TITLE:   GCN CIRCULAR
NUMBER:  29894
SUBJECT: GRB 210420B: Zeiss-1000 of Koshka optical observations
DATE:    21/04/23 18:31:49 GMT
FROM:    Alexei Pozanenko at IKI, Moscow  <apozanen@iki.rssi.ru>

N. Pankov (HSE, IKI), S. Naroenkov (INASAN), A. Pozanenko (IKI),  S. 
Belkin (IKI), M. Ibrahimov (INASAN)  report  on behalf of IKI GRB FuN:

We observed the field of  GRB 210420B (Moss et al., GCN 29833; Lien et 
al., GCN 29854; Osborne et al. GCN 29876; Stamatikos et al., GCN29879) 
with Zeiss-1000 telescope of Koshka observatory (INASAN). We marginally 
detect the optical afterglow  (Lipunov et al., GCN 29845; Oksanen et 
al., GCN 29846; Hu et al., GCN 29847; Tianrui Sun et al., GCN 29848; 
Leonini et al., GCN 29851; de Ugarte Postigo et al., GCN 29852, 29853; 
Watson et al., GCN 29858; Zheng et al., GCN 29861; Breeveld et al., GCN 
29862; Stecklum et al., GCN 29863; Butler et al.,GCN 29864; Jelinek et 
al., GCN 29866; Watson, GCN 29867; Leonini et al., GCN 29872; Guelbenzu 
et al., GCN 29890).

Preliminary photometry of the afterglow is following

Date       UT start   t-T0       Filter Exp.    OT    Err.  UL(3sigma)
                      (mid, days)        (s)
2021-04-21  20:02:55    15*180    1.03507 R    21.0   0.3  21.0
2021-04-22  20:16:45    24*180    2.05405 R     n/d   n/d  20.5


The photometry is based on the nearby USNO-B1.0 stars
USNO_B1.0_id R2
1325-0345119 16.12
1326-0349499 16.57