TITLE: GCN CIRCULAR NUMBER: 29866 SUBJECT: GRB 210420B: Ondrejov D50 optical afterglow detection DATE: 21/04/21 15:32:16 GMT FROM: Martin Jelinek at Astro.Inst-AVCR,Ondrejov M. Jelinek, J. Strobl, R. Hudec, C. Polasek (ASU CAS Ondrejov) report: We observed the position of the bright GRB 210420B (Moss et al., GCN 29833 & Lien et al., GCN 29854) with the D50 telescope of the Astronomical Institute Ondrejov, near Prague, Czech Republic. We performed a series of 120 s unfiltered exposures starting 21:10 UT, between 2.6 and 4.9 h after the trigger. The optical afterglow (Lipunov et al., GCNC 29845; Oksanen et al., GCNC 29846; Hu et al., GCNC 29847; Tianrui Sun et al., GCNC 29848; Leonini et al., GCNC 29851; de Ugarte Postigo et al., GCNC 29852; de Ugarte Postigo et al., GCNC 29853; Watson et al., GCNC 29858; Zheng et al., GCNC 29861; Breeveld et al., GCNC 29862; Stecklum et al., GCNC 29863 and Butler et al., GCNC 29864) is clearly detected in single 120 s images, at least at the beginning of the series. We can confirm a decaying nature of the object - the afterglow faded > 0.4 mag during the course of our observations, providing an alpha decay of 0.69 +- 0.17.