Left panel: accuracy of the cross-correlation method using the errors of a single cross-correlation function. Right panel: accuracy of the refined method using the standard deviation between several cross-correlation functions for identical slit orientation as error estimate. The absolute stellar position angle is estimate for a high resolution spectra (R = 120 000) and for increasing apparent stellar radii. Filled squares: R_star = 1 mas; filled triangles: R_star = 2.5 mas; circles: R_star = 5 mas; filled diamonds: R_star = 10 mas and stars: R_star = 15 mas. To improve the visibility of the error bars, the points for a given plate scale are slightly dispersed around that value.