Schematic view showing the method used to compute the brightness distribution of the dust jet. Left panel (Fig. 1c): the original image. Right panel (computational steps): the comet nucleus is masked from the center to the cut radius. The background brightness distribution that excluded the jet and artificial regions (the ghost formed because of the WAC optics) is calculated in a ring with a certain width. A specific narrow region covering the dust jet is carved out, and the jet value is obtained by background subtraction.