Velocity shifts in H₂ 1–0 S(1) between symmetric positions on either side of the jet axis, after averaging across 3 slicing mirrors with . The 1σ error bars include the fitting error due to noise and the 1 km s^-1 rms instrumental drifts from spaxel to spaxel. Blue lines illustrate the typical range of spurious instrumental velocity gradients along slicing mirrors, as measured from OH lines (see Sect. 2). Residual errors in absolute calibration and correction for uneven-slit illumination cancel out here.