A sagittally focusing X-ray crystal monochromator based on diffractive–refractive optics is a four-crystal ($+,-,-,+$) monochromator with parabolic longitudinal grooves on the crystals surface. Such a monochromator is simultaneously able of focusing and monochromatizing a white beam. The focusing was experimentally proved in earlier papers but the size of the focus was always larger than expected. This discrepancy has not been properly discussed. Such a discussion is presented in this paper.

中文翻译：

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衍射折射光学元件：矢状聚焦X射线单色仪的聚焦大小

基于衍射-折射光学器件的矢状聚焦X射线晶体单色仪是四晶体（$+，-，-，+$）单色仪，在晶体表面上有抛物线形的纵向凹槽。这种单色仪能够同时聚焦和单色化白光束。在较早的论文中已经通过实验证明了聚焦，但是聚焦的大小始终大于预期。这种差异尚未得到适当讨论。本文讨论了这种讨论。