Development of broadband reflection coatings for future X-ray telescopes

In contrast to visible light astronomy, space based X-ray telescopes use mirrors with reflecting surfaces oriented almost parallel to the incoming radiation. To obtain broadband X-ray reflection, bilayer systems with a thin, low-Z overcoating (like carbon) on top of usual high-Z materials (gold, iridium or platinum) have been proposed more than ten years ago to enhance the reflectivity at low X-ray energies (between 0.5 and 4 keV). The trick here is to use the high reflection from low density material at low energies, while photons at higher energies are transmitted by this material and then reflected by the high density material below. For several future projects, for example ATHENA and eXTP, the use of low density overcoatings is considered to increase the effective telescope area. In this poster we will discuss the development of new overcoating material combinations different from the ones considered so far, in particular based on a thin layer of chromium followed by another layer of a carbon-based material, and also novel approaches for their deposition.