Comparison of pupil orientation in pupil-tracking mode predicted by the control law of the derotator with pupil orientation actually measured in a simulated observation using the distortion grid located in the SPHERE calibration unit. Top panel: absolute pupil orientation, while the bottom panel shows the residuals between the predicted and measured values. The average constant offset of ∼1° is not a problem and is simply caused by the arbitrary zero-point of the derotator. However, the drop ∼0.05° right after meridian passage is the result of a mechanical backlash of the derotator mechanism. The repeatability of this error has not been investigated in detail, but if it is stable it could be compensated for in the data processing of SPHERE pupil-tracking data.