TITLE: GCN CIRCULAR NUMBER: 30834 SUBJECT: GRB 210912A: CAHA 2.2m Detection DATE: 21/09/13 09:35:41 GMT FROM: Alexander Kann at IAA-CSIC <kann@iaa.es> D. A. Kann (HETH/IAA-CSIC), A. de Ugarte Postigo (HETH/IAA-CSIC, DARK/NBI), C. Thoene, M. Blazek, J. F. Agui Fernandez (all HETH/IAA-CSIC), and S. Gongora (CAHA) report: We observed GRB 210912A (Swift detection: Troja et al., GCN #30820) with CAFOS mounted on the 2.2m CAHA telescope at Calar Alto, Almeria, Spain. We obtained 7 x 60 s in the SDSS i' band under passing clouds, starting at 20:04:34 UT (9.056 hrs after the trigger; further observations at a later time were prevented by cloud cover). The afterglow (Hu et al., GCN #30823; Pankov et al., GCN #30825; Kuin & Troja, GCN #30832) is faintly detected in the stacked image. Compared to a nearby Pan-STARRS comparison star, we measure (AB mag) i' = 21.66 +/- 0.15 mag at 0.38146 d after the GRB, further confirming the decaying afterglow. This is still significantly brighter than the potential underlying host galaxy (Kann, GCN #30829).