B. Salmaso

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.

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].

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

Future X-ray telescopes with very large collecting area, like the proposed Athena with more than 2 m2 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.

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’’.

LAMP: a micro-satellite based soft x-ray polarimeter for astrophysics

The Lightweight Asymmetry and Magnetism Probe (LAMP) is a micro-satellite mission concept dedicated for astronomical X-ray polarimetry and is currently under early phase study. It consists of segmented paraboloidal multilayer mirrors with a collecting area of about 1300 cm2 to reflect and focus 250 eV X-rays, which will be detected by position sensitive detectors at the focal plane. The primary targets of LAMP include the thermal emission from the surface of pulsars and synchrotron emission produced by relativistic jets in blazars. With the expected sensitivity, it will allow us to detect polarization or place a tight upper limit for about 10 pulsars and 20 blazars. In addition to measuring magnetic structures in these objects, LAMP will also enable us to discover bare quark stars if they exist, whose thermal emission is expected to be zero polarized, while the thermal emission from neutron stars is believed to be highly polarized due to plasma polarization and the quantum electrodynamics (QED) effect. Here we present an overview of the mission concept, its science objectives and simulated observational results.

Production of thin glass mirrors by hot slumping for x-ray telescopes: present process and ongoing development

Thin glass foils are considered good candidates to build a segmented X-ray telescope with effective area as large as 2 m2 and angular resolution better than 5 arcsec. In order to produce thin glass mirror segments, we developed a direct hot slumping technique assisted by pressure, in which the shape of a mould is replicated onto the optical surface of the glass. In this paper we present the result obtained with AF32 (by Schott) and EAGLE XG (by Corning) glass types. The selected mould material is Zerodur K20, as it does not require any anti-sticking layer and has a good matching, in terms of Coefficient of Thermal Expansion, with both glass types. Our group already produced a few prototypes, reaching angular resolution near 20 arcsec. In this work, relevant steps forward aimed at attaining a 5 arcsec angular resolution are described, along with the tuning of few key parameters in the slumping process. The results obtained on a newly procured cylindrical Zerodur K20 mould are presented.

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.

Active shape correction of a thin glass/plastic x-ray mirror

Optics for future X-ray telescopes will be characterized by very large aperture and focal length, and will be made of lightweight materials like glass or plastic in order to keep the total mass within acceptable limits. Optics based on thin slumped glass foils are currently in use in the NuSTAR telescope and are being developed at various institutes like INAF/OAB, aiming at improving the angular resolution to a few arcsec HEW. Another possibility would be the use of thin plastic foils, being developed at SAO and the Palermo University. Even if relevant progresses in the achieved angular resolution were recently made, a viable possibility to further improve the mirror figure would be the application of piezoelectric actuators onto the non-optical side of the mirrors. In fact, thin mirrors are prone to deform, so they require a careful integration to avoid deformations and even correct forming errors. This however offers the possibility to actively correct the residual deformation. Even if other groups are already at work on this idea, we are pursuing the concept of active integration of thin glass or plastic foils with piezoelectric patches, fed by voltages driven by the feedback provided by X-rays, in intra-focal setup at the XACT facility at INAF/OAPA. In this work, we show the preliminary simulations and the first steps taken in this project.

Accurate modeling of periodic and graded multilayer x-ray scattering from surface microroughness characterization

Several hard X-ray imaging telescopes of the next future will be characterized by a high angular resolution. To this end, it is necessary to produce optics with a very low surface microroughness, as this is responsible of X-ray scattering, which results in image quality degradation especially at the higher energies. To the smooth surface approximation, it is possible to compute the X-Ray Scattering (XRS) from the Power Spectral Density (PSD) of the surface roughness. Indeed, multilayers coatings will be used to reflect X-rays beyond 10 keV; in this case the scattering pattern is more complicated but it can still be computed from the PSDs of each interface and the cross-correlation functions of the rough profiles. A growth model able to describe the roughness evolution of the surfaces enables us to compute the XRS of the multilayer, which can be directly compared to the experimental data. With this approach we aim to validate the roughening model assumed and to accurately predict the scattering pattern we expect on the focal plane. In this work we show the application of this formalism: direct measurements of the PSDs for the substrate and the outermost layer of a multilayer sample are used as input for the code to model the PSD growth. XRS measurements of that sample, performed at the energy of 8.05 keV, are presented to validate the modeling achieved.

Manufacturing an active X-ray mirror prototype in thin glass

Adjustable mirrors equipped with piezo actuators are commonly used at synchrotron and free-electron laser (FEL) beamlines, in order to optimize their focusing properties and sometimes to shape the intensity distribution of the focal spot with the desired profile. Unlike them, X-ray mirrors for astronomy are much thinner in order to enable nesting and reduce the areal mass, and the application of piezo actuators acting normally to the surface appears much more difficult. There remains the possibility to correct the deformations using thin patches that exert a tangential strain on the rear side of the mirror: some research groups are already at work on this approach. The technique reported here relies on actively integrating thin glass foils with commercial piezoceramic patches, fed by voltages driven by the feedback provided by X-rays, while the tension signals are carried by electrodes on the back of the mirror, obtained by photolithography. Finally, the shape detection and the consequent voltage signal to be provided to the piezoelectric array will be determined by X-ray illumination in an intra-focal setup at the XACT facility. In this work, the manufacturing steps for obtaining a first active mirror prototype are described.