From zone plate to microcalorimeter . 50 years of cosmic X-ray spectroscopy at SRON

The first method used by the SRON Laboratory for Space Research at Utrecht to spectroscopically image the Sun in X-rays employed Fresnel zone plates. Four Fresnel plates, covering four specific wavelengths, were flown on an Aerobee rocket in 1967 and gave a first useful X-ray image of the Sun in the Si-X line at 51 Å. The technique developed for the solar X-ray images enabled SRON to become the Lead Investigator for the grating spectrographs on several major X-ray satellites, i.e. on the Einstein and EXOSAT satellites, launched in November 1978 and May 1983 respectively, and on the Chandra and XMM-Newton observatories both launched in 1999. Since then, a considerable effort was put into the development of cryogenically cooled, non-dispersive X-ray spectrometers as model payload elements for the XEUS, IXO and Athena mission studies. This paper briefly reviews these developments, highlights some of the resulting scientific insights and offers a few thoughts on the present outlook for a next generation X-ray observatory. The biggest challenge for the realization of such a mission is not primarily technical: global coordination and collaboration, both among scientists and the major space agencies, is a prerequisite for a successful next major leap in this discipline.