TITLE: GCN CIRCULAR NUMBER: 13700 SUBJECT: GRB 120817B: Deep limit on the optical afterglow from du Pont and Magellan observations DATE: 12/08/22 18:26:16 GMT FROM: Wen-fai Fong at CFA W. Fong, E. Berger (Harvard), D. Kelson, R. Williams (Carnegie) report: "We re-observed the BAT-refined location of the IPN short-duration GRB 120817B (GCNs 13670, 13692) with the Inamori-Magellan Areal Camera and Spectrograph (IMACS) mounted on the Magellan/Baade 6.5-m telescope starting on 2012 Aug 21.410 UT, 4.2 days post-burst and 3.3 days after our initial LCO/WFCCD observations (GCN 13672). We obtained 2x300-sec exposures in R-band in 0.92" seeing. Digital image subtraction with our first observations using the ISIS software package reveals no evidence for a fading optical source in or around the BAT error circle. Calculating the zeropoint using a Stetson standard star field at the same airmass, we place a 3-sigma limit of R(Vega)>23.3 mag on the optical afterglow of GRB 120817B at 23.7 hours after the burst."