The CGH, showing the ‘largest energy bin detectable with Fermi-LAT’ versus redshift. The ‘largest energy bin’ is calculated as the mean energy of the four highest energy source-type events associated with each source, excluding PSF0 events (i.e. evclass = 128 and evtype = 56). The opacity regimes for EBL optical depths |$\tau _{(E,z)} = 1.0$| and |$\tau _{(E,z)} = 0.1$| are represented by grey dashed and magenta dot–dashed lines, respectively, corresponding to the Saldana-Lopez model (Saldana-Lopez et al. 2021). This plot highlights sources in the |$\tau _{(E,z)} > 0.1$| regime where absorption may be detectable at the highest energy bin observed with Fermi-LAT, while sources in the |$\tau < 0.1$| regime are represented by grey markers. Different markers indicate the quality of the redshift determination: robust (spectroscopic) redshifts are shown 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 unknown quality) are represented by blue crosses. Approximately one-third of the 1CGH sources lack redshift assignments and are therefore not included in this plot. Note: The |$\tau _{(E,z)}$| values regarding the largest energy bin are listed in the 1CGH catalogue to help identify interesting targets.