Improving the quality of x-ray mirrors: An inter-lab, optical metrology collaboration to guide deterministic polishing

The surface quality of x-ray mirrors is a major constraint on optical performance at synchrotron light and free electron laser facilities. A limiting factor for creating state-of-the-art optics is the accuracy of metrology data to deterministically guide the polishing tool to correct surface errors. The “MooNpics” (Metrology On One-Nanometer-Precise Optics) collaboration aims to improve optical metrology capabilities at European facilities to enable reproducible measurement of long or curved optics with height errors <1 nm rms and slope errors <100 nrad rms. Three challenging x-ray optics were measured by several labs using a variety of instruments. The mirrors, chosen to challenge and explore different aspects of optical metrology, were as follows: a 1 m-long, ultra-flat (radius of curvature R > 100 km); an ellipse with added parabolic arcs; and a strongly curved sphere (R ∼ 9.3 m) with an added spatially varying chirp. This study highlighted calibration issues with several instruments, which were subsequently corrected. In this paper, we present results about the ellipse mirror. Based on metrology data provided by the collaboration, two cycles of ion beam figuring improved all aspects of the mirror, including correcting the ellipse parameters, reducing high- and mid-frequency spatial polishing errors, and refining the shape of the parabolic arcs. Overall, the slope and height errors were improved by a factor of ∼10. We also show how the round-robin measurement exercise helped refine “best practice” procedures for mounting optics, alignment, and data acquisition and analysis methods. It is hoped that this collaborative project will ignite further improvements in the production quality of x-ray optics to benefit many scientific communities around the world.