TITLE: GCN CIRCULAR NUMBER: 18464 SUBJECT: GRB 151023A: RATIR Observations DATE: 15/10/24 03:59:24 GMT FROM: Nat Butler at UC berkeley Nat Butler (ASU), Alan M. Watson (UNAM), Alexander Kutyrev (GSFC), William H. Lee (UNAM), Michael G. Richer (UNAM), Ori Fox (UCB), J. Xavier Prochaska (UCSC), Josh Bloom (UCB), Antonino Cucchiara (GSFC/STScI), Eleonora Troja (GSFC), Owen Littlejohns (ASU), Enrico Ramirez-Ruiz (UCSC), José A. de Diego (UNAM), Leonid Georgiev (UNAM), Jesús González (UNAM), Carlos Román-Zúñiga (UNAM), Neil Gehrels (GSFC), Harvey Moseley (GSFC), John Capone (UMD), V. Zach Golkhou (ASU), and Vicki Toy (UMD) report: We observed the field of GRB 151023A (D'Avanzo, et al., GCN 18450) 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 2015/10 24.09 to 2015/10 24.14 UTC (12.35 to 13.66 hours after the BAT trigger), obtaining a total of 1.07 hours exposure in the r, i, and z bands. We detect the GROND source (Knust, et al., GCN 18461) at a similar brightness level. In comparison with the USNO-B1 catalog, we find: r 20.27 +/- 0.04 i 19.10 +/- 0.02 z 18.29 +/- 0.09 These magnitudes are in the AB system and are not corrected for Galactic extinction in the direction of the GRB. We note that this source is present in the DSS at a comparable brightness level and may not, therefore, be the GRB afterglow. We thank the staff of the Observatorio Astronómico Nacional in San Pedro Mártir.