TITLE: GCN CIRCULAR NUMBER: 30988 SUBJECT: GRB 210610B: Sintesz-Newton/CrAO optical observations DATE: 21/10/25 12:07:05 GMT FROM: Alexei Pozanenko at IKI, Moscow <apozanen@iki.rssi.ru> N. Pankov (HSE), S. Nazarov (CrAO), A. Pozanenko (IKI), S. Belkin (IKI, HSE) report on behalf of IKI GRB FuN: We observed the GRB 210610B (Page et al., GCN 30170) with Sintesz-Newton 350mm f/5 telescope of CrAO observatory. Observation started on 2021-06-10 (UT) 22:07:10, and continued on 2021-06-11 (UT) 20:22:38, 2021-06-12 (UT) 20:39:03, and 2021-06-13 (UT) 19:38:28. The series consists of images with an exposure of 300 s in a Clear filter. The optical afterglow first reported by UVOT team (Page et al., GCN 30170) is clearly detected in each epoch. Preliminary photometry of the stacked images is following Date UT start t-T0 Filter Exp. OT err UL(3) (mid, days) (s) 2021-06-10 22:07:10 0.10293 Clear 5*300 17.20 0.03 20.5 2021-06-10 n/d 0.12658 Clear 5*300 17.28 0.08 21.1 2021-06-10 n/d 0.14409 Clear 5*300 17.28 0.06 21.1 2021-06-10 n/d 0.16159 Clear 5*300 17.29 0.06 21.1 2021-06-11 20:22:38 1.02165 Clear 8*300 19.58 0.10 21.2 2021-06-11 20:50:32 1.04103 Clear 6*300 19.50 0.09 21.2 2021-06-11 21:33:21 1.07077 Clear 17*300 19.69 0.09 21.5 2021-06-12 20:39:03 2.03479 Clear 14*300 20.57 0.19 21.5 2021-06-13 19:38:28 3.02571 Clear 20*300 21.26 0.17 21.8 The photometry is based on nearby USNO-B1.0 (R2) stars USNO_B10-1043-00282648 USNO_B10-1043-00282621