Spline fit to the Si II feature of 2020bvc. The wavelength at which the spline fit reaches its minimum value is used in the Doppler formula to compute the expansion velocity of the feature. The ‘input spectrum’ (shown in grey) was generated by adding random noise to the SG-filtered observed spectrum (i.e. the reduced 1D spectrum from the source catalogue). The solid orange line represents the ‘smoothed spectrum’, created by applying an SG filter to the input spectrum. A solid red line illustrates the ‘spline fit’ applied to the smoothed spectrum between the wavelengths λ_blue and λ_red. The wavelength at which the spline fit reaches its minimum was taken as the observed wavelength of the feature and is denoted as λ_obs (purple line).