Upper panel: blurred image used for the detection of pronounced Hα regions. Lower panel: original CTIO image of NGC 5236. Overplotted in each panel are the estimated locations of the arms. In the lower panel the two fields observed with the HST/ACS are marked as yellow regions. Green dot-dashed line is the corotation radius located at 170′′ (Lundgren et al. 2004). Dash-dotted lines in blue denote annuli at 2000, 1400 and 1100, see text for details.