{
  "bibcode": "2009GCN..9293....1H",
  "body": "A. Henden (AAVSO), J. Gross (SRO), B. Denny (DC-3),\nD. Terrell (SwRI), and W. Cooney (SRO) report:\n\nWe have performed an all-sky calibration of the field\nfor the GRB090418A afterglow reported by Chornock,\net al. (GCN 9148) using the Sonoita Research\nObservatory (SRO) 35cm telescope in southern\nArizona.  Two photometric nights were used for the\ncalibration, with multiple Landolt fields observed\nduring each night along with an extinction\nstar for the calibration.  The calibration file has\na limiting magnitude around V=18, with good standards\nbrighter than V=16.  The file is available at\nftp://ftp.aavso.org/public/calib/grb/grb090418a.dat\n\nWe estimate the external zeropoint error of this calibration\nto be about 0.02mag.  In particular, the star used by\nHenden, et al. (GCN 9211) as a comparison star:\n17:57:42.53 +33:25:47.2 J2000\nhas an R magnitude of 12.82, so all of the photometry in\nGCN 9211 should be adjusted 0.30mag brighter.\n\nOur system is available for any other bright UBVRI\ncalibrations (4<V<19) for this field or any\nother field; contact the first author for such requests.\n\nThe AAVSO thanks the Curry Foundation for their continued support of the\nAAVSO International High Energy Network,",
  "circularId": 9293,
  "createdOn": 1241016594000,
  "email": "arne@aavso.org",
  "subject": "GRB 090418A BVRcIc field calibration",
  "submitter": "Arne A. Henden at AAVSO  <arne@aavso.org>",
  "eventId": "GRB 090418A"
}