TITLE:   GCN CIRCULAR
NUMBER:  9293
SUBJECT: GRB 090418A BVRcIc field calibration
DATE:    09/04/29 14:49:54 GMT
FROM:    Arne A. Henden at AAVSO  <arne@aavso.org>

A. Henden (AAVSO), J. Gross (SRO), B. Denny (DC-3),
D. Terrell (SwRI), and W. Cooney (SRO) report:

We have performed an all-sky calibration of the field
for the GRB090418A afterglow reported by Chornock,
et al. (GCN 9148) using the Sonoita Research
Observatory (SRO) 35cm telescope in southern
Arizona.  Two photometric nights were used for the
calibration, with multiple Landolt fields observed
during each night along with an extinction
star for the calibration.  The calibration file has
a limiting magnitude around V=18, with good standards
brighter than V=16.  The file is available at
ftp://ftp.aavso.org/public/calib/grb/grb090418a.dat

We estimate the external zeropoint error of this calibration
to be about 0.02mag.  In particular, the star used by
Henden, et al. (GCN 9211) as a comparison star:
17:57:42.53 +33:25:47.2 J2000
has an R magnitude of 12.82, so all of the photometry in
GCN 9211 should be adjusted 0.30mag brighter.

Our system is available for any other bright UBVRI
calibrations (4<V<19) for this field or any
other field; contact the first author for such requests.

The AAVSO thanks the Curry Foundation for their continued support of the
AAVSO International High Energy Network,