2D histogram of 390 000 simultaneous spacecraft potential estimates V_ph versus V_z or V_f in 200 × 200 bins, corresponding to bin widths of ∼0.3 and 0.5 V, coloured by log₁₀ counts for the entire cometary mission. As V_ph is sensitive to noise, there are several artefacts at e.g. +22 V and −25 V, but there is otherwise a clear agreement, especially for large negative values as far as V_ph can measure. As the spacecraft potential approaches zero, V_f and V_z diverge non-linearly from V_ph, the latter being considered the better estimate for the spacecraft potential at these ranges. An empirical model was fitted to move from V_f and V_z to V_ph and is plotted in red.