Ca II IRT cross-correlation results using several atmospheric models computed under different assumptions (each column). Top panels: K_p-velocity map. The colour bar indicates the significance (S/N) of the measurement. The white-dashed lines indicate the expected K_p = 196.52 km s⁻¹. Bottom panels: cross-correlation result at the predicted K_p value. The blue-dashed lines show the 0 km s⁻¹ radial velocity and the grey-dotted lines the ±3σ significance level. The red-dotted horizontal lines show the significance of the peak, obtained by fitting a Gaussian profile to the transmission signal.