Extended [OIII] 5007 emission region south-east of M 31 from D2023. Hα isocontours from Finkbeiner (2003, VTSS) are over-plotted in log scale (in red). These contours are described in Fig. 3. The two yellow rectangles indicate the MISTRAL long-slit positions, the one called “onset” is located to the south, and the comparison rectangle, called the “offset” slit, lies to the north, where both Hα and [OIII] emissions are fainter. Their widths are enlarged by a factor 10 for visibility. North is up and east left. The displayed FoV is ∼5.1° × 1.6°.