 1990 the x-ray astronomy satellite ROSAT was launched into a near-earth orbit and has been in operation since then. ROSAT carries two grazing incidence telescopes covering the XUV (≈ 50-750 Å) and the soft x-ray (≈ 50-750 Å) spectral regions. The Wolter type I x-ray telescope consists of four nested mirror pairs with a 835-mm maximum aperture and 2.4-m focal length, characterized by a half energy width of <4 arcsec and extremely low scattering wings due to the mirror surface microroughness of <2.8 Å. By now, ROSAT has finished the first all-sky survey performed with an imaging x-ray telescope and >60,000 new cosmic x-ray sources were discovered. Since 7 February 1991, pointed observations of preselected targets are carried out in an international guest-observer program. X-ray images have been obtained at an unprecedented level of angular resolution, contrast, and background noise. 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. Development and status of SOHO-CDS, which is a Wolter type II solar EUV telescope of 2-arcsec resolution, and XMM, which consists of three telescopes of fifty-eight-nested light-weight mirror pairs for 0.1-10 keV, are presented.