TITLE: GCN CIRCULAR NUMBER: 7411 SUBJECT: GRB 080310: ROTSE-III Observations of Optical Counterpart DATE: 08/03/12 03:32:31 GMT FROM: Fang Yuan at ROTSE F. Yuan (U Mich), R. Quimby (Caltech), H. Swan (U Mich), C. Akerlof (U Mich), report on behalf of the ROTSE collaboration: ROTSE-IIIb, located at McDonald Observatory, Texas, responded to GRB 080310 (Swift trigger 305288; Cummings et al., GCN 7382) and began imaging at UT 08:38:25.7 (5.7 sec after the GCN notice time) under cloudy conditions. The first usable image with a detection of the OT (Chornock et al. GCN 7381) starts at UT 09:03:03.4. Observations continued in fluctuating weather conditions until about 3 hours after the trigger. The OT is slightly blended with the two nearby stars in ROTSE images. To reduce the contamination, we tried two different methods to obtain the OT magnitudes, first by subtracting the scaled PSF of the two nearby stars, and, second, by subtracting a reference image constructed from images taken on Mar. 11 between UT 05:26:28.5 and 06:00:53.6 (when the OT has dropped below our detection threshold). The two methods yield similar results. From 0.42 to 1.72 hour after the burst, the OT is observed to decay with a power-law index 0.6+/-0.1, consistent with the observation by the LBT (Garnavich et al., GCN 7390). The later images have degraded seeing and don't constrain the time dependence of the OT very well. The magnitudes reported below are unfiltered calibrated to SDSS r using standard stars in the pre-burst SDSS observations (Cool et al., GCN 7396). Start_UT End_UT mag magerror mlim(of image) --------------------------------------------------------------- 09:03:03.4 09:07:31.2 17.2 0.1 18.4 09:56:17.5 10:09:49.8 18.0 0.1 19.4