GRB 210321A: OSN Upper Limit

We observed the optical afterglow (Perri et al. GCN #29678; Siegel et al. GCN #29679; Butler et al. GCN #29680; Heintz et al. GCN #29684; Xin et al., GCN #29687; Pankov et al., GCNs #29689, #29692; Laporte et al., GCN #29697) of GRB 210321A (Swift detection: Dichiara et al., GCN #29677) with the 1.5m T150 telescope of the Observatorio de Sierra Nevada (OSN), Granada, Spain. Observations could not be taken due to high winds in the first night after the GRB. We obtained 11 x 180 s exposures in Rc starting at 2021-03-22 19:59:44.93 UT, before high winds shut the telescope down again. The afterglow is not detected in the combined image. We determine an upper limit Rc > 23.3 mag (AB mag, vs. a nearby Pan-STARRS star converted to Rc following the Lupton transformations, then transformed back to AB mag) at 1.5437 days after the GRB.