Top panel: spatially averaged bisector velocity (SABV) for the 630.25 nm line in the observations (solid lines), the HD-FR (solid black line) and the HD-SPAT-SPEC (dashed lines). Bottom panel: comparison between SABVs and bisector velocities of average profiles (BVAP) for the 630.25 nm line. Solid/dash-dotted black lines: SABV/BVAP in the HD-FR. Solid red/blue lines: SABV/BVAP for the SP data. Red/blue dashed lines: SABV/BVAP for the simulation degraded to SP resolution. The purple line gives the bisector in the FTS atlas profile. Positive velocities correspond to redshifts relative to the average velocity at 50% line depression.