TITLE: GCN CIRCULAR NUMBER: 22033 SUBJECT: GRB 171020A: LT observations DATE: 17/10/21 06:30:48 GMT FROM: Cristiano Guidorzi at Ferrara U,Italy C. Guidorzi (U. Ferrara), S. Kobayashi (LJMU), C.G. Mundell (U. Bath), A. Gomboc (U. Nova Gorica), I.A. Steele (LJMU) on behalf of a large collaboration report: The 2-m Liverpool Telescope automatically began observing Swift GRB 171020A (Page et al. GCN 22028) on October 20, 23:10:12 UT (3.0 minutes from the GRB trigger time) with the RINGO3 polarimeter and the IO:O camera in the SDSS-R filter. We clearly detect the optical afterglow (Malesani et al, GCN 22029; Moskvitin et al., GCN 22031) with the following magnitude: Mid Time      Exposure       Filter       Magnitude (AB) (min)           (s) ------------------------------------------------------- 35.8            6x10         SDSS-R       21.12 +- 0.14 ------------------------------------------------------- as calibrated against nearby Pan-STARRS objects.