TITLE: GCN GRB OBSERVATION REPORT NUMBER: 2495 SUBJECT: GRB031220: ROTSE-IIIb Optical Limits DATE: 03/12/20 18:02:47 GMT FROM: Eli Rykoff at Univ. of Michigan/ROTSE E. S. Rykoff, R. Quimby, C. W. Akerlof and B. E. Schaefer report on behalf of the ROTSE collaboration: The ROTSE-IIIb telescope at McDonald Observatory, Texas, responded automatically to HETE Trigger 2976 (GRB031220). Our first image of the final SXC localization was taken 6 seconds after the receipt of HETE Trigger 2976-6 on December 20, 05:16:14 UT, 1.77 hours after the burst. The system took 10x5 sec, 10x20 sec and 160x60 sec exposures. After co-adding sets of ten images, comparison to the USNO-B catalog reveals no new sources in the SXC error box. Furthermore, visual comparison to the DSS-2 Red and IR Plates reveal no new sources to an unfiltered limiting magnitude of 19.2 at 1.9 hours after the burst.