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