11 March 2003 Normal incidence multilayer telescope for soft X-ray beam expander
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Abstract
Normal incidence optics have been used with multilayers in EUV region. The 2d of the multilayers has to be equal to the wavelength of interest. At the same time, the reflectivity of the multilayers should decrease with the increase of the interfacial roughness much faster than grazing optics. In general, 2d of 10 nm is the shortest d-spacing available for multilayers in normal incidence. As a challenge of shorter wavelength application, we made NiCr/C multiplayer mirror for the laboratory use at 4.47 nm(carbon K alpha line). The main dish of the Cassegrain optics is 20 cm diameter spherical mirror and the secondary mirror is a reflector in aspherical shape to correct astigmatisms. Its focal point is placed at the X-ray source to create a broad parallel beam of 20 cm in diameter. The flux of the parallel beam is slightly less than the expected value, and gradually decreases of 40% toward the outer region. The measured parallelism is about 25 arcsec, which is a little larger than the designed value. More pricise positioning of the focal point to the X-ray generator may reduce such divergence. The beam profile through a slit shows a core of about 20 arcsec and an extended tail which might be due to scattering tail by the roughness of 0.3 nm. An application of this system is demonstrated with the Astro-E X-ray telescope. The image core is sharper but the scattering tail is considerable. Even after the subtraction of the tail, still some wing is left. This system is bright and parallel enough to examine the optical alignment much faster than previous method, while careful measurements are necessary for quantitative calibration of X-ray telescopes.
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Hideyo Kunieda, Kazutami Misaki, Yoshito Haba, Manabu Ishida, Kei Itoh, Hideyuki Mori, Ryo Shibata, "Normal incidence multilayer telescope for soft X-ray beam expander", Proc. SPIE 4851, X-Ray and Gamma-Ray Telescopes and Instruments for Astronomy, (11 March 2003); doi: 10.1117/12.461513; https://doi.org/10.1117/12.461513
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