A Fizeau interferometry stitching system to characterize X-ray mirrors with sub-nanometre errors

Abstract

We present the development of a Fizeau interferometer stitching system to characterize the surface profile of state-of-the-art X-ray optics for synchrotron and free electron laser sources. Controls and acquisition software precisely translate and rotate the surface under test in synchrony with data capture by a Zygo Verifire HDX interferometer. Overlapping sub-aperture images are combined into a composite, high-spatial resolution topographical image of the entire optical surface using PyLOSt stitching software produced by the MooNpics collaboration. After minimization of random and systematic measurement errors, system performance was quantified by characterizing two challenging optics. These optics represent extreme cases for synchrotron X-ray mirrors: an aspheric elliptical profile with a slope error of ∼50 nrad rms, and a corresponding height error of 0.2 nm rms; and a chirped periodic structure superimposed upon a strong curvature (radius ∼ 9.33 m). Results are in very good agreement with the Diamond-NOM slope profilometer and Bruker GTX stitching micro-interferometer. Without a specialist transmission reference, both optics could not be measured by the interferometer without pitch and translation stitching. Any individual HDX scan, relative to the average of the ensemble, has an average slope error repeatability of < 15 nrad rms. After calibration of the transmission reference flat and zoom factor, a reproducibility of ∼ 27 nrad rms compared to the Diamond-NOM, was achieved for the elliptically curved mirror.