TITLE: GCN CIRCULAR NUMBER: 23017 SUBJECT: GRB 180720B: COATLI Optical Detection DATE: 18/07/22 20:52:09 GMT FROM: Alan M. Watson at Instituto de Astronomia UNAM Alan M. Watson (UNAM), Nat Butler (ASU), Rosa L. Becerra (UNAM), William H. Lee (UNAM), Carlos Román-Zúñiga (UNAM), Alexander Kutyrev (GSFC), and Eleonora Troja (GSFC) report: We observed the field of GRB 180720B (Siegel et al., GCN Circ. 22973) with the COATLI 50-cm telescope and interim imager at the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir (http://coatli.astroscu.unam.mx) from 2018-07-22 06:25 to 11:23 UTC (from 40.07 to 45.03 hours after the trigger), obtaining a total of 4.06 hours of exposure in the w filter. We detect the optical counterpart (Martone et al., GCN Circ. 22976) with w = 20.07 +/- 0.02 These magnitudes are calibrated against the USNO-B1 catalog (adjusted to an approximate AB system) and are not corrected for Galactic extinction in the direction of the GRB. We thank the COATLI technical team (Fernando Ángeles, Oscar Chapa, Salvador Cuevas, Alejandro Farah, Jorge Fuentes, Rosalía Langarica, Fernando Quirós, and Carlos Tejada) and the staff of the Observatorio Astronómico Nacional.