The CGH, showing the highest energy photon versus redshift, based on Fermi-LAT ultra-clean events with PSF0 events removed (evclass = 512 and evtype = 56). Sources detected above 10 GeV are represented, with robust (spectroscopic) redshifts marked as filled circles; lower limit redshifts are represented by right arrows, with the reported redshift value corresponding to the centre of the arrow, and uncertain redshifts (photometric or reported as doubtful) indicated by blue crosses. The EBL optical depth |$\tau _{(E,z)} = 1.0$| is depicted for different models: Finke et al. (2010) (blue dashed line), Domínguez et al. (2011) (black dotted line), Franceschini & Rodighiero (2017) (grey line), and Saldana-Lopez et al. (2021) (magenta dot–dashed line). For better visualization, the photon energy of sources with robust redshift is colour-coded. Note that approximately one-third of the 1CGH sources lack assigned redshift and are therefore not included in this plot.