TITLE: GCN CIRCULAR NUMBER: 9830 SUBJECT: GRB 090820: MASTER-Net optical polarization observations DATE: 09/08/20 16:35:57 GMT FROM: Vladimir Lipunov at Moscow State U/Krylov Obs E. Gorbovskoy, V. Lipunov, V.Kornilov, A.Belinski, N.Shatskiy, N.Tyurina, D.Kuvshinov, P.Balanutsa, V.V.Chazov, P.V.Kortunov, A.Kuznetsov Sternberg Astronomical Institute, Moscow State University K.Ivanov, S.Yazev Irkutsk State University A. Tlatov, A.V. Parhomenko, Kislovodsk Solar Station of the Pulkovo Observatory V.Krushinski, I.Zalognih, T.Kopytova Ural State University, Kourovka There are 6 MASTER Very Wide Field cameras located at Kislovodsk and Irkutsk with common FOW = 2000 + 1680 square degrees (http://observ.pereplet.ru/). Four MASTER Very Wide Field Cameras located at Kislovodsk (4 Carl Zeiss Planar T* 85/1.4 ZF, 4x420 square degrees, 11 Mpx, 22" per pix with 4 polarizers) responded to the bright GRB 090820 (Fermi TRIGGER_NUM:272421498, V. Connaughton, GCN 9829), producing images beginning 9 s after the GRB Time (first alert notice). We took 1300 images with 5 sec exposition in different polarizations. Individual images have limiting magnitudes~11. Our unfiltered images are calibrated relative to Tycho stars (V) and very close to V band. Other observations are required for independ error box elaboration. The reduction is continued. The message may be cited. mailto: lipunov@sai.msu.ru