A highly precise small angle generator (HPSAG) has been developed in UME to realize the SI unit of plane angle, radian (rad), more precisely and calibrate high precision autocollimators. The device can easily generate small angles in measurement steps of 0.001 arcsec in the measurement range of ±8 arcsec. This enables calibration of high-resolution electronic autocollimators in very small measurement steps close to the autocollimators’ resolution with an expanded uncertainty of 0.01 arcsec (k = 2). Description of the device, method, uncertainty budget and the first results are reported.

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Calibration of high-resolution electronic autocollimators is carried out in TUBITAK UME using an angle comparator to ensure direct traceability to the SI unit of plane angle, radian (rad). The device is a specially designed air-bearing rotary table fitted with a commercially available angular encoder utilizing a single reading head. It is shown that high-resolution electronic autocollimators in the large measurement range (e.g. ±1000 arcsec) can be calibrated with an expanded uncertainty of 0.035 arcsec (k = 2) in conventional dimensional laboratory conditions, applying good measurement strategy for single reading head angle encoders and taking simple but smart precautions. Description of the angle comparator is presented with various test results derived using different high-precision autocollimators, and a detailed uncertainty budget is given for the calibration of a high-resolution electronic autocollimator.

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.

Interpolation errors at small angular scales are caused by the subdivision of the angular interval between adjacent grating lines into smaller intervals when radial gratings are used in angle encoders. They are often a major error source in precision angle metrology and better approaches for determining them at low levels of uncertainty are needed. Extensive investigations of interpolation errors of different angle encoders with various interpolators and interpolation schemes were carried out by adapting the shearing method to the calibration of autocollimators with angle encoders. The results of the laboratories with advanced angle metrology capabilities are presented which were acquired by the use of four different high precision angle encoders/interpolators/rotary tables. State of the art uncertainties down to 1 milliarcsec (5 nrad) were achieved for the determination of the interpolation errors using the shearing method which provides simultaneous access to the angle deviations of the autocollimator and of the angle encoder. Compared to the calibration and measurement capabilities (CMC) of the participants for autocollimators, the use of the shearing technique represents a substantial improvement in the uncertainty by a factor of up to 5 in addition to the precise determination of interpolation errors or their residuals (when compensated). A discussion of the results is carried out in conjunction with the equipment used.