TITLE: GCN CIRCULAR NUMBER: 31004 SUBJECT: GRB 211023A: Mondy optical observations DATE: 21/10/26 12:06:17 GMT FROM: Alexei Pozanenko at IKI, Moscow <apozanen@iki.rssi.ru> S. Belkin (IKI), A. Pozanenko (IKI), E. Klunko (ISTP), N. Pankov (HSE), report on behalf of GRB IKI FuN: We observed the GRB 211023A (Fermi GBM team, GCN 30958) with AZT-33IK telescope of Sayan observatory (Mondy) starting on 2021-10-25 (UT) 15:16:32. We obtained 17 images in R-filter. The afterglow of the GRB 211023A (Lipunov et al., GCN 30977; Kann et al. GCN 30982; Hu et al. GCN 30990) is clearly detected. Preliminary photometry of the afterglow in a stacked image is following Date UT start t-T0 Filter Exp. OT Err. UL(3sigma) (mid, days) (s) 2021-10-25 15:16:32 2.10265 R 17*120 19.87 0.15 21.0 The photometry is based on the nearby USNO-B1.0 stars RA DEC R2 04:53:23.10336 +85:19:26.4720 14.09 04:52:47.26344 +85:22:52.1508 15.75 Based on photometry reported early (Kann et al. GCN 30982; Hu et al. GCN 30990) we can approximate a light curve of the afterglow as a power law with the index of 1.49+/-0.03. See also the figure of the light curve in http://grb.rssi.ru/GRB211023A/GRB211023A_LC.png