Top: intensity spectrum extracted from the synthetic 2D spectrum of the cool giant for ψ = 90°. Middle: corresponding position spectrum. The pixel plate scale is P_s = 0.2′′/pixel, the seeing is set at 1′′(S_eeing = 5 P_s) and the S/N of the 2D spectrum is fixed to 150. In this example the apparent stellar diameter is set to be as large as Aldebaran’s: 20 mas, which permits a visual identification of the displacement of the photocentre at the line position. Bottom: errors on the position spectrum in mas.