Convergence behaviour of the standard spherically symmetric model calculated with the 3D FVM (blue) and the 3D SC method using linear (red) or Bézier (green) interpolations. Left and right panels: convergence behaviour of the continuum and line transfer for ϵ_C = ϵ_L = 10⁻⁶, respectively. While the upper row displays the optically thin models with k_C = 10 and k_L = 10, the lower row has been calculated using k_C = 100 and k_L = 10⁵. Different acceleration techniques have been applied, where the NN-ALO is implemented only for the SC method.