Top panel: photon–photon absorption correction (exp[τ_γγ]) for the source MKN 501 at z = 0.034 based on the EBL model by FR2017. Bottom panel: observed (open black) and absorption-corrected (filled red) spectral data. Data are taken from Aharonian et al. (2001); see also Aharonian et al. (2006c). The intrinsic spectrum is fitted by a broken power-law with relatively flat photon spectral indices, as indicated in the insert. The energy-binned sensitivity limits for 50-h 5σ and 100-h 2σ of CTA South are also shown.