Direct hot slumping and accurate integration process to manufacture prototypal x-ray optical units made of glass

Abstract

X-ray telescopes with very large collecting area, like the proposed International X-ray Observatory (IXO, with around 3 m 2 at 1 keV), need to be composed of a large number high quality mirror segments, aiming at achieving an angular resolution better than 5 arcsec HEW (Half-Energy-Width). A possible technology to manufacture the modular elements that will compose the entire optical module, named X-ray Optical Units (XOUs), consists of stacking in Wolter-I configuration several layers of thin foils of borosilicate glass, previously formed by hot slumping. The XOUs are subsequently assembled to form complete multi-shell optics with Wolter-I geometry. The achievable global angular resolution of the optic 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. The Brera Astronomical Observatory (INAF-OAB) is leading a study, supported by ESA, concerning the implementation of the IXO telescopes based on thin slumped glass foils. In addition to the opto-mechanical design, the study foresees the development of a direct hot slumping thin glass foils production technology. Moreover, an innovative assembly concept making use of Wolter-I counter-form moulds and glass reinforcing ribs is under development. The ribs connect pairs of consecutive foils in an XOU stack, playing a structural and a functional role. In fact, as the ribs constrain the foil profile to the correct shape during the bonding, they damp the low-frequency profile errors still present on the foil after slumping. A dedicated semirobotic Integration MAchine (IMA) has been realized to this scope and used to build a few integrated prototypes made of several layers of slumped plates. In this paper we provide an overview of the project, we report the results achieved so far, including full illumination intra-focus X-ray tests of the last integrated prototype that are compliant with a HEW of around 17''.

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

“…In green are indicated the data of the central part of the glass at the corresponding energy. These results are quite in line with the expectation from the metrological characterization of the external layer [10] where the expected HEW of PPO was inferred around 17''. We can expect few arcsec difference due to metrology accuracy.…”

“…This time the layers were mounted in co-focal configuration in order to check the alignment performance of the IMA. The metrological data (available in summer 2013) have been reported in [10]. With respect to the final X-ray calibration, the 20 meters focal length of the model posed some constrain in the facility schedule, as an extension of the normal set-up has to be mounted [12].…”

“…This alignment feature can be used to check the correct alignment of the system. Initially the alignment has been checked trough UV data on the UV bench available at INAF/OAB and the results were in line with expectations [10]. A final assessment has been possible during the X-ray calibration activities performed at Panter/MPE.…”

“…This achievement was inferred thought the metrological data acquired on UV bench. Moreover the expected optical quality of the external layer was evaluated around 17'' trough metrological characterization [10] and looking at the intra-focal images acquired at Panter/MPE [11].…”

A high resolution large x-ray mission based on thin glass: optomechanical design

“…NUSTAR demonstrated the success of such approach and in Europe several modules have been produced to demonstrate the feasibility of mirror stacks with high spatial resolution [3] : they are based essentially on two technologies, the Silicon Pore Optics [4] (SPO) and Slumped Glass Optics [5] [6] (SGO). In INAFOsservatorio Astronomico di Brera (OAB), the slumping glass technology has been developed and several prototypes have been produced in the latest years [7] integrating separated slumped glass foils with glass ribs as spacers. The advantage of the SPO is the compactness of the optical stack, even if it does not mean a lower mass because the gain of a smaller optical mirror shells, and therefore lighter mechanical structure and ribs, can be balanced by an increase of the mass due to the higher ribs vignetting into the optics.…”