X-ray optics for the WFXT telescope

Wide Field X-ray Telescope (WFXT) is the core instrument of the Wide Angle X-ray Survey (WAXS) mission, which is aimed at conducting a high angular resolution, high sensitivity X-ray survey over a large solid angle of the sky. The project has been developed as a feasibility study in the frame of the Agenzia Spaziale Italiana (ASI) program for small-medium satellite missions. WFXT uses grazing incidence optics based on a new design where the Wolter I profile is substituted by a five term polynomial profile, The coefficients of the polynomium are optimized to obtain high spatial resolution (< 15 arcsec half energy width) over a,field of view (FOV) of about I degree. The WFXT optics consist of 25 nested shells. In order to have both a large effective area at low energies (similar to 360 cm(2) at 1.5 keV) and a meaningful area at higher energies (similar to 85 cm(2) at 4.5 keV), a design consisting of 9 large mirror shells and 16 smaller shells, contributing mainly at higher energies, has been developed. The outermost and innermost mirror shells have a diameter of 600 and 226 mm, respectively. The total length of the mirror shells is 120+120 mm, while the focal length of the optical system is 3000 mm. For the WFXT optics, in addition to the well proved manufacturing process by nickel electroforming, we considered a novel replication technique for the manufacture of the mirrors which make use of ceramic materials like Silicon Carbide (SiC) in order to meet the stringent requirements of high spatial resolution and low weight. In this paper we give the details of the optical design and report the results of the X-ray measurements of the prototypes of the outermost mirror shell manufactured with nicker and SiC.