Maximum intensity of the first and main emission of the Hα versus the corresponding shock velocity (red dots: 1990s, blue dots: 2015; see Table 1). Mean uncertainty of residual flux is 2%. The straight linear regression lines through the points between 110 ⩽ V_shock ⩽ 135 and 135 ⩽ V_shock ⩽ 150 are shown. The uncertainty on the shock velocity induced by using Eq. (2) is certainly not smaller than 10 km s⁻¹ when the shock velocity varies respectively from 100 to 150 km s⁻¹.