TITLE: GCN CIRCULAR NUMBER: 29861 SUBJECT: GRB 210420B: KAIT Optical Observations DATE: 21/04/21 14:39:40 GMT FROM: Weikang Zheng at UC Berkeley WeiKang Zheng and Alexei V. Filippenko (UC Berkeley) report on behalf of the KAIT GRB team: The 0.76-m Katzman Automatic Imaging Telescope (KAIT), located at Lick Observatory, responded to the Swift GRB 210420B (Moss et al., GCN 29844) starting at ~0.61 days after the Swift trigger (Lien et al. GCN 29854). A total of 30x60s images were obtained in the clear (roughly R) filters. We detect the optical afterglow (Lipunov et al., GCN 29845; Oksanen, GCN 29846; Hu et al., GCN 29847; Sun et al., GCN 29848; Leonini et al., GCN 29851; de Ugarte Postigo et al., GCN 29852; Watson et al., GCN 29858) in the coadd image with a mag of 19.9 +/- 0.2, calibrated to the Pan-STARRS1 catalog.