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arXiv 2004.13001astro-ph.IM
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衍射极限X射线望远镜的光学设计

Optical design of diffraction-limited X-ray telescopes

Brandon D. Chalifoux, Ralf K. Heilmann, Herman L. Marshall, Mark L. Schattenburg

摘要:

具有微角秒($μ$ as)角分辨率的天文成像能够促成突破性的科学发现。先前提出的作为X射线成像器设计的$μ$是具有有限有效收集面积的干涉仪。在这里,我们描述的X射线望远镜实现衍射限制的性能在一个宽的能量带大的有效面积,采用嵌套的外壳结构与掠入射镜,同时匹配的所有外壳之间的光程长度。我们提出了两个紧凑的嵌套壳沃尔特2型掠入射望远镜设计的衍射限制X射线成像:角分辨率为14 $μ$,在5 keV光子能量下有效面积为2.9 m $^2$的微弧秒望远镜设计($λ$ =0.25 nm),和较小的毫角秒望远镜设计,分辨率为525 $μ$,在1 keV($λ$ =1.24 nm)下的有效面积为645 cm $^2$。我们描述了如何在一个紧凑的反射镜组件中匹配所有外壳之间的光路长度,并研究色差和离轴像差。色差的结果,从全反射镜表面的外部,我们大大减轻其影响,稍微调整每个镜壳的路径长度。反射镜表面高度误差和衍射限制性能的对准要求是具有挑战性的,但可以说在未来几十年是可行的。由于衍射极限X射线望远镜的焦比非常大($f/D$ ~10 $^5$),唯一重要的离轴像差是场曲,因此使用平面探测器实现1弧秒视场是可行的。探测器必须与反射镜组件一起编队飞行,但相对定位公差在数十至数百公里的距离上为1米的量级。虽然实现衍射极限X射线成像存在许多挑战,但我们没有发现任何根本障碍。

英文摘要:

Astronomical imaging with micro-arcsecond ($μ$as) angular resolution could enable breakthrough scientific discoveries. Previously-proposed $μ$as X-ray imager designs have been interferometers with limited effective collecting area. Here we describe X-ray telescopes achieving diffraction-limited performance over a wide energy band with large effective area, employing a nested-shell architecture with grazing-incidence mirrors, while matching the optical path lengths between all shells. We present two compact nested-shell Wolter Type 2 grazing-incidence telescope designs for diffraction-limited X-ray imaging: a micro-arcsecond telescope design with 14 $μ$as angular resolution and 2.9 m$^2$ of effective area at 5 keV photon energy ($λ$=0.25 nm), and a smaller milli-arcsecond telescope design with 525 $μ$as resolution and 645 cm$^2$ effective area at 1 keV ($λ$=1.24 nm). We describe how to match the optical path lengths between all shells in a compact mirror assembly, and investigate chromatic and off-axis aberrations. Chromatic aberration results from total external reflection off of mirror surfaces, and we greatly mitigate its effects by slightly adjusting the path lengths in each mirror shell. The mirror surface height error and alignment requirements for diffraction-limited performance are challenging but arguably achieveable in the coming decades. Since the focal ratio for a diffraction-limited X-ray telescope is extremely large ($f/D$~10$^5$), the only important off-axis aberration is curvature of field, so a 1 arcsecond field of view is feasible with a flat detector. The detector must fly in formation with the mirror assembly, but relative positioning tolerances are on the order of 1 m over a distance of some tens to hundreds of kilometers. While there are many challenges to achieving diffraction-limited X-ray imaging, we did not find any fundamental barriers.

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