Fused silica segments: a possible solution for x-ray telescopes with very high angular resolution like Lynx/XRS

Abstract

In order to look beyond Chandra, the Lynx/XRS mission has been proposed in USA and is currently studied by NASA. The optic will have an effective area of 2.5 m<SUP>2</SUP> and an angular resolution of 0.5 arcsec HEW at 1 keV. In order to fulfill these requirements different technologies are considered, with the approaches of both full and segmented shells (that, possibly, can be also combined together). Concerning the production of segmented mirrors, a variety of thin substrates (glass, metal, silicon) are envisaged, that can be produced using both direct polishing or replication methods. Innovative post-fabrication correction methods (such as piezoelectric or magneto-restrictive film actuators on the back surface, differential deposition, ion implantation) are being also considered in order to reach the final tolerances. In this paper we are presenting a technology development based on fused silica (SiO<SUB>2</SUB>) segmented substrates, owing the low coefficient of thermal expansion of Fused Silica and its high chemical stability compared to other glasses. Thin SiO<SUB>2</SUB> segmented substrates (typically 2 mm thick) are figured by direct polishing combined with final profile ion figuring correction, while the roughness reduction is reached with pitch tools. For the profile and roughness correction, the segments are glued to a substrate. In this paper we present the current status of this technology.