This study presents a deep Hα kinematical analysis of the Sculptor Group galaxy NGC 253. The Fabry–Perot data were taken with the 36-cm Marseille Telescope in La Silla, Chile, using an EMCCD detector. Typical emission measures of ∼0.1 cm−6 pc are reached. The observations allow the detection of the diffuse ionized gas component through [N ii] emission at very large radii of 11.5, 12.8 and 19.0 arcmin, on the receding side of the galaxy. No Hα emission is observed at radii larger than the neutral component (11.5 arcmin). The very extended rotation curve confirms previous results and shows signs of a significant decline, on the order of 30 per cent vmax. Using the rotation data, mass models are constructed with and without the outer [N ii] data points, and similar results are found. The declining part of the rotation curve is very well modelled, and seems to be truly declining.