Study of the relationship between metrological characterization and angular resolution in grazing-incidence X-ray optics

Abstract

The grazing-incidence optics with Wolter-I type geometry is commonly used in X-ray astronomy. The manufacturing technologies are still under development for future space missions to fulfill the stringent performance requirements with reduced weight and cost, e.g. the planned enhanced X-ray Timing and Polarimetry Mission. To improve the manufacturing process, it is necessary to study the relationship between metrological characterization and angular resolution via ray-optics or wave-optics models. The model calculations produce inconsistent results depending on the characteristics of wide-band surface errors, which require validation before their application in the Wolter-I type optics. In this work, two samples of the single-reflection mirrors with an elliptical shape are produced to validate the models. The first sample uses the Aluminum alloy substrate and the second sample uses the Aluminum alloy coated with Nickel-Phosphorous as the substrate. Tungsten is coated on both substrates to increase the X-ray reflectivity. The metrological characterization is inspected using the Fizeau interferometer and 3D optical profiler. The X-ray calibration of the mirror is performed in the 100-m X-ray Test Facility of Institute of High Energy Physics using the Color X-ray Camera. Both ray-optics and wave-optics models are validated in a wide scope of applications from smooth to relatively rough surfaces. The proper treatments of the metrological data are required as input to the model calculations: the post-fit distribution of figure errors, the micro-roughness defined in a specific frequency band, and the smoothed power spectral density of the surface errors. The validated models can be further applied in Wolter-I optics to predict mirror performances or to provide precision processing requirements.