TITLE: GCN CIRCULAR NUMBER: 21044 SUBJECT: GRB 170428A: MASTER-OAFA early optical observations DATE: 17/04/28 15:19:42 GMT FROM: Vladimir Lipunov at Moscow State U/Krylov Obs R.Podesta, C.Lopez, F. Podesta Observatorio Astronomico Felix Aguilar (OAFA) , National University of San Juan, Argentina H. Levato, C. Saffe Instituto de Ciencias Astronomicas,de la Tierra y del Espacio (ICATE), San Juan, Argentina V. Lipunov, E. Gorbovskoy, V.Kornilov, N.Tyurina, D.Kuvshinov, A.V.Krylov, I.Gorbunov, P.Balanutsa, A.Kuznetsov, V.V.Chazov, D. Vlasenko Lomonosov Moscow State University, Sternberg Astronomical Institut of MSU K.Ivanov, S.Yazev, N.M.Budnev, O.Gres, O.Chuvalaev, V.A.Poleshchuk, O. Ershova Irkutsk State University V.Yurkov, Yu.Sergienko Blagoveschensk Educational State University, Blagoveschensk A. Tlatov, V.Senik, A.V. Parhomenko, D. Dormidontov Kislovodsk Solar Station of the Pulkovo Observatory D.Buckley, S. Potter, A.Kniazev, M.Kotze South African Astronomical Observatory R. Rebolo, M. Serra, N. Lodieu, G. Israelian, L. Suarez-Andres The Instituto de Astrofisica de Canarias MASTER II robotic telescope (MASTER-Net: http://observ.pereplet.ru) located in OAFA was pointed to the GRB170428.38 17 sec after notice time and 31 sec after trigger time at 2017-04-28 09:14:14 UT. On our first (10s exposure) set we haven`t found optical transient within SWIFT error-box (ra=330.062 dec=26.9172 r=0.05). The 5-sigma upper limit has been about 16.5 mag The observations made on zenit distance = 72 degrees, galaxy latitude b = -22 degree. The moon ( 5 % bright part) below the horizon (The altitude of the Moon is -49 degree ). The sun altitude is -24.2 degree. The object can be observed till sunrise at 2017-04-28 11:06:50 The message may be cited.