TITLE: GCN GRB OBSERVATION REPORT NUMBER: 3382 SUBJECT: GRB 050509B: ROTSE-III Optical Limits DATE: 05/05/09 06:00:23 GMT FROM: Eli Rykoff at U of Michigan/ROTSE E.S. Rykoff (U Mich), H. Swan (U Mich), B. Schaefer (Louisiana State), R. Quimby (U Texas), report on behalf of the ROTSE collaboration: ROTSE-IIIb, located at McDonald Observatory, Texas, responded to GRB 050509B (Swift trigger 118749), producing images beginning 10.0 s after the GCN notice time. An automated response took the first image at 04:00:43.0 UT, 23.7 s after the burst, under excellent conditions. We took 10 5-sec, 10 20-sec and 60+ 60-sec eposures. These unfiltered images are calibrated relative to USNO A2.0 (R). Imaging is ongoing. Comparison to the DSS (second epoch) reveals no new sources within the 3-sigma error circle, nor do we detect the tentative UVOT source (Hurkett et al, GCN 3381). Individual images have limiting magnitudes ranging from 17.07-18.48. In particular, we set a limit of magnitude 17.25 in a single 5 s exposure, starting 23.7 s after the burst. Coadding the images into sets of 10 reveals no new sources down to limits of 18.63 (77 s effective length (t_end - t_start), beginning 23.7s post-burst), 18.72 (298.5s effective length, beginning 100.6s post-burst), and 19.46 (695.6s effective length, beginning 399.1s post-burst). [GCN OPS NOTE (09may05): Per author's request, the two references to 050509A were changed to the correct value of 050509B.]