Fund Project:Project supported by the National Key R&D Program of China (Grant No. 2017YFB0503300)
Received Date:22 September 2019
Accepted Date:17 October 2019
Published Online:05 February 2020
Abstract:On November 10, 2016, China launched an X-ray pulsar navigation test satellite (XPNAV-1) to investigate the X-ray pulsar navigation technology, and a lot of scientific observation data have been obtained. The X-ray grazing incidence optics is a critical component of the focusing pulsar telescope. It plays an important role in increasing the effective area and enhancing the sensitivity of the telescope. It is also the first grazing incidence optics verified in orbit in China. According to the characteristic that the times of arrival (TOA) of X-ray photons are measured in pulsar navigation, the grazing incidence focusing optics based on single-reflection paraboloid mirror is designed, and manufacturable mirror design parameters are obtained through theoretical calculation and derivation. The theoretical effective area of the designed optics is 15.6 cm2 at 1 keV. The designed optics is then simulated to evaluate its focusing performance. It meets the focusing requirement in the full field of view. The electroforming nickel replication process used for manufacturing the mirrors for XMM-Newton and eRosita missions is investigated. A super-smooth mandrel is firstly fabricated and used for follow-up replication. An about-100 nm-thick gold layer is deposited on the mandrel, and serves as the reflection and release layer of the mirror. The nickel substrate of the mirror is electroformed on the gold layer. The mirror is finally obtained by releasing the nickel and gold layer from the mandrel. The patterns and roughness of the mandrel are then replicated onto the inner surface of the mirror. The 4-layered mirror is fabricated for the optics. The reflectivity for each layer of the 4-layered mirror is then measured with a dedicated facility on 4B7B beamline of BSRF. The effective area of the optics based on the above-measured reflectivity is 13.2 cm2 at 1 keV. Finally, according to the in-orbit observation data, the effective area of the optical system is evaluated to be a typical value of 4.22 cm2 at 1 keV, which is less than the ground-tested value. The reason for this is analyzed and it turns out to be due to the thermal deformation of mechanical structure and contamination of the mirrors. Therefore, in our future work, we will strictly control the environmental factors and implement space environmental adaptability design, while increasing the accuracy of the optics. Keywords:multi-layer nested/ grazing incidence optics/ development/ performance evaluation
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2.1.光学设计
对于探测大量X射线光子到达时间的脉冲星导航而言, 需要牺牲成像所需的高角分辨率, 提高光子通量, 获得仪器的高灵敏度. 使用多层嵌套的抛物面反射镜, 可获得更大的有效面积, 更适合于脉冲星自主导航. NICER和XPNAV-1的在轨性能表明, 单次反射掠入射聚焦型光学系统满足脉冲星导航的需求, 在提高导航精度方面起到了核心作用. 单层抛物面反射镜光路如图1所示. 图 1 单层反射镜光路图 Figure1. Schematic of optical path of a parabolic mirror.
以表面粗糙度为0.5 nm(RMS)和反射面镀金为输入条件, 计算反射镜在平均掠入射角下的反射率, 结合图7中各层反射镜的实测反射率, 以及反射镜的几何集光面积与结构对反射镜的遮挡面积, 得到光学系统的理论有效面积曲线和基于实测反射率的有效面积, 结果如图8所示. 图 8 基于实测反射率与理论反射率的有效面积 Figure8. Effective areas based on measured and theoretical reflectivity.