Future X-ray telescopes will require measuring thousands of grazing-incidence mirrors with nanometer accuracy and high throughput. Fizeau interferometry will require accurately separating the reference surface from the surface under test, a challenging proposition for optical surfaces that are highly off-axis conics. We present a lateral shifting interferometry approach called Axial Shift Mapping (ASM), in which we measure the surface under test at multiple positions and numerically separate the reference and test surfaces. In this paper, we estimate the measurement accuracy that would be required for an exemplary 0.5 arcsecond half-power diameter angular resolution telescope. We mathematically describe the procedure for using ASM, plus a lateral shift, to extract the most important quantities for X-ray telescope mirrors: axial profile (including the quadratic component) and cone angle variation. We also conclude that the radius and cone angle of a mirror are unknowable with ASM alone.
The next generation of x-ray telescopes will require mirror segments to be characterized to a surface uncertainty of 5 nm RMS or better. We present axial shift mapping, a Fizeau interferometry method to characterized near-cylindrical null correctors and surfaces. We extend our previously tested technique to cylindrical optics of similar dimensions to x-ray telescope mirrors. We report on progress towards full surface extraction of a cylindrical optic using axial shift mapping.
Concentrated solar power (CSP) plants need to monitor the surface slope error of thousands of heliostats with sub-milliradian accuracy. We present grating embedded mirrors (GEMs) and the accompanying Diffractive AutoStigmatic Hartmann Camera (DASHCam) metrology system for measuring heliostat surface slopes. GEMs have multiple phase diffraction gratings written within a glass mirror substrate using an ultrafast laser. The gratings direct a small fraction of incident light to non-specular directions, which the DASHCam senses from a virtual center of curvature to measure the facet slopes at each grating location. Our results show, in a laboratory environment, 24 μrad RMS measurement repeatability and 47 µrad RMS accuracy, with single-shot image capture. GEMs and DASHCam are a compact, accurate, and high-speed heliostat slope error metrology system that is robust to the harsh environmental conditions at CSP plants.
The next generation of high-resolution x-ray telescopes will require mirror segments characterized to 5 nm uncertainty or better. This is difficult to achieve due to the mirror segment’s off-axis hyperbolic and parabolic shape and the challenge of manufacturing and testing a cylindrical null lens. In a typical Fizeau interferometer setup, errors in the assumed perfect null lens will be coupled into the final surface figure, increasing uncertainty. To combat the higher uncertainty of the cylindrical null corrector, we have been developing lateral shift mapping, an absolute metrology technique using a Fizeau interferometer. In this technique, the surface under test is laterally shifted between measurements while the reference surface does not move. Contributions to the interferogram due to the surface under test will move, while contributions due to the reference will stay static. Using this information, we can extract the true surface under test with low uncertainty. There is a quadratic ambiguity that arises due to the extraction method being akin to an integration. We have shown in the past our ability to utilize lateral shift mapping to extract flat surfaces to sub-nanometer uncertainties by comparing our results to a three-flat test. We also demonstrated that we can eliminate the quadratic ambiguity in flats using an external measurement with an autocollimator. We are expanding this method from optical flats to cylindrical surfaces, creating axial shift mapping. We will report on progress toward sub-nanometer measurements of cylindrical mirrors using axial shift mapping.
Currently, high-resolution X-ray telescope mirrors, such as for the Lynx X-Ray Observatory concept, are measured using a Fizeau interferometer with a cylindrical null corrector. Uncertainties in the null wavefront directly couple into the surface measurement uncertainty, including the axial figure and cone angle variation. We extend the absolute surface metrology method of lateral shift mapping for measuring X-ray telescope mirror segments. Lateral shift mapping involves laterally shifting the surface under test relative to the null to multiple positions. The null wavefront can be extracted from the difference between these shifted measurements, leaving only the surface under test. Accurately extracting quadratic terms of the surface under test requires measuring its tilt during shifting. We will show surface metrology results of optical flats measured by Fizeau-based lateral shift mapping with the required angle measured using an autocollimator and compare these results against a three-flat test. We will show how we plan to extend this method to conical X-ray telescope mirror metrology. The lateral shift mapping method reduces the uncertainty introduced by the cylindrical null, a critical step toward making high-resolution X-ray telescope mirrors.
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