Carbon-based Fresnel optics for hard X-ray astronomy

We investigate the potential of large-scale diffractive-refractive normal-incidence transmission lenses for the development of space-based hard X-ray telescopes with an angular resolution in the range of micro- to milli-arcseconds over a field of view which is restricted by the available detector size. Coherently stepped achromatic lenses with diameters up to 5 m for compact apertures and 13 m in the case of segmentation provide an access to spectrally resolved imaging within keV-wide bands around the design energy between 10 keV and 30 keV. Within an integration time of 10⁶ s, a photon-limited 5σ sensitivity down to 1/10⁹ -1/10⁷ per (s cm² keV) can be achieved, depending on the specific design. An appropriate fabrication strategy, feasible with nowadays micro-optical technologies, is considered and relies on the availability of high-purity Carbon or polymer membranes. X-ray fluorescence measurements of various commercially available Carbon-based materials prove for most of them a virtually negligible contamination by critical trace elements such as transition metals on the ppm level.