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Grazing incidence imaging x-ray optics: the return of Kirkpatrick-Baez

Kirkpatrick-Baez multi-foil optics offer a thinner, lighter, less-expensive alternative to conventional Wolter designs for a range of space-based-telescope applications.
27 October 2011, SPIE Newsroom. DOI: 10.1117/2.1201110.003859

X-ray optics is a key element of a range of telescopes, microscopes, and other sophisticated imaging instruments. Grazing incidence (reflective) x-ray lenses represent an important class of the technology. Most such systems used in astronomy and in other (laboratory) applications are based on the Wolter 1 (or modified) configuration, which consists of double reflection at a glancing angle on parabolic and hyperbolic surfaces. But other designs and arrangements have been proposed for future applications, including in space.1 Kirkpatrick-Baez (KB) lenses, and various types of lobster-eye (LE) and micropore optics, are just some of many examples. In micropore systems, as in Wolter lenses, the x-rays are generally reflected twice to create focal images.

Non-Wolter-type grazing incidence x-ray optical elements, mainly KB and LE, offer alternative solutions to conventional designs. At the same time, new computer-based systems make previously inconceivable configurations possible. Some of these systems such as KB optics, have already found wide application in synchrotron laboratories, on the basis of their superior imaging performance and accuracy.1 KB telescopes present a different case. Used in the past on sounding rockets in the 1970s, and subsequently planned for satellite applications, these instruments were ultimately never flown on a satellite. The reasons were various but include difficulty in achieving precise alignment of reflecting plates.1 Their future could, however, change with the introduction of new substrates and better alignment control. Furthermore, funding pressures may lead to serious consideration of lower-cost KB optics as an alternative to Wolter-1 systems for large-area x-ray telescopes.

Historically, KB telescope applications were based on thin sheets of float glass because the quality of substrates for silicon wafers was inadequate for x-ray optics. The recent availability of substrates with considerably improved parameters and flatness now makes silicon wafers a more promising choice, especially for segmented telescopes.3–5 Novel methods also exist for further improving the quality of thin float glass (e.g., by thermally shaping the sheets on flat mandrels).3–5 Next-generation materials and substrates for glass foils and silicon wafers must also be thin and light. Shaping them to small radii, as called for in Wolter designs, is not an easy task. KB arrangements represent a less-laborious and hence less-expensive alternative.

Marsikova and co-workers2 and, more recently, Willingale and Spaan6 investigated the feasibility of a KB system for the proposed joint NASA/European Space Agency (ESA)/Japan Aerospace Exploration Agency International X-ray Observatory (IXO). These studies indicate that employing superior-quality reflecting plates and a large focal length could achieve angular resolution of the order of a few arcseconds (see Figure 1). Recent simulations further suggest that, in comparison to the Wolter configuration, KB optics have a reduced on-axis collecting area but a larger field of view, at comparable angular resolution.6 Note that to achieve a similarly effective area, the focal length of the KB system must be about twice the focal length of the Wolter system. Figure 2 shows the principle of KB multi-foil optics (MFO).

Figure 1. Comparison of focal images between the KB and Wolter systems.2 The upper four panels show the full width at half-maximum (FWHM) in arcseconds, and the bottom four panels the focal peak intensity.

Figure 2. The principle of the Kirkpatrick-Baez multi-foil optics (KB-MFO) telescope.2

Figure 3. Laboratory samples of advanced KB MFO modules designed and developed at Rigaku Innovative Technologies Europe in Prague.2

Advanced KB telescopes based on the MFO approach to x-ray grazing incidence imaging optics include numerous thin reflecting substrates and foils.5 At Rigaku Innovative Technologies Europe (RITE) in Prague, we recently designed and constructed KB test modules based on novel materials, namely, glass foils and high-quality silicon wafers. We began with a model based on ray tracing (11 or more profiles). We constructed two sets of mirrors from silicon chips 100×100×0.525mm (see Figure 2). The total length of the optics is 600mm, and the aperture measures 40×40mm. The preliminary results of full-aperture x-ray optical tests of KB modules are promising, with full width half-maximum of complete stacks on the order of 30 arcsec in 2D arrangement. These results justify further efforts to improve KB optics for use in low-cost, high-performance space-borne astronomical imaging. A more detailed description is provided elsewhere.7

