Laura Proserpio

Accurate integration of segmented x-ray optics using interfacing ribs

Abstract. Future lightweight and long-focal-length x-ray telescopes must guarantee a good angular resolution (e.g., 5 arc sec HEW) and reach an unprecedented large effective area. This goal can be reached with the slumping of borosilicate glass sheets that allow the fabrication of lightweight and low-cost x-ray optical units (XOU). These XOUs, based on mirror segments, have to be assembled together to form complete multishell Wolter-I optics. The technology for the fabrication and the integration of these XOUs is under development in Europe, funded by European Space Agency, and led by the Brera Observatory (INAF-OAB). While the achievement of the required surface accuracy on the glass segments by means of a hot slumping technique is a challenging aspect, adequate attention must be given to the correct integration and coalignment of the mirror segments into the XOUs. To this aim, an innovative assembly concept has been investigated, based on glass reinforcing ribs. The ribs connect pairs of consecutive foils, stacked into a XOU, with both structural and functional roles, providing robust monolithic stacks of mirror plates. Moreover, this integration concept allows the correction of residual low-frequency errors still present on the mirror foil profile after slumping. We present the integration concept, the related error budget, and the results achieved so far with a semi-robotic integration machine especially designed and realized to assemble slumped glass foils into XOUs.

A wide field X-ray telescope for astronomical survey purposes: from theory to practice

X-ray mirrors are usually built in the Wolter I (paraboloid-hyperboloid) configuration. This design exhibits no spherical aberration on-axis but suffers from field curvature, coma and astigmatism, therefore, the angular resolution degrades rapidly with increasing off-axis angles. Different mirror designs exist in which the primary and secondary mirror profiles are expanded as a power series in order to increase the angular resolution at large off-axis positions, at the expanses of the on-axis performances. Here we present the design and global trade off study of an X-ray mirror systems based on polynomial optics in view of the Wide Field X-ray Telescope (WFXT) mission. WFXT aims at performing an extended cosmological survey in the soft X-ray band with unprecedented flux sensitivity. To achieve these goals the angular resolution required for the mission is very demanding, ~5 arcsec mean resolution across a 1° field of view. In addition an effective area of 5-9000 cm 2 at 1 keV is needed.

X-ray optics developments at ESA

Future high energy astrophysics missions will require high performance novel X-ray optics to explore the Universe beyond the limits of the currently operating Chandra and Newton observatories. Innovative optics technologies are therefore being developed and matured by the European Space Agency (ESA) in collaboration with research institutions and industry, enabling leading-edge future science missions. Silicon Pore Optics (SPO) [1 to 21] and Slumped Glass Optics (SGO) [22 to 29] are lightweight high performance X-ray optics technologies being developed in Europe, driven by applications in observatory class high energy astrophysics missions, aiming at angular resolutions of 5” and providing effective areas of one or more square meters at a few keV. This paper reports on the development activities led by ESA, and the status of the SPO and SGO technologies, including progress on high performance multilayer reflective coatings [30 to 35]. In addition, the progress with the X-ray test facilities and associated beam-lines is discussed [36].

Progress on indirect glass slumping for future x-ray telescope optics

Large X-ray telescopes for future observations need to combine a big collecting area with good angular resolution. Due to the mass limits of the launching rocket, light-weight materials are needed in order to enhance the collecting area in future telescopes. We study the development of mirror segments made from thin glass sheets which are shaped by thermal slumping. At MPE we follow the indirect approach which enables us the production of the parabolic and hyperbolic part of the Wolter type I mirrors in one piece. In our recent research we have used a test mould made of CeSiC™ for slumping processes in our lab furnace as well as in a heatable vacuum chamber, to avoid oxidation and air enclosure. Additional slumping tests in the vacuum furnace have been carried out using a Kovar mould and are compared with results under air. We describe the experimental set-up, the slumping process and the metrology methods and give an outlook on future activities.

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

X-ray telescopes with very large collecting area, like the proposed International X-ray Observatory (IXO, with around 3 m2 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’’.

Slumping technique for the manufacturing of a representative x-ray grazing incidence mirror module for future space missions

The Astronomical Observatory of Brera (INAF-OAB, Italy), with the financing support of the European Space Agency (ESA), has concluded a study regarding a glass shaping technology for the production of grazing incidence segmented x-ray optics. This technique uses a hot slumping phase, in which pressure is actively applied on thin glass foils being shaped, to form a cylindrical approximation of Wolter I x-ray segments, and a subsequent cold slumping phase, in which the final Wolter I profile is then freeze into the glass segments during their integration in elemental X-ray Optical Units. The final goal of this study was the manufacturing of a prototype containing a number of slumped pair plates (meaning parabola and hyperbola couples) having representative dimensions to be tested both in UV light and in x-rays at the Panter facility (Germany). In this paper, the INAF-OAB slumping technique, comprising a shaping step and an integration step is described, together with the results obtained on the manufactured prototype modules: the first prototype was aimed to test the ad-hoc designed and built semi-automatic Integration MAchine (IMA) and debug its control software. The most complete module comprises 40 slumped segments of Schott D263 glass type of dimension 200 mm x 200 mm and thickness of 0.4 mm, slumped on Zerodur K20 mould and stacked together through glued BK7 glass structural ribs to form the first entire x-ray optical module ever built totally composed by glass. A last prototype was aimed at demonstrate the use of Schott glass AF32 type instead of D263. In particular, a new hot slumping experimental set-up is described whose advantage is to permit a better contact between mould and glass during the shaping process. The integration procedure of the slumped segments into the elemental module is also reviewed.

