TITLE: GCN GRB OBSERVATION REPORT NUMBER: 3304 SUBJECT: GRB 050421: ROTSE-III Optical Limits DATE: 05/04/21 15:24:42 GMT FROM: Eli Rykoff at U of Michigan/ROTSE E.S. Rykoff (U Mich), S.A. Yost (U Mich), H. Swan (U Mich) report on behalf of the ROTSE collaboration: ROTSE-IIIb, located at McDonald Observatory, Texas, responded to GRB 050421 (Swift trigger 115135), producing images beginning 9.7 s after the GCN notice time. An automated response took the first image at 04:12:58.8 UT, 67.2 s after the burst, under good conditions. These observations were affected by the bright moon. We took 10 5-sec, and 350 20-sec eposures. These unfiltered images are calibrated relative to USNO A2.0 (R). Comparison to the DSS (second epoch) reveals no new sources within the 3-sigma BAT error circle including the 3-sigma XRT error circle (GCN 3301). Individual images have limiting magnitudes ranging from 16.49-17.34. In particular, we set a limit of magnitude 16.49 in a single 5 s exposure, starting 67.1 s after the burst. Coadding images into sets of 10 reveals no new sources down to limits of 17.8 (t0+61.7s - t0+130.7s) and 18.4 (t0+144s - t0+431s)