Top left: ratio map of [OIII] λ5007/Hβ with the [OIII] λ5007 contours overplotted as reference and the position of CAL 83 is noted with a white star. The colour range is 0.05–6.5.Top right: ratio map of [SII] λλ6716,6731/Hα with the Hα contours overplotted. The colour range is 0.15–1.2. The white shapes show the regions from which we measured line ratios: the triangle corresponds to the faint nebula region, the oval to the central region and the L-shape to the He II region.Bottom left: ratio map of [NII] λ6583/Hα with the Hα contour overplotted. The colour range is 0.05–0.3.Bottom right: ratio map of [OI] λ6300/Hα overplotted by the Hα contours. The colour range is 0.03–0.47. The colour bar represents the normalised colour range. All maps are Gaussian-smoothed over a 3    ×    3 pixel box. E is up and N is right and the FoV is 25.5′′ ×  25.5′′ (7.5    ×    7.5 pc²).