TITLE: GCN CIRCULAR NUMBER: 29935 SUBJECT: Fermi GRB 210503A: DDOTI Upper Limits on the Afterglow DATE: 21/05/04 22:48:23 GMT FROM: Emma Margarita Pereyra Talamantes at IA-UNAM Ensenada Margarita Pereyra (UNAM), Nat Butler (ASU), Alan M. Watson (UNAM), Eleonora Troja (GSFC/UMD), Simone Dichiara (GSFC/UMD), Rosa L. Becerra (UNAM), William H. Lee (UNAM), Oc��lotl L��pez, Diego Gonzalez (UNAM), Alexander Kutyrev (GSFC/UMD), and Srihari Ravi (ASU), report: We observed the field of the likely long Fermi GRB 210503A (trigger 641763254, GCN #29927, Fermi GBM Team) with the DDOTI/OAN wide-field imager at the Observatorio Astron��mico Nacional on Sierra San Pedro M��rtir ( http://ddoti.astroscu.unam.mx) on 2021-05-04 from 07:02 to 11:38 UTC (11.5 to 16 hours after the event). We observed a region of 20.4 degrees in RA by 20.4 degrees in declination, with a 3 x 2 grid, centered on the Fermi GBM Final Position 328.93d +66.03d (J2000). This region contains 6 instrumental fields or about 400 square degrees. We obtained 1560 seconds of exposure per instrumental field in the w filter. We obtained AB photometry by calibration against the APASS catalog. We detect no likely candidate for the afterglow to our 10-sigma upper limits of w = 19.32 to 19.93 (interquartile range). We thank the staff of the Observatorio Astron��mico Nacional in San Pedro M��rtir. --