Visualisation of the determination of the continuum emission mask needed because of high continuum optical depth towards Sgr B2 (N1). From the channel-intensity distribution of the continuum-included spectra, the intensity at the peak T_peak and the maximum intensity T_max were determined. The maps in colour-scale and the contours show the difference T_max − T_peak normalised by T_max. This ratio was only computed when T_peak > 3σ, where σ is the average noise level in the continuum emission map. The contour steps (black) are 0.3, 0.4, and 0.5. The red dashed contour at a value of 0.43 indicates the boundary of the adopted mask. Green crosses show positions N1S-1 and N1S to the south and N1W-1 to the west. The observational setup and spectral window are shown in the lower right corner together with the respective beam sizes. Only those spectral windows are shown from which molecular transitions are used to create integrated intensity maps (cf. Figs. 3 and 4).