Kotska Wallace

X-Ray Pore Optics Technologies and Their Application in Space Telescopes

Silicon Pore Optics (SPO) is a new X-ray optics technology under development in Europe, forming the ESA baseline technology for the International X-ray Observatory candidate mission studied jointly by ESA, NASA, and JAXA. With its matrix-like structure, made of monocrystalline-bonded Silicon mirrors, it can achieve the required angular resolution and low mass density required for future large X-ray observatories. Glass-based Micro Pore Optics (MPO) achieve modest angular resolution compared to SPO, but are even lighter and have achieved sufficient maturity level to be accepted as the X-ray optic technology for instruments on board the Bepi-Colombo mission, due to visit the planet Mercury. Opportunities for technology transfer to ground-based applications include material science, security and scanning equipment, and medical diagnostics. Pore X-ray optics combine high performance with modularity and economic industrial production processes, ensuring cost effective implementation.

Silicon Pore Optics Development

Future X-ray astrophysics missions, such as the International X-ray Observatory, IXO, require the development of novel optics in order to deliver the missions large aperture, high angular resolution and low mass requirements. A series of activities have been pursued by ESA, leading a consortium of European industries to develop Silicon Pore Optics for use as an x-ray mirror technology. A novel process takes as the base mirror material commercially available silicon wafers, which have been shown to possess excellent x-ray reflecting qualities. These are ribbed, curved and stacked concentrically in layers that have the desired shape at a given radii of the x-ray aperture. Pairs of stacks are aligned and mounted into doubly reflecting mirror modules that can be aligned into the x-ray aperture without the very high angular and position alignment requirements that need to be achieved for mirror plates within the mirror module. The use of this silicon pore optics design substantially reduces mirror assembly time, equipment and costs in comparison to alternative IXO mirror designs. This paper will report the current technology development status of the silicon pore optics and the roadmap expected for developments to meet an IXO schedule. Test results from measurements performed at the PTB lab of the Bessy synchrotron facility and from full illumination at the Panter x-ray facility will be presented.

Production of silicon mirror plates

Silicon pore optics are currently under development for missions such as the International X-ray Observatory (IXO) as an alternative to the glass or nickel shell mirrors that were used in previous generation X-ray telescopes. The unprecedented effective area requirement of the IXO requires a modular optics design suitable for mass production. In this paper we discuss the current state-of-the-art in plate manufacturing technology. We provide examples of process innovations that have directly impacted the cost per mirror plate and have reduced the manufacturing cost of a mirror module. We show how a switch from silicon to silica as the reflective surface results in a simplified process flow without a corresponding change in the optical performance. We demonstrate how standard photolithographic techniques, applied in the semiconductor industry, can be used to pattern a reflective layer. The 5 arc-second angular resolution requirement of the IXO has stimulated a theoretical analysis of engineering tolerances in relation to angular resolution. We prove that improved control of the wedge angle by means of etch rate monitoring results in improved angular resolution. The results of this investigation will be used as the basis for future development in design for mass production.

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].

Performance characterization of silicon pore optics

The characteristics of the latest generation of assembled silicon pore X-ray optics are discussed in this paper. These very light, stiff and modular high performance pore optics (HPO) have been developed [1] for the next generation of astronomical X-ray telescopes, which require large collecting areas whilst achieving angular resolutions better than 5 arcseconds. The suitability of 12 inch silicon wafers as high quality optical mirrors and the automated assembly process are discussed elsewhere in this conference. HPOs with several tens of ribbed silicon plates are assembled by bending the plates into an accurate cylindrical shape and directly bonding them on top of each other. The achievable figure accuracy is measured during assembly and in test campaigns at X-ray testing facilities like BESSY-II and PANTER. Pencil beam measurements allow gaining information on the quality achieved by the production process with high spatial resolution. In combination with full beam illumination a complete picture of the excellent performance of these optics can be derived. Experimental results are presented and discussed in detail. The results of such campaigns are used to further improve the production process in order to match the challenging XEUS requirements [2] for imaging resolution and mass.

