Disentangling the instrumental and the stellar contribution to the line tilt. Left panel: the stellar rotation induces a line tilt in the opposite direction to the instrumental tilt. Right panel: the stellar rotation induces a line tilt in the same direction as the instrumental tilt. The simulated stellar position angle is PA_star = 75°. Maximum displacement is expected for φ = −15°and195°. Each cross-correlation function is calculated by selecting only telluric and stellar lines respectively. The instrumental contribution remained constant between the two slit orientations.