A comparison of traceable spatial angle autocollimator calibrations performed by PTB and VTT MIKES

Autocollimators are versatile devices for angle metrology used in a wide range of applications in engineering and manufacturing. A modern electronic autocollimator generally features two measuring axes and can thus fully determine the surface normal of an optical surface relative to it in space. Until recently, however, the calibration capabilities of the National Metrology Institutes were limited to plane angles. Although it was possible to calibrate both measuring axes independently of each other, it was not feasible to determine their crosstalk if angular deflections were present in both axes simultaneously. To expand autocollimator calibrations from plane angles to spatial angles, PTB and VTT MIKES have created dedicated calibration devices which are based on different measurement principles and accomplish the task of metrological traceability in different ways. Comparing calibrations of a transfer standard makes it possible to detect systematic measurement errors of the two devices and to evaluate the validity of their uncertainty budgets. The uncertainty levels of the devices are comparable to each other, with an expanded uncertainty U = 0.014 arcsecond (95.5% coverage probability) over a measuring range of +/- 1000 arcsecond in the case of PTB and U = 0.015 arcsecond over a range of +/- 500 arcsecond and U = 0.020 arcsecond over +/- 1000 arcsecond in the case of VTT MIKES. Over a range of +/- 1000 arcsecond with regards to both measuring axis of an Elcomat 3000 autocollimator as a transfer standard, no statistically significant differences between the calibrations were detected. The results thus confirm the calibration capabilities of PTB and VTT MIKES as stated in the calibration and measuring capability database of Bureau International des Poids et Mesures.