TITLE: GCN CIRCULAR NUMBER: 22460 SUBJECT: GRB 180305A: RATIR Optical Afterglow Candidate DATE: 18/03/06 11:24:41 GMT FROM: Eleonora Troja at GSFC Eleonora Troja (GSFC), Nat Butler (ASU), Alan M. Watson (UNAM), Alexander Kutyrev (GSFC), William H. Lee (UNAM), Michael G. Richer (UNAM), Ori Fox (STScI), J. Xavier Prochaska (UCSC), Josh Bloom (UCB), Antonino Cucchiara (UVI), 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 the LAT GRB 180305A (Axelsson, et al., GCN 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 6.11 to 2018/03 6.20 UTC (17.28 to 19.45 hours after the burst), obtaining a total of 1.36 hours exposure in the r and i bands. At the position of source #2 found by Swift-XRT  (Evans, et al., GCN 22459), we detect a faint uncatalogued optical source. In comparison with the USNO-B1 catalog, we obtain the following detections:   r    = 21.95 +/- 0.12   i    = 21.48 +/- 0.08 These magnitudes are in the AB system and are not corrected for Galactic extinction in the direction of the GRB. The source is located at RA,Dec = 03:18:28.33,+32:06:36.2 (J2000, +/-0.5"). Further observations to establish fading of the optical source are planned. We thank the staff of the Observatorio Astronómico Nacional in San Pedro Mártir.