Two examples of the procedure followed to derive H_α maps. Upper panels: MACS 1423-00446; bottom panels: MACS 0717-00596. (a) and (b) refer to the two distinct PAs of the same galaxy. Panels (1a) and (1b) show the rest-frame flux-calibrated galaxy 2D spectra, after the sky background and the contamination have been subtracted. The dashed horizontal black lines show the y-position of the continuum. The box shows the position of the emission lines. Panels (2a) and (2b) show the rest-frame 2D spectra after the continuum has also been subtracted. Panels (3a) and (3b) show the map of the galaxy in the light of the Hα line. Panels 4 show the two maps overplotted as contours on an image of the galaxy in the FW475 filter. Blue and red lines represent different levels of ΣSFR, as described in Figure 5. The arrows indicates the direction of the cluster center. See text for details.