Development of soft and hard x-ray optics for astronomy: progress report II and considerations on material properties for large-diameter segmented optics of future missions

In this paper we will review the activities devoted to the development of soft (0.1-10 keV) and hard (10-100 keV) X-ray optics for future astronomical missions that were carried out at the Brera Astronomical Observatory (OAB, Italy) during the last year. Concerning the soft X-ray optics, we are studying the approach based on the use of ceramic carriers for making monolithic Wolter I mirror shells of large diameter by epoxy replication. The ceramic materials investigated in our study are SiC and Alumina (Al2O3), respectively produced by Chemical Vapor Deposition and plasma spray. We fabricated a number of mirror shell prototypes ($PHI equals 60 cm) using carriers based on both materials. X-ray imaging tests performed at the PANTER X-ray facility (Germany) with a full illumination of the optics demonstrated that the mirror shells based on SiC show much better performances than in the case of Alumina. These results can be explained in terms of the thermal-mechanical parameters of the two materials, being in the case of SiC much more performing than for Alumina. Concerning the development of hard X-ray multilayer optics, we are exploring the approach based on Ni electroforming replication. In the last period of activity we in particular concentrated our work on the surface superpolishing methods for the mandrel to be used in the replication process, to be much improved with respect the case Au coated single layer mirrors for soft X-rays. Concerning the specific aspect of the mandrel superpolishing, the results that we obtained can be considered very good and it is possible to claim that we achieved the goal prefixed at the beginning of the development program. The last part of the paper is dedicated to theoretical considerations on large-size and low-weight optics based on segmented mirrors like e.g., those under study for the petals of the XEUS project. In particular, the expected imaging performances by segmented optics produced using different kinds of materials will be compared.