Upper panel: comparison between RVs estimated from spectra obtained with the HR13 and HR11 grisms. Different symbols correspond to different clusters. The continuous line is drawn at a typical value for the mean difference (ΔV_r =  −0.5 km s^-1), and the dotted lines enclose the  ± 2σ range about the mean, where a typical value of σ = 0.7 km s^-1 has been adopted (see Table 1). Lower panel: comparison with the external dataset by L11 for two representative clusters, NGC 104 (filled circles) and NGC 6218 (empty circles). The continuous line marks ΔV_r = 0.0 km s^-1, the dotted lines enclose the  ± 2σ range. The fraction of stars lying within  ± 2σ is also reported [F(Δ < 2σ)].