TITLE: GCN CIRCULAR NUMBER: 23198 SUBJECT: GRB 180904A: SAO RAS optical observations DATE: 18/09/05 00:46:37 GMT FROM: Moskvitin Alexander at SAO RAS Moskvitin, A. S., Spiridonova O. I., Maslennikova O. A. (SAO RAS), report on behalf of larger GRB follow-up team. We observed the field of the GRB 180904A (D'Ai et al., GCNC 23194) with the Zeiss-1000 telescope of SAO RAS (+ CCD UBVRcIc photometer) on the 2018 September, 4.926 (mid. time). The observations started since 7.2 minutes after the Swift trigger (21:35:44--22:46:03 UT). 12 x 300 sec. exposures in Rc band were obtained. We can not see any object inside the XRT enhanced 1".8 error circle (Osborne et al., GCNC 23196) down to the limiting magnitude of R_lim = 22.7, but at the edge of the circle we detected the object mentioned by Malesani et al. (GCNC 23195). The preliminary magnitude of the object is R = 21.9 +/- 0.2 (Vega system) compared to nearby USNO-B1 stars. The finding chart can be found at ftp://ftp.sao.ru/pub/grb/GRB/GRB180904A_SAORAS.png