Pushing the limits: latest developments in angle metrology for the inspection of ultra-precise synchrotron optics

Abstract

The requirements on the quality of ultra-precise X-ray optical components for application in the Synchrotron Radiation (SR) community are increasing continually and strongly depend on the quality of the metrology devices available to measure such optics. To meet the upcoming accuracy goal of 50 nrad rms for slope measuring profilers, a dedicated project, SIB58 Angles, consisting of 16 worldwide partners and supported by the European Metrology Research Programme (EMRP) was started in Sep 2013. The project covers investigations on autocollimators under extremely challenging measuring conditions, ray-tracing models, 2D autocollimator calibration (for the first time worldwide), determination of error sources in angle encoders providing traceability by 'sub-division of 2π rad' with nrad uncertainty, angle generation by 'ratio of two lengths' in nrad level, and on the development of portable precise Small Angle Generators (SAGs) for regular in-situ checks of autocollimators' performance. Highlights from the project will be reported in the paper and the community of metrology for X-Ray and EUV Optics will be informed about its progress and the latest work in angle metrology.

Environmental influences on autocollimator-based angle and form metrology

“…Note that, in the divisor of Eq. (14), p m can usually be replaced by either p 1 or p 2 with negligible numerical impact. the outgoing autocollimator beam by 90 • toward the SUT.…”

“…[10][11][12] Deflectometric profilometry poses stringent demands on the quality, alignment, and characterization of the profilometer's optomechanical components, including the autocollimator and the pentaprism. [13][14][15][16][17][18][19][20][21][22][23][24][25][26][27] When properly aligned, the beam deflection angle of the pentaprism is highly robust regarding changes in its angular orientation. [16][17][18][19] The reflectivity and curvature of the surface under test (SUT) influence the angle response of the autocollimator.…”

Investigations of interpolation errors of angle encoders for high precision angle metrology

“…Each head furnishes two sinusoidal signals per grating line, i.e. 2 18 signals in 360°, corresponding to angular intervals of 4.94 arcsec. These intervals are further subdivided by the reading heads' electronics and software by a factor of 4096 = 2 12 to obtain 2 30 intervals in 360°, in other words, 0.0012 arcsec final resolution is achieved (see [7] for technical details).…”

“…Calibration of autocollimators (ACs) at an uncertainty level, of less than 50 nrad (0.01 arcsec) is demanded for autocollimator-based slope measuring profilers. This is because autocollimators have recently become very popular with the x-ray & EUV optics metrology community since autocollimatorbased slope measuring profilers have proven to be capable of characterizing ultra-precise beam shaping optics, particularly for applications in synchrotron radiation (SR) and x-ray free electron lasers (XFEL) [13][14][15][16][17][18]. They are needed for the creation of the next-generation of x-ray optics for SR and XFEL beamlines.…”

Application of the differential Fabry–Perot interferometer in angle metrology

“…The nanoradian (nrad) angle measurement demands continue to grow following their unprecedented accuracy for applications in synchrotron radiation (SR), x-ray free electron lasers (XFELs) and gamma ray spectroscopy-fundamental physics and scientific space missions [1][2][3][4]. State-of-the-art uncertainty requirements for the traceable calibration of angle measurement devices such as autocollimators are also increasing with the extension of their application areas [2]. Autocollimators are optical instruments which measure the angular displacement of reflecting surfaces [5].…”