TITLE: GCN CIRCULAR NUMBER: 22465 SUBJECT: GRB 180305A: RATIR Optical Observations DATE: 18/03/07 14:59:49 GMT FROM: Alan M. Watson at Instituto de Astronomia UNAM Alan M. Watson (UNAM), Nat Butler (ASU), Alexander Kutyrev (GSFC), William H. Lee (UNAM), Michael G. Richer (UNAM), Ori Fox (STScI), J. Xavier Prochaska (UCSC), Josh Bloom (UCB), Antonino Cucchiara (UVI), Eleonora Troja (GSFC), Owen Littlejohns (ASU), Enrico Ramirez-Ruiz (UCSC), Jesús González (UNAM), Carlos Román-Zúñiga (UNAM), Harvey Moseley (GSFC), John Capone (UMD), V. Zach Golkhou (U. Wash.), and Vicki Toy (UMD) report: We observed the field of GRB 180305A (Axelsson, et al., GCN Circ. 22457) with the Reionization and Transients Infrared Camera (RATIR; www.ratir.org) on the 1.5m Harold Johnson Telescope at the Observatorio Astronómico Nacional on Sierra San Pedro Mártir from 2018/03 7.11 to 2018/03 7.20 UTC (41.30 to 43.46 hours after the BAT trigger), obtaining a total of 1.64 hours exposure in the r and i bands. The source reported by Troja et al. (GCN Circ. 22460) is still detected, but has faded to: r = 23.15 +/- 0.21 i = 22.80 +/- 0.15 These magnitudes are in the AB system, are calibrated against the USNO-B1 catalog, and are not corrected for Galactic extinction in the direction of the GRB. The combination of the optical fading reported here and the X-ray fading reported by D’Avanzo et al. (GCN Circ. 22462) confirm that this is the afterglow of the GRB. We thank the staff of the Observatorio Astronómico Nacional in San Pedro Mártir.