TITLE: GCN CIRCULAR NUMBER: 30187 SUBJECT: GRB 210610B: SAO RAS optical observations DATE: 21/06/11 03:16:36 GMT FROM: Moskvitin Alexander at SAO RAS A. S. Moskvitin (SAO RAS) on behalf of the GRB follow-up team report. We observed the field of the GRB 210610B with the SAO RAS 1-m telescope Zeiss-1000 + CCD-photometer in BVRc Johnson-Cousins filters. The observations started on June 10, 20:21:53 UT, since half an hour after the burst detection by the Swift/BAT (Page et al., GCN #30170). We clearly detected the OT (Page et al., GCN #30170; Kumar et al., GCN #30174; Rumyantsev et al., GCNs #30175, #30178; Hu et al., GCN #30177; de Wet et al., GCN #30180; Romanov, GCN #30181; Fynbo et al., GCN #30182) with the following brightness: UT_mid T_mid-T0, h Exp., s Rc mag (+/- 0.01 mag) 20:22:03 0.510 20 16.15 20:32:40 0.687 60 16.43 20:48:12 0.946 60 16.75 21:19:20 1.448 60 17.13 21:28:10 1.612 60 17.22 21:53:21 2.032 60 17.26 22:13:03 2.360 60 17.19 22:36:03 2.743 180 17.17 23:04:20 3.215 120 17.20 23:52:38 4.020 120 17.19 The preliminary photometry is based on the USNO-A2.0 star from GCN #30178 (Rumyantsev et al.). Magnitudes were not corrected for MW extinction.