TITLE:   GCN CIRCULAR
NUMBER:  21189
SUBJECT: GRB 170531B: ISON/Terskol optical observations
DATE:    17/06/02 15:22:42 GMT
FROM:    Alexei Pozanenko at IKI, Moscow  <apozanen@iki.rssi.ru>

E. Mazaeva (IKI),   A. Mokhnatkin (KIAM),  A. Pozanenko (IKI), A. 
Volnova (IKI),   I. Molotov (KIAM)  report on behalf of  larger GRB 
follow-up collaboration:

We observed the field of GRB 170531B (Evans et al.,GCN  21171) with 
K-800 (0.8m) telescope  of ISON/Terskol   observatory starting  on May, 
31 (UT) 22:21:31.  We obtained several unfiltered images of 40 s 
exposure. The optical afterglow (Lipunov et al., GCN 21173 Pozanenko et 
al.  GCN 21174)  is clearly visible in combined images. Preliminary 
photometry of the afterglow is following


Date       UT start  t-T0   Filter   Exp.  OT   Err. UL
                      (mid, days)     (s)

2017-05-31 22:21:31 0.01810 CR     18*40  18.90 0.10 21.0
2017-05-31 22:35:41 0.02949 CR     21*40  19.10 0.10 21.0

Photometry is based on nearby USNO-B1.0 stars
USNO-B.1_id R2
0736-0718845 16.77
0735-0729626 17.35
0735-0729622 16.96
0735-0729439 16.59
0735-0729327 17.22