TITLE: GCN CIRCULAR NUMBER: 29238 SUBJECT: GRB 210104A: Assy optical observations DATE: 21/01/04 14:22:05 GMT FROM: Alexei Pozanenko at IKI, Moscow V. Kim (FAI, Pulkovo Observatory), A. Pozanenko (IKI), M. Krugov (FAI), S. Belkin (IKI) report on behalf of GRB-IKI-FuN: We observed the GRB 210104A (Fermi GBM team, GCN 29232; Troja et al., GCN 29233) with AZT-20 telescope of Assy-Turgen observatory starting on Jan. 04 (UT) 12:55:33 in r'-filter. We detect the optical afterglow (Troja et al., GCN 29233; Xin et al., GCN 29235; Hu et al., GCN 29236; Hosokawa et al., GCN 29237). Preliminary photometry of the afterglow obtained in a few initial imaging is following Date UT start t-T0 Filter Exp. OT Err. UL (3sigma) (mid, days) (s) 2021-04-01 12:55:33 0.0632 r'(AB) 5*60 17.97 0.06 22.7 The photometry is based on nearby stars of PanSTARRS-PS1. Observations is continuing.