Future investigations of astronomical X-ray sources require light weight telescope systems with large collecting areas and good angular resolution. The Wolter I type telescope design offers a suitable possibility for obtaining performant X-ray mirrors with high collecting areas. The technology based on replicated slumped glass optics using thin glasses thereby provides the opportunity to fulfil the light weight and mass production requirements. In NASA's telescope NuSTAR this technology has been proven as advantageous compared to previous systems. Coating thin glasses with iridium, gold or platinum enhances the reflectivity of X-ray mirrors.
Large X-ray telescopes for future observatories need to combine a big collecting area, meaning thin mirrors with large diameter, with good angular resolution. Structures have to be stiff enough to guarantee the correct profiles and positioning of such mirrors. Due to the mass limits of the launching rockets, lightweight materials and configurations are required.. The Slumped Glass Optic (SGO) group of the Max-Planck-Institute for Extraterrestrial physics (MPE) is developing the indirect slumping technology to comply with this need. This technique foresees the shaping at high temperature of thin glass foils, originally flat, to Wolter I design X-ray mirror segments, by using suitable moulds. During the thermal cycle inside an electrical oven the glass viscosity is such reduced that it allows its bending onto the mould. So the mould’s shape is replicated while still maintaining the original micro-roughness of the glass on the non-contact side that is of fundamental importance for X-ray reflections. This replication process is particularly suitable for the manufacturing of several identical optical elements, which must successively be coated with the necessary reflective layer and then aligned and integrated into supporting structures. Numerous aspects of the technology have been studied in the past, such as the selection of mould and glass materials, and the corresponding optimization of the thermal cycle parameters. During the last year, we focused on different process set-ups. The current results and status of activities will be presented in the paper.
The Slumped Glass Optic (SGO) group of the Max Planck Institute for Extraterrestrial physics (MPE) is studying the indirect slumping technology for its application to X-ray telescope manufacturing. Several aspects of the technology have been analyzed in the past. During the last months, we concentrated our activities on the slumping of Schott D263 glass on a precise machined Fused Silica mould: The concave mould was produced by the Italian company Media Lario Technologies with the parabola and hyperbola side of the typical Wolter I design in one single piece. Its shape quality was estimated by optical metrology to be around 6 arcsec Half Energy Width (HEW) in double reflection. The application of an anti-sticking Boron Nitride layer was necessary to avoid the adhesion of the glass on the mould during the forming process at high temperatures. The mould has been used for the slumping of seven mirror segments 200 mm long, 100 mm wide, and with thickness of 200 μm or 400 μm. The influence of the holding time at maximum temperature was explored in this first run of tests. The current results of the activities are described in the paper and plans for further investigations are outlined.
Previously used mirror technologies are not suitable for the challenging needs of future X-ray telescopes. This is why the required high precision mirror manufacturing triggers new technical developments around the world. Some aspects of X-ray mirrors production are studied within the interdisciplinary project INTRAAST, a German acronym for "industry transfer of astronomical mirror technologies". The project is embedded in a cooperation of Aschaffenburg University of Applied Sciences and the Max-Planck-Institute for extraterrestrial Physics. One important task is the development of low-stress Iridium coatings for X-ray mirrors based on slumped thin glass substrates. The surface figure of the glass substrates is measured before and after the coating process by optical methods. Correlating the surface shape deformation to the parameters of coating deposition, here especially to the Argon sputtering pressure, allows for an optimization of the process. The sputtering parameters also have an influence on the coating layer density and on the micro-roughness of the coatings, influencing their X-ray reflection properties. Unfortunately the optimum coating process parameters seem to be contrarious: low Argon pressure resulted in better micro-roughness and higher density, whereas higher pressure leads to lower coating stress. Therefore additional measures like intermediate coating layers and temperature treatment will be considered for further optimization. The technical approach for the low-stress Iridium coating development, the experimental equipment, and the obtained first experimental results are presented within this paper.