TITLE: GCN CIRCULAR NUMBER: 30757 SUBJECT: GRB 210820A: TSHAO optical upper limit DATE: 21/09/03 17:11:38 GMT FROM: Alexei Pozanenko at IKI, Moscow <apozanen@iki.rssi.ru> S. Belkin (IKI), I. Reva (FAP), A. Kusakin (FAP), A. Pozanenko (IKI), N.Pankov (HSE), V. Kim (FAP) report on behalf of GRB-IKI-FuN: We observed GRB 210820A (Beardmore et al., GCN 30664) with Zeiss-1000 telescope of Tien Shan Astronomical Observatory starting on Aug. 21 (UT) 21:32:02. The optical afterglow (Beardmore et al., GCN 30664; Lipunov et al., GCN 30666; Fu et al., GCN 30669; Zhu et al., GCN 30670; Butler et al., GCN 30672; Strausbaugh et al., GCN 30673; Blazek et al., GCN 30674; Laha et al., GCN 30675; LaPorte and Beardmore, GCN 30686; Komesh et al., GCN 30688; Guelbenzu et al., GCN 30695; Kann et al., GCN 30731) was not detected in the stacked image down to a limiting magnitude of 21.8 in R filter. Preliminary photometry of the field in a stacked image is following. Date UT start t-T0 Filter Exp. OT Err. UL(3 sigma) (mid, days) (s) 2021-08-21 21:32:02 1.17659 R 27*90 n/d n/d 21.8 The photometry is based on nearby USNO-B1.0 stars. USNO-B1.0 RA Dec R2 01:22:01.52088 +04:35:30.2028 17.48 01:22:14.95944 +04:43:31.2204 17.89