TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  3845
SUBJECT: GRB050820a BVRcIc field photometry
DATE:    05/08/21 16:44:01 GMT
FROM:    Arne A. Henden at AAVSO  <arne@aavso.org>

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.