GCN Circular 3845
A. Henden (AAVSO/USNO) reports on behalf of the USNO GRB Team: We have acquired BVRcIc all-sky photometry for a 23x23arcmin field centered on the coordinates of the optical afterglow (Fox and Cenko, GCN 3829) for the Swift burst GRB050820a (Page et al., GCN 3830) with the USNOFS 1.0-m telescope on one photometric night but with bright moonlight. Stars brighter than V=12.0 are saturated and should be used with care. We have placed the photometric data on our anonymous ftp site: ftp://ftp.nofs.navy.mil/pub/outgoing/aah/grb/grb050820.dat The astrometry in this file is based on linear plate solutions with respect to UCAC2. The external errors are about 100mas. The estimated external photometric error is about 0.03mag. As always, you should check the dates on the .dat file prior to final publication to get the latest photometry. There is a README file on the ftp directory to give you information about the procedures used to calibrate these fields.