TITLE: GCN CIRCULAR NUMBER: 7496 SUBJECT: GRB 080319B: UVOT Observations DATE: 08/03/21 11:34:39 GMT FROM: Stephen Holland at USRA/NASA/GSFC/SSC S. T. Holland (CRESST/USRA/GSFC) & J. L. Racusin (PSU) report on behalf of the Swift/UVOT team: The Swift/UVOT observed the field of GRB 080319B starting 51 s after the BAT trigger (Racusin et al., GCN Circ. 7427). We detect the optical afterglow all filters. Preliminary magnitudes are reported below. Filter T_start (s) T_stop Exposure Mag Err Comment v 175 575 393 <11.5 Saturated b 654 664 10 13.39 0.01 u 630 649 19 12.44 0.01 uvw1 605 625 19 12.49 0.02 uvm2 581 600 19 13.39 0.05 uvw2 685 705 19 13.36 0.04 white 67 169 98 <13.9 Saturated The above magnitudes are not corrected for the Galactic extinction corresponding to a reddening of E_{B-V} = 0 .01 mag (Schlegel et al., 1998, ApJS, 500, 525). The photometry is on the UVOT flight system described in Poole et al. (2008, MNRAS, 383,627).