Development and in-orbit performance evaluation of multi-layered nested grazing incidence optics

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 cm<sup>2</sup> 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 cm<sup>2</sup> 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 cm<sup>2</sup> 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.

Precision polishing of the mandrel for x-ray grazing incidence mirrors in the Einstein probe

“…The symmetrical full-shell structure of the nickel replica lens barrel exhibits excellent mechanical properties and can achieve high surface accuracy with an angular resolution of 10″-15″. Thus, grazing-incidence optical elements fabricated using nickel-based replication methods have been widely used [5][6][7]. However, in practical applications, it is necessary to consider the influence of factors such as the thickness of the mirror, the focal length, and the axial length to achieve the best optical characteristics.…”