This test is performed using the Renishaw XR20-W rotary axis calibrator mounted to the rotating stage table. The XR20-W consists of an integrated angular retroreflector mounted on a precision servo-controlled axis. The angular position of this axis, and the optics relative to the main body housing, is controlled by a very high accuracy encoder system with the scale directly machined onto the main bearing. The angular interferometer optic is placed at the end of the stage and is held stationary. The angular optic setup creates two beam paths. As the stage rotates, the XR20-W counter-rotates by the same amount. The angular error is calculated based residual rotation measures by the laser.
In the same way that the linear accuracy data is collected, the stage is moved in fixed increments (typically 5° - 30° per step), and a measurement is taken at each point. The stage is moved through its full range of travel, in both directions, for multiple passes. The data that is collected is plotted and the overall errors are calculated. The results are typically reported in arc-seconds or microradians.