TITLE:   GCN CIRCULAR
NUMBER:  23124
SUBJECT: GRB 180809B: CrAO optical observations
DATE:    18/08/10 19:50:36 GMT
FROM:    Alexei Pozanenko at IKI, Moscow  <apozanen@iki.rssi.ru>

E. Mazaeva (IKI), V. Rumyantsev (CrAO),  A. Pozanenko (IKI),   A. 
Volnova (IKI), report on behalf of larger GRB follow-up collaboration:

We observed the field of the GRB 180809B (Moss  et al., GCN 23105)
with ZTSH 2.6m telescope of CrAO observatory starting on Aug. 9 (UT) 
21:14:58. We took several images in BVRI filters  with exposures of 120 
s. We  detect  the afterglow (e.g., Moss et al., GCN 23105; Lipunov et 
al., GCN 23109; Schweyer et al. GCN 23111; Guidorzi et al., GCN 23112, 
Tiurina et al., GCN 23113).
Preliminary photometry of the afterglow in R- filter is following

UT start              t-T0    Filter  Exp.     OT    err
                      (mid, days)       (s)

21:14:58.579          0.03276 R       1*120    19.80 0.05
21:17:05.489          0.03423 R       1*120    19.86 0.04
21:19:12.801          0.03570 R       1*120    19.93 0.05
21:25:54.597          0.04035 R       1*120    20.35 0.05
21:34:27.347          0.04629 R       1*120    20.86 0.08
21:43:00.178          0.05519 R       2*120    21.51 0.11

The photometry is based on nearby  USNO-B1.0 stars:

Ref.stars	
USNO-B1.0_id	R2
0747-0765959	19.65
0746-0776364	18.76
0746-0776499	19.18