Performance prediction and measurement of silicon pore optics

We present the latest results of X-ray metrology performed on Silicon Pore Optics, a novel type of lightweight X-ray optics made from silicon and developed for future, large area space based X-ray telescopes. From these so-called pencil beam measurements, performed at the PTB laboratory of the BESSY synchrotron radiation facility, the overall performance in terms of half energy width (HEW) of the optics has been calculated. All measurements are performed at an intrafocal distance, but due to the nature of this measurement method, the results in terms of HEW can be extrapolated to the focal plane. In the near future, upgrades of the X-ray facilities will allow measuring the performance of the optics in the actual focal plane. We also present the newest development of our X-ray tracer tool, which is used to retrieve performance and imaging prediction from single plate level up to a full optic by use of the mirror figure, as recorded during the fabrication process. We furthermore present results of AFM imaging and X-ray reflectivity measurements performed to determine the surface roughness of the base material (polished Si wafers) and of fully processed and coated mirror plates.

ESA-led ATHENA/IXO optics development status

The International X-ray Observatory (IXO) is a candidate mission in the ESA Space Science Programme Cosmic Vision 1525, and was studied as a joint mission with NASA and JAXA. Considering the programmatic evolution of the international context, the mission is being reformulated as an ESA-led mission, under the name of ATHENA (Advanced Telescope for High Energy Astrophysics), with possible participation of NASA and JAXA. The mission is building on the novel Silicon Pore Optics (SPO) technology to achieve the required performance for this demanding astrophysics observatory. This technology is being developed by an industrial consortium, and involves also several research institutes [1-12]. A second optics technology, slumped glass optics (SGO), which is being developed in Europe and the USA, was the backup technology for IXO, and additionally work is progressing on improved reflective coatings and X-ray test facilities [13-17].

X-ray imaging glass micro-pore optics

Glass micro-pore optics technology, developed over the last years for planetary X-ray imagers, has been used to assemble optical modules in approximation of a Wolter-I configuration. These tandems of glass sectors consist of hundreds of square, millimetre sized, multi-fibres that each contain more than a thousand, 3 μm thin, X-ray mirrors with a surface roughness suitable for application at medium X-ray energies. The performance of the tandems can be traced back to the quality of the individual fibres. Extensive X-ray testing has been done on all constituents, from several fibres up to tandem level, using pencil beam and, for the first time, full beam illumination at PANTER. The results of these campaigns and of reflectometry measurements are discussed in this paper and have been used throughout the technology development program to monitor the X-ray performance. It will be shown that the quality of focussing micro-pore X-ray optics is now high enough to achieve an angular resolution of several arc minutes and that the multi-fibres are as good as 20 arc seconds, demonstrating the potential of this technology. The tandems can be combined and assembled into larger geometries, hence forming a very light and compact X-ray lens of ~200 mm diameter and a focal length of 1 m. This is part of an ESA breadboard program discussed elsewhere in this conference.

Assembling silicon pore optics into a modular structure

The XEUS petals encompass the optical bench structure of the stand alone X-Ray Optical Units (XOU) based on the high performance and light weight Silicon Pore Optics technology. The performance aspects under consideration of the design drivers, the related trade offs (e.g. mechanical concepts, material selection, XOU butting efficiency etc.) and the current development activities wrt. the design, manufacturing, assembly and the functional and environmental test verification approach of the Form Fit Function Model are described in this paper. Special emphasis is given to the critical external optical and mechanical interfaces coherent to the mission design, e.g. the Mirror S/C frame work structure and the Detector S/C. The technology program is based on the heritage achieved within the context of the XMM/Newton telescope development. The investigations of the correlated programmatic aspects towards the FM production by application of effective robot system supported assembly procedures shall be illustrated.

Metrology, integration, and performance verification of silicon pore optics in Wolter-I configuration

It has been demonstrated that silicon pore optics can serve as the new technology for building the next generation of X-ray telescopes for astronomical missions. In order to build up an optic in Wolter-I configuration, the high performance pore optics (HPO) have to be co-aligned and integrated into pairs, forming so-called X-ray optical units (XOU). The stringent co-alignment requirements for a 50 m focal length telescope like XEUS (e.g. 1 arcsecond between parabolic and hyperbolic HPO) demand holistic alignment concepts, which integrate the metrology, the fixation and the performance verification. The application in space and the resulting thermal requirements in combination with launch loads and other mechanical restrictions must also be considered. Finite element modelling of different fixation mechanisms and XOU configurations allow one both to assess difficulties at an early stage and to validate solution strategies. This paper reports on the concepts, which have been developed. The most promising candidate has been selected to build a form fit function model. The experimental set-up to align the HPOs, the required metrology and first results of the performance verification at test facilities will be shown and discussed.