Industrialization scenario for X-ray telescopes production based on glass slumping

Large X-ray segmented telescopes will be a key element for future missions aiming to solve still hidden mysteries of the hot and energetic Universe, such as the role of black holes in shaping their surroundings or how and why ordinary matter assembles into galaxies and clusters as it does. The major challenge of these systems is to guarantee a large effective area in combination with large field of view and good angular resolution, while maintaining the mass of the entire system within the geometrical and mass budget posed by space launchers. The slumping technology presents all the technical potentiality to be implemented for the realization of such demanding systems: it is based on the use of thin glass foils, shaped at high temperature in an oven over a suitable mould. Thousands of slumped segments are then aligned and assembled together into the optical payload. An exercise on the mass production approach has been conducted at Max Planck Institute for Extraterrestrial Physics (MPE) to show that the slumping technology can be a valuable approach for the realization of future X-ray telescopes also from a point of view of industrialization. For the analysis, a possible design for the ATHENA mission telescope was taken as reference.

Light-weight glass mirror systems for future x-ray telescopes

Future X-ray telescopes need to combine large collecting area with good angular resolution. In order to achieve these aims within the mass limit, light-weight materials are needed for mirror production. We are developing a technology based on indirect hot slumping of thin glass segments; this method enables the production of the parabolic and hyperbolic part of the Wolter type I mirrors in one piece. Currently we use a combination of a porous ceramic for the slumping mould and the glass type D263 for the mirror material. In this study we use glasses that have been polished on one side to remove thickness variations in the glass, in order to investigate their influence on the results. We describe the experimental set-up, the slumping process and the metrology methods. Finally we present the results of an X-ray test of several integrated glass sheets, and give an outlook on future activities.

Slumped glass option for making the XEUS mirrors: preliminary design and ongoing developments

The XEUS mission (X-ray Evolving-Universe Spectroscopy Mission) of ESA, in the present configuration has a mirror collecting area in the order of 5-6 m2 @ 1 keV, 2 m2 @ 7 keV and 1 m2 @ 10 keV. These large collecting areas could be obtained with a mirror assembly composed of a large number of high quality segments each being able to deliver the angular resolution requested by the mission or better. The XEUS telescope will fit in the fairing of an Ariane 5 ECA launcher and hence its diameter is presently of about 4.5 m. The request in terms of angular resolution of the telescope has been set to 5 arcsec with a goal of 2 arcsec. Due to the large size of the optics it is impossible to create closed shells like those used for XMM or Chandra and hence it will be necessary to assemble a large number of segments (for example of ~0.6 m x ~0.3 m size) to recreate the mirror shells. These segments will form a module, an optical sub-unit of the telescope. The modules will be assembled to form the whole mirror system. As for all the space missions, the limits imposed on the payload mass budget by the launcher is the main driver that force the use of very lightweight optics and this request is of course very challenging. For example, the current design for XEUS foresees a geometric-area/mass ratio better than about 30 cm2/kg. In this article is illustrated a possible approach for the realization of large size and lightweight X-ray mirrors that derive from an experience gained from a previous work made in INAF-OAB on the thermal slumping of thin glass optics. The process foresees the use of a mould having a good optical figure but opposite shape respect to the segment to be slumped. On the mould is placed an initially flat glass sheet. With a suitable thermal cycle the glass sheet is conformed to the mould shape. Once tested for acceptance the glass sheet it is then integrated into a module by means of a robotic arm having a feedback system to confirm the correct alignment. A study on different optical geometries using the classical Wolter I and Kirkpatrick-Baez configurations has been also performed to investigate the theoretical performances obtainable with optics made using very thin glass shells.

Profile reconstruction of grazing-incidence x-ray mirrors from intra-focal x-ray full imaging

The optics of a number of future X-ray telescopes will have very long focal lengths (10 – 20 m), and will consist of a number of nested/stacked thin, grazing-incidence mirrors. The optical quality characterization of a real mirror can be obtained via profile metrology, and the Point Spread Function of the mirror can be derived via one of the standard computation methods. However, in practical cases it can be difficult to access the optical surfaces of densely stacked mirror shells, after they have been assembled, using the widespread metrological tools. For this reason, the assessment of the imaging resolution of a system of mirrors is better obtained via a direct, full-illumination test in X-rays. If the focus cannot be reached, an intra-focus test can be performed, and the image can be compared with the simulation results based on the metrology, if available. However, until today no quantitative information was extracted from a full-illumination, intra-focal exposure. In this work we show that, if the detector is located at an optimal distance from the mirror, the intensity variations of the intra-focal, full-illumination image in single reflection can be used to reconstruct the profile of the mirror surface, without the need of a wavefront sensor. The Point Spread Function can be subsequently computed from the reconstructed mirror shape. We show the application of this method to an intra-focal (8 m distance from mirror) test performed at PANTER on an optical module prototype made of hot-slumped glass foils with a 20 m focal length, from which we could derive an expected imaging quality near 16 arcsec HEW.