*5.1. Observing Conditions*

Low zenith angles, fast repointing, dark nights, low redshift, and highly energetic events have always been considered as optimal, if not necessary, conditions to have some chances for GRB detections with IACTs. On the other hand, these first VHE GRBs have demonstrated that GRBs can have a level of TeV emission large enough to be detected by the current generation of IACTs, even under non-optimal conditions. GRB 190114C was observed with a zenith angle > 55◦ and in the presence of the moon. Both conditions imply a higher energy threshold (typically & 0.2 TeV) and require a dedicated data analysis. Another example is GRB 160821B, that was observed with a NSB 2–8 times higher than the standard dark night conditions. Moreover, significant VHE excess was found not only in case of short delays (less than hundreds of seconds) from the burst trigger but, somewhat surprisingly, also at quite late times, i.e., with delays of several hours or even days, as in the case of GRB 180720B and GRB 190829A, respectively. This showed the importance of pointing a GRB also at relatively late times, in cases fast follow-up observations are not feasible.

Optimal observing conditions and short delays remain, however, crucial to detecting GRBs at a higher redshift, for which the impact of EBL is already large at a few hundreds GeV. This explains how the detection of a GRB has been possible at *z* = 1.1 (GRB 201216C): in this case, optimal observing conditions allowed us to reach a low energy threshold of the sensitivity window (∼70 GeV). The excess of signal was indeed found only below 200 GeV (more precisely, between 70 and 200 GeV) where the attenuation by the EBL is still limited.
