[S ii]/Hα flux-ratio image created out of the flux-calibrated, continuum-subtracted MCELS Hα and [S ii] data (see Fig. 3). A circular structure is visible, indicated by the dashed black circle. The north-eastern part of this structure has a [S ii]/Hα flux ratio greater than 0.67 and thus seems to be shock-ionised. Therefore, this structure is indicative of an SNR. The position of the Hα cavity is marked in white. The flux ratio contours are shown with contour levels at 0.46 and 0.67. The area of the sky is the same as in Fig. 3.