Visualization of the moving-ring filter operation used to estimate the emission spectrum, T_B(ν), at the positions affected by absorption features. The left panel shows a single plane (ν_lsr = 45.5 km s^-1) of the VGPS data cube of W 51 located in the center of the map. In the middle panel the filtered plane is shown. The size of the ring filter is marked with the two black circles in the upper left. The right panel contains the residual image. It is used to estimate the error of the emission spectrum by calculating the RMS. To improve the RMS calculation, a mask was applied (see text). The mask is indicated by the black contours.