TITLE: GCN CIRCULAR NUMBER: 18199 SUBJECT: GRB 150821A: LCOGT Siding Springs early observations DATE: 15/08/24 15:29:32 GMT FROM: Cristiano Guidorzi at Ferrara U,Italy C. Guidorzi (U. Ferrara), S. Kobayashi (LJMU), A. Gomboc (U. Ljubljana), C. Mundell (U. Bath) on behalf of a larger collaboration report: The 1-m LCOGT telescopes in Siding Springs (Australia) began observing Swift GRB 150821A (Troja et al. GCN 18186) on August 21, 10:22 UT (38 minutes after the burst trigger) with SDSS r and i filters under initially poor sky conditions. We detect the optical afterglow (Troja et al. GCN 18186; D'Elia et al. GCN 18187; Kuin et al. GCN 18192; Kruehler et al. GCN 18195) with the following magnitudes: Mid Time Exposure Filter Magnitude (min) (s) ----------------------------------------------------- 135 1680 r' 19.1 +- 0.4 233 480 r' 19.7 +- 0.3 398 720 r' 20.3 +- 0.2 ----------------------------------------------------- Calibration is done against R2 and I values of USNOB-1 nearby stars.