TITLE: GCN CIRCULAR NUMBER: 23024 SUBJECT: GRB 180720B: D50 optical observations DATE: 18/07/23 20:21:57 GMT FROM: Jan Strobl at AI AS CR,Ondrejov M. Jelinek, J. Strobl, R. Hudec, C. Polasek (ASU CAS Ondrejov) report: We observed the position of the bright GRB 180720B (Siegel et al., GCN 22973 & GCN 22975) with the D50 telescope of the Astronomical Institute Ondrejov, near Prague, Czech Republic. We performed a series of ~300x 20s unfiltered exposures as soon as the position became accessible above local horizon, between 9.8 and 12h after the trigger. The optical afterglow (Martone et al., GCN 22976; Sasada et al., GCN 22977; Reva et al., GCN 22979; Itoh et al., GCN 22983) is clearly detected in single 20 s images. We confirm the stationary behaviour during our observations as reported by Kann et al. (GCN 22985) - the afterglow might have faded as few as 0.04 mag between 10 and 11h after the GRB trigger.