On the design of wide-field x-ray telescopes
X-ray telescopes having a relatively wide field-of-view and spatial resolution vs. polar off-axis angle curves much flatter than the parabolic dependence characteristic of Wolter I designs are of great interest for surveys of the X-ray sky and potentially for study of the Sun's X-ray emission. We discuss the various considerations affecting the design of such telescopes, including the possible use of polynomial mirror surface prescriptions, a method of optimizing the polynomial coefficients, scaling laws for mirror segment length vs. intersection radius, the loss of on-axis spatial resolution, and the positioning of focal plane detectors.
Mathematical formalism for designing wide-field x-ray telescopes: mirror nodal positions and detector tilts
We describe a mathematical formalism for determining the mirror shell nodal positions and detector tilts that optimize the spatial resolution averaged over a field-of-view for a nested x-ray telescope, assuming known mirror segment surface prescriptions and known detector focal surface. The results are expressed in terms of ensemble averages over variable combinations of the ray positions and wavevectors in the flat focal plane intersecting the optical axis at the nominal on-axis focus, which can be determined by Monte-Carlo ray traces of the individual mirror shells. This work is part of our continuing efforts to provide analytical tools to aid in the design process for wide-field survey x-ray astronomy missions.
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 mission. 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 over a field of view of about 1 degree. The WFXT optics consist of 25 nested shells. In order to have both a large effective area at low energies and a meaningful area at higher energies, 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 material like Silicon Carbide 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 rest of the x-ray measurements of the prototypes of the outermost mirror shell manufactured with nickel and SiC.