TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  1503
SUBJECT: GRB020813, UBVRI field calibration
DATE:    02/08/17 18:47:11 GMT
FROM:    Arne A. Henden at USNO/USRA  <aah@nofs.navy.mil>

A. Henden (USRA/USNO) reports on behalf of the USNO GRB team:

We have acquired UBVRcIc all-sky photometry for
an 11x11 arcmin field centered at the optical transient coordinates
for GRB020813 (Villasenor et al. GCN 1471; Fox et al. GCN 1470)
with the USNOFS 1.0-m telescope on one photometric night.  Stars
brighter than V=14.0 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/grb020813.dat
The astrometry in this file is based on linear plate solutions
with respect to UCAC2.  The external errors are less than 100mas.

While the night was photometric, the extinction was high
due to nearby forest fires.  The GRB field is towards
our southern horizon and so there is a greater risk of
zeropoint errors due to nonuniform smoke.  We estimate the
present zeropoint errors to be about 0.03mag.
The current calibration also has a brighter limiting magnitude
than normal due to the moonlit sky.  We will be extending this
calibration fainter and with additional nights to ensure against
a systematic zeropoint error.  You should check the dates on
the .dat file prior to final publication to get the latest photometry.