GCN Circular 11191
M. Andreev, A. Sergeev (Terskol Branch of Institute of Astronomy) and A.Pozanenko (IKI) on behalf of larger GRB follow up collaboration report: We are continuing observations of the Swift GRB 100901A (Immler et al., GCN 11159) with the Z-600 telescope of Mt.Terskol observatory in R-filter (Andreev et al., GCN 11166). In a combined image at mid time Sep. 03 (UT) 00:36 we detect afterglow (Immler et al., GCN 11159; Guidorzi et al., GCN 11160; Ivanov et al., GCN 11161; Klunko et al., GCN 11162). In comparison with our previous photometry (Andreev et al., GCNs 11166, 11168) below we used several reference stars of SDSS DR7 and Lupton transformation ugriz -> BVRI: T0+ Filter, Exposure, OT (d) (s) 1.4599 R 10x120 19.22 +/- 0.05 The photometry above is consistent with steady decay based on early photometry (Kann et al, GRB 11187). We note however the systematic faintness of about ~0.25m in SDSS DR7 in comparison with USNO-B1.0 R2 which can explain possible contradiction of MITSUME observation calibrated against SDSS (Kuroda et al, GCN 11189) and photometry reported in Yoshida et al (GCN 11190). Detailed calibration of the field stars is necessary.