Thin glass shell oriented to wide field x-ray telescope

Abstract

ABSTRACT The next generation wide-field X-ray telescope (WFXT), to be implemented beyond eRosita and proposed within the NASA RFI call 2011, requires an angular resolution of less than 10 arcsec (with goal of 5” ) constant across a wide field of view (1 deg 2 ). To achieve this requirement the design is based on nested modified grazing incidence Wolter-I mirrors with polynomial profiles. Our goals in te rms of mass and stiffness can be meet with the use of fused silica glass, a well-known material with good thermo-mechanical properties and polishability characteristics, together with an innovative polishing approach. Here we present the X-ray calibration results obtained for a prototypal shell tested in full-illumination mode at the Panter/MPE facility. Keywords: X-ray Telescopes, Wide Field X-ray missions, X-ray calibrations, X-Ray shells, Deterministic polishing 1. INTRODUCTION The Wide Field X-ray Telescope (WFXT) is a proposed medium-class mission dedicated to survey the sky in the soft X-ray band (0.2–7 keV; Murray et al. 2008). It foresees a X-ra y telescope assembly whose main requirement is to be orders of magnitude more effective than previous and planned X-ray missions in carrying out surveys. This is obtained through a telescope design that makes use of polynomial proÞ les for the mirror shells, focal plane curvature and plate scale corrections (Burrows et al. 1992, Conconi et al. 2001, Conconi et al. 2010). In or der to reach the effe ctive area within the allocated mass, the shells need to be very thin, with wall thickness of few mm. Moreover, th e shells are characterized by a very small Length-to-Diameter ratio (L/D), e.g. three times smaller than for XMM-Newton mirrors, making their fabrication more difficult. In the last years, a research activ ity has been carried out at the Osservatorio Astronomico di Brera (OAB) with the aim of defining a process for the realization of these glass shells with the direct polishing technique. Given that fused silica thermo-mechanical (T/M) properties (in particular low density and high Young modulus) are suitable to reach the necessary stiffness, tubes of fused silica, already availabl e on the market, are used as raw material for the shell production. The deterministic direct polishing method is chosen to figure the shell to the needed accuracy: the technological challenge is to apply this method, already used for previous missions such as Einstein, Rosat, Chandra, to almost ten times thinner shells. To this aim, a temporary stiffening concept has been designed and realized. A first prototypal shell, integrated in this stiffening structure, has undergone the out-of-roundness correction through fine grinding at Lt-Ultra and a first phase of polishing/superpolis hing at Zeeko Ltd (Citterio et al. 2011). In the last months,