TITLE: GCN CIRCULAR NUMBER: 23358 SUBJECT: GRB 181020A: LCO Cerro Tololo observations DATE: 18/10/21 15:54:44 GMT FROM: Cristiano Guidorzi at Ferrara U,Italy A. Cucchiara (U. Virgin Islands), C. Guidorzi, R. Martone (U. Ferrara), S. Kobayashi (LJMU), C.G. Mundell (U. Bath), A. Gomboc (U. Nova Gorica), I.A. Steele (LJMU), D. Morris (U. of Virgin Islands) on behalf of a large collaboration report: We observed Swift GRB 181020A (Moss et al. GCN 23349) with one of the LCO 1-m unit at Cerro Tololo Observatory on October 20, from 23:55 to Oct 21, 00:49 UT (corresponding to 4.9 to 5.8 hours from the GRB trigger time) with the SDSS r' and i' filters. We clearly detect the optical afterglow (Moss et al. GCN 23349) in both filters with the following magnitudes for the first half of the observation run: Mid time from GRB      Exposure         Filter          Magnitude trigger time (hrs)     (s) --------------------------------------------------------------------- 5.0                   5x120             SDSS r'         17.55 +- 0.03 5.2                   5x120             SDSS i'         17.2  +- 0.1 --------------------------------------------------------------------- as calibrated against nearby USNO UCAC4 objects.