One very important factor in the success of the technology is the ease of constructing highly segmented modules based on multiply nested thin reflecting substrates compared with the Wolter design. For example, whereas the Wolter design for IXO/ATHENA (Advanced Telescope for High Energy Astrophysics) requires the substrates to be precisely formed with curvatures as small as 0.25m (and 0.15m for ATHENA), the alternative KB arrangement uses almost flat or only slightly bent sheets. Yet it still has the potential to achieve the required angular resolution.

In summary, the availability of high-quality novel materials such as superior silicon wafers and/or glass foils will enable the design and construction of KB x-ray optical systems with very high angular resolution at reasonable cost for various applications both in space, astronomy, as well as in the laboratory. In the near future, we plan to continue with assembling and testing KB modules with longer focal length and improved performance.

The work on novel x-ray optics performed in the Czech Republic, and partly described here, represents a broad collaboration of several Czech institutions, namely, the teams from Czech Technical University, Faculty of Nuclear Science (Prague), the Institute of Chemical Technology (Prague), RITE (Prague), ON Semiconductor Czech Republic, the Astronomical Institute of the Academy of Sciences of the Czech Republic (Ondřejov), and many others. We acknowledge the support provided by the Academy of Science of the Czech Republic (grant IAAX01220701) and by the Ministry of Education and Youth of the Czech Republic (projects ME918 and ME09028). The investigations related to the ESA IXO/ATHENA project are supported by the ESA Plan for European Cooperating States (project 98039). KB optics development is supported by the Ministry of Education and Youth of the Czech Republic (project ME09004).

René Hudec
Astronomical Institute of ASCR
Ondřejov, Czech Republic
Czech Technical University
Faculty of Electrical Engineering
Prague, Czech Republic

René Hudec heads the High-Energy Astrophysics Group at the Astronomical Institute and is an associate professor at Czech Technical University in Prague. He is involved in the design and development of innovative x-ray imaging optics and related technologies.

1. R. Hudec, Kirkpatrick-Baez (KB) and lobster eye (LE) optics for astronomical and laboratory applications, Adv. X-Ray Opt. Instrum., pp. 139148, 2010. doi:10.1155/2010/139148
2. V. Maršiková, X-ray optics: Wolter versus KB, Proc. AXRO2009, 2009. http://axro.cz/axro09
3. R. Hudec, L. Pina, A. Inneman, L. Sveda, V. Semencova, M. Skulinova, V. Brozek, M. Mika, R. Kacerovsky, J. Sik, Novel technologies for x-ray multi-foil optics, Proc. SPIE 5900, pp. 276-287, 2005. doi:10.1117/12.620425
4. R. Hudec, V. Marsikova, M. Mika, J. Sik, M. Lorenc, L. Pina, A. Inneman, M. Skulinova, Advanced x-ray optics with Si wafers and slumped glass, Proc. SPIE 7437, pp. 74370S, 2009. doi:10.1117/12.827978
5. R. Hudec, L. Pina, V. Maršiková, A. Inneman, M. Skulinová, M. Mika, New technologies for future space x-ray telescopes, Proc. AIP Conf. 1248, pp. 587-588, 2010.
6. R. Willingale, F. H. P. Spaan, The design, manufacture, and predicted performance of Kirkpatrick-Baez silicon stacks for the International X-ray Observatory or similar applications, Proc. SPIE 7437, pp. 74370B, 2009. doi:10.1117/12.826225
7. L. Pina, V. Marsikova, R. Hudec, Full-aperture x-ray tests of Kirkpatrick-Baez modules: preliminary results, Proc. SPIE 8076, pp. 807609, 2011.