(upper panels) Elliptic flow relative to the spectator plane, $v_2\left\{\Psi_{\rm SP}\right\}$, and to the participant plane, $v_2\left\{4\right\}$, as a function of transverse momentum in different centrality classes for Pb-Pb collisions. The linear fits to the ratio $v_2\left\{\Psi_{\rm SP}\right\}/v_2\left\{4\right\}$ are shown as red lines for the individual centrality classes. The error bars (open boxes) indicate statistical (systematic) uncertainties. The correlated uncertainties, related to the ZDC, and bin-to-bin uncorrelated ones are combined for $v_2\left\{\Psi_{\rm SP}\right\}$ (upper panels). For the ratio $v_2\left\{\Psi_{\rm SP}\right\}/v_2\left\{4\right\}$ (lower panels), the correlated uncertainty is shown by the grey band at unity.