TITLE: GCN CIRCULAR NUMBER: 10475 SUBJECT: GRB 100305A Optical Observations DATE: 10/03/05 14:57:01 GMT FROM: Arne A. Henden at AAVSO A. Henden (AAVSO), J. Gross (SRO), B. Denny (DC-3), D. Terrell (SwRI), and W. Cooney (SRO) report: We obtained imagery of the GRB 100305A burst reported by Troja et al. (GCN 10470) using the Sonoita Research Observatory (SRO) 35cm telescope in southern Arizona, utilizing an automatic VOevent trigger. The exposures began about 3 minutes after the burst and continued for approximately two hours. 20 Rc-band, 10 V-band, and a set of BVRI exposures for field calibration were acquired. Inspection of the enhanced Swift-XRT position given by Osborne, et al. (GCN 10474) does not show any optical afterglow to the following limits, based on using star USNOB 1323-0268523 at 11:13:38.45 +42:23:45.7 as having magnitudes R=15.98 and V=16.67): UTmid delT exp filt lim (hrs) (min) (# x s) 09.1544 003.63 01x060 Rc 19.1 09.2379 008.64 10x060 Rc 20.5 09.4821 023.29 05x180 Rc 21.3 09.8411 044.83 05x300 Rc 21.6 10.8564 105.75 10x300 V 21.1 The AAVSO thanks the Curry Foundation for its support of the AAVSO International High Energy Network.