Continuum-subtracted Hα contours (blue) overplotted on the r-band image (grey) of the 4 × 1 deg² (1.1 × 0.3 Mpc²) strip of the core of the cluster north of M87. Given that the survey still does not reach its full sensitivity, contours are measured at different surface brightness levels: Σ (Hα) ≃ 10⁻¹⁸ erg s⁻¹ cm⁻² arcsec⁻² for NGC 4569 and IC 3583 (from Boselli et al. 2016a), Σ(Hα) ≃ 3 × 10⁻¹⁸ erg s⁻¹ cm⁻² arcsec⁻² for IC 3355, NGC 4388, and NGC 4531, and Σ(Hα) ≃ 6 × 10⁻¹⁸ erg s⁻¹ cm⁻² arcsec⁻² for M86, IC 3258, NGC 4402, NGC 4413, NGC 4425, and NGC 4438.