Calibration of the rotary scale of the primary standard for position angle

A calibration of primary standard rotary scale was carried out based on the establishment of new calibration system. The angle metrological performance including index error, interpolation error, repeatability, and resolution were calibrated separately by comparison with optical polygon and accompanying-calibration angle encoder. The methods of crosscalibration and self-calibration were applied to make the calibration results traceable to circle-closure. The calibration results reveal that the index error is within ±0.04", the interpolation error is within ±0.076", the repeatability is better than 0.005", and the resolution is better than 0.002", which meet NIM's recent requirement of metrological performance for the reproduction of position angle at primary level.