X-ray optical units made of glass: achievements and perspectives

Abstract

Future X-ray telescopes with very large collecting area, like the proposed Athena with more than 2 m 2 effective area at 1 keV, need to be realized as assemblies of a large number of X-ray optical units, named X-ray Optical Units (XOUs). The Brera Astronomical Observatory (INAF-OAB) is developing a new technology to manufacture these modular elements, compatible with an angular resolution of 5 arcsec HEW (Half-Energy-Width). This technique consists in stacking in a Wolter-I configuration several layers of thin foils of glass, previously formed by direct hot slumping. The achievable global angular resolution of the optics relies on the required surface shape accuracy of slumped foils, on the smoothness of the mirror surfaces and on the correct integration and co-alignment of the mirror segments operated trough a dedicated Integration Machine (IMA). In this paper we provide an overview of the project development, reporting on the very promising results achieved so far, including in-focus full illumination X-ray tests of the prototype (Proof of Concept, POC#2, integrated at the beginning of 2013) for which an HEW of 22.1'' has been measured at Panter/MPE. Moreover we report on the on-going activities, with a new integrated prototype (PoC#3). X-ray test in pencil beam revealed that at least a segment between two external ribs is characterized by an HEW well below 10''. Lastly, the overall process up-grade to go from 20 m to 12m focal length (to be compatible with Athena+ configuration) is presented.

Manufacturing and testing a thin glass mirror shell with piezoelectric active control

“…The ribs also provide the mechanical stiffness to the XOU needed to maintain its shape during the operation . For the ATHENA (formerly IXO) X-ray telescope, the SGO technology has been developed at INAF/OAB under ESA/ESTEC contract in 2009-2013, and in parallel at MPE (Garching, Germany), following two d ifferent methodologies (direct slumping at OA B [4] and indirect slumping at MPE [5] ). In both cases, as well as for SPOs, it was proven that an optical design based on SGO was able to provide the requested effective area/mass ratio.…”

“…The other source of imaging degradation is represented by deformations of the optical surface. In the direct slumping methodology, the longitudinal profile of the X-ray mirror is subject to non-negligible errors that have been continuously reduced [6] by a proper optimization of the slu mping setup (mould height, intensity and duration of the pressure exerted, surface cleanliness, settings of the slump ing cycle...), and part ly corrected by the develop ment of an integration process [4] , which enforces the slumped foils with the Wolter-I profile of accurately figured integration moulds. Fixing the stack on the g lass via stiffen ing glass ribs, the no minal shape is imparted to the optical surface at the rib locations.…”

Direct hot slumping of thin glass foils for future generation x-ray telescopes: current state of the art and future outlooks

“…Despite the overall HEW higher with respect to the PoC#2, the X-ray analysis, performed at the PANTER/MPE facility in pencil beam mode, showed a best HEW value, in a portion of the module, of 5.5 arcsec at 0.27 keV (Fig. 4) [9]. Moreover, the impact of the roughness in the 0.27-1.49 keV range was considerably reduced with respect to the previous prototypes, denoting a better surface roughness.…”

“…The glasses were then integrated using newly procured BK7 integration moulds, with much more accurate Wolter-I profiles with respect to the previous Metapore moulds. These modifications have allowed the complete module to reach a HEW of ~ 22 arcsec and the better plate pair a HEW of ~ 20 arcsec, directly measured in-focus at 0.27 keV at the PANTER facility [9]. Finally, a last prototype (PoC#3) was realized with Corning Eagle glass foils slumped on a newly procured Zerodur K20 slumping mould, with better surface roughness with respect to the previous one [10].…”

“…Moreover, the impact of the roughness in the 0.27-1.49 keV range was considerably reduced with respect to the previous prototypes, denoting a better surface roughness. The detailed results for the last two prototypes can be found elsewhere [9]: this paper concentrates on presenting the currently achieved know-how and performance of the hot slumping technology @ INAF-OAB.…”