TITLE: GCN CIRCULAR NUMBER: 17780 SUBJECT: GRB 150428B: MASTER-net prompt optical observation DATE: 15/04/28 15:33:45 GMT FROM: Vladimir Lipunov at Moscow State U/Krylov Obs D.Buckley, S. Potter, A.Kniazev, M.Kotze South African Astronomical Observatory E. Gorbovskoy, V. Lipunov, N.Tyurina, V.Kornilov, P.Balanutsa, A.Kuznetsov, D.Kuvshinov, Lomonosov Moscow State University, Sternberg Astronomical Institute K.Ivanov, S.Yazev, N.M.Budnev, O.Gres, O.Chuvalaev, V.A.Poleshchuk Irkutsk State University A. Tlatov, A.V. Parhomenko, D. Dormidontov, V.Sennik Kislovodsk Solar Station of the Pulkovo Observatory V.Yurkov, Yu.Sergienko, D.Varda, E.Sinyakov Blagoveschensk Educational State University, Blagoveschensk V.Krushinski, I.Zalozhnih, A. Popov Ural Federal University, Kourovka Hugo Levato and Carlos Saffe Instituto de Ciencias Astronomicas, de la Tierra y del Espacio (ICATE) Claudio Mallamaci, Carlos Lopez and Federico Podest Observatorio Astronomico Felix Aguilar (OAFA) MASTER II robotic telescope (MASTER-SAAO: http://observ.pereplet.ru) located in SAAO was pointed to the GRB150428B 24 sec after notice time and 59 sec after trigger time at 2015-04-28 03:13:03 UT in two polarizations. On our first (10s exposure) set we haven`t found optical transient within SWIFT BAT (Page et. al GCN 17766) and XRT (Beardmore et al. GCN 17773) error-boxes . We have 3x2 images during Gamma Ray activity (see Page et al., GCN 17777) in two polarizations. So our observations is prompt observations. We obtain a followin upper limits on single and coadd images: Date start T_start -T_trig T_mid-T_trig Expt. Limit Coadd 03:13:03.3 59 64 10 16.5 no 03:13:03.3 59 97 50 17.4 3 03:13:03.3 59 529 810 19.3 12 Data analysis was hampered by a high density of stars in the error box. XRT position covered by bright (~15m) star USNO B1 0941-0468603. The message may be cited.