Upper panel: strength of the WDH as a function of time extracted from the 51 Eri data taken with SPHERE (at 1 μm) on September 25, 2015, starting at 8h19m40s and ending at 9h35m25s (UTC). Lower panel: measured turbulence coherence time τ₀ as a function of time during the 51 Eri observation sequence. The AO-telemetry data are from SPHERE-SPARTA (red solid lines) and the atmospheric profiling data are from the MASS–DIMM (blue solid lines). In both cases, the full data are shown by the thin line, and the thick line shows the data interpolated at the image rate (black diamonds in the upper panel). In both panels, the meridian crossing of the target star is indicated by the gray dot-dashed line.