TITLE: GCN CIRCULAR NUMBER: 17831 SUBJECT: GRB 150518A: RATIR optical observations DATE: 15/05/20 06:28:43 GMT FROM: Owen Littlejohns at Az State U Owen Littlejohns (ASU), Nat Butler (ASU), Alan M. Watson (UNAM), Alexander Kutyrev (GSFC), William H. Lee (UNAM), Michael G. Richer (UNAM), Chris Klein (UCB), Ori Fox (UCB), J. Xavier Prochaska (UCSC), Josh Bloom (UCB), Antonino Cucchiara (ORAU/GSFC), Eleonora Troja (GSFC), Enrico Ramirez-Ruiz (UCSC), José A. de Diego (UNAM), Leonid Georgiev (UNAM), Jesús González (UNAM), Carlos Román-Zúñiga (UNAM), Neil Gehrels (GSFC), and Harvey Moseley (GSFC) report: We observed the field of GRB 150518A (Kawamuro, et al., GCN 17825) 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 2015/05 20.15 to 2015/05 20.24 UTC (29.94 to 32.08 hours after the BAT trigger), obtaining a total of 1.78 hours exposure in the r, i and z bands. For a source within the Swift-XRT error circle (Sbarufattim et al., GCN 17827), in comparison with the SDSS DR9, we obtain the following detections and upper limit (3-sigma): r 21.26 +/- 0.04 i 21.04 +/- 0.04 z > 20.08 These magnitudes are in the AB system and are not corrected for Galactic extinction in the direction of the GRB. This source is spatially consistent with the catalogued galaxy SDSS J153648.25+161946.9, as previously reported by Xinglong Station (Xu, et al., GCN 17829). We note that the RATIR detections in the r and i bands are brighter than the SDSS catalogue values, indicating that the GRB may indeed be within the galaxy, providing an excess flux. We also note that our detections are consistent with those from Xinglong Station. We thank the staff of the Observatorio Astronómico Nacional in San Pedro Mártir.