TITLE: GCN CIRCULAR NUMBER: 29254 SUBJECT: GRB 210104A: DOAO optical afterglow observations DATE: 21/01/05 03:31:37 GMT FROM: Gregory SungHak Paek at SNU Gregory S.H. Paek (SNU ARC/SNU), Myungshin Im (SNU ARC/SNU), Taewoo Kim, and Wonseok Kang (DOAO) on behalf of a larger collaboration We observed the optical counterpart of GRB 210104A (E. Troja et al., GCN #29233) with the 1-m class telescope in Deokheung Optical Astronomy Observatory (DOAO). We calibrated flux with the SDSS catalog and used an AB magnitude system. ---------------------------------------------------------- Filter Date-obs[UT] Exp.time[s] Mag Mag Error ---------------------------------------------------------- R 2021-01-04T13:37:29.100 300*4 18.34 0.07 I 2021-01-04T13:42:37.050 300*4 18.04 0.05 ---------------------------------------------------------- The magnitudes are not corrected for galactic extinction.