TITLE: GCN CIRCULAR NUMBER: 29858 SUBJECT: GRB 210420B: DDOTI Detection of the Afterglow DATE: 21/04/21 09:15:45 GMT FROM: Alan M Watson at UNAM Alan M. Watson (UNAM), Nat Butler (ASU), Rosa L. Becerra (UNAM), Simone Dichiara (GSFC/UMD), Alexander Kutyrev (GSFC/UMD), William H. Lee (UNAM), Océlotl Lopez (UNAM), Diego Gonzalez (UNAM), Margarita Pereyra (UNAM), Srihari Ravi (ASU), and Eleonora Troja (GSFC/UMD) report: We observed the field of the Swift/BAT GRB 210420B (Moss et al., GCN Circ. 29844) with the DDOTI wide-field imager at the Observatorio Astronómico Nacional on Sierra San Pedro Martir (http://ddoti.astroscu.unam.mx) from 2021-04-21 04:36 to 08:15 UTC (from 10.0 to 13.7 hours after the trigger) and obtained 10380 seconds of exposure in the w filter. We observed a region covering aproximately 7 degrees in RA and 10 degrees in declination (about 70 square degrees), including the XRT position. We calibrated our images against the APASS catalog. We detect a source at 16:57:17.97 +42:34:12.3 J2000 with an AB magnitude of w = 21.47 +/- 0.09. This source is outside the XRT error region reported by Moss et al. (GCN Circ. 29844), but is not present in the Pan-STARRS DR1 g image of the field and is consistent with the afterglow coordinates reported by Oksanen (GCN Circ. 29846), Lipunov et al. (GCN Circ. 29845), Leonini et al. (GCN Circ. 29851), and de Ugarte Postigo et al. (GCN Circ. 29852). We thank the staff of the Observatorio Astronomico Nacional in San Pedro Martir.