TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  4979
SUBJECT: GRB060418, BVRcIc field calibration
DATE:    06/04/18 21:48:53 GMT
FROM:    Arne A. Henden at AAVSO  <arne@aavso.org>

A. Henden (AAVSO) reports on behalf of the AAVSO
High Energy Network and Sonoita Research Observatory (SRO):

We have acquired BVRcIc all-sky photometry for a 20x20arcmin
field centered on the coordinates of the afterglow for
GRB060418 (Falcone et al. GCN 4966) with the SRO 0.35m telescope
on one marginally photometric night.  The current zeropoint
error is approximately 0.04mag and will be improved with
additional nights of photometry.  As opposed to previous
calibrations, this one is quite shallow, saturating at
about V=11.  Further reductions will extend this calibration
about two magnitudes fainter.

We have placed the photometric data on our anonymous ftp site:

ftp://ftp.aavso.org/public/grb/grb060418.dat

Note#1:  the USNO-FS ftp site (ftp.nofs.navy.mil) that used
to contain all of the calibrations by Henden had a hard disk
failure about a month ago.  All of the calibrations have been
moved to the AAVSO site, where they will be maintained for
the forseeable future.

Note#2: the grb060418 file has a modified format that includes
(V-Ic).  This will be the format for subsequent calibrations.

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.