TITLE:   GCN GRB OBSERVATION REPORT

NUMBER:  4898

SUBJECT: GRB060218:  Refined photometric calibration of comparison stars

DATE:    06/03/20 20:19:10 GMT

FROM:    Malcolm Hicken at Harvard/Physics  <mhicken@cfa.harvard.edu>

Malcolm Hicken (CFA), Maryam Modjaz (CFA), Peter Challis (CFA), Robert

Kirshner (CFA), Jose Luis Prieto (OSU), Krzystof Stanek (OSU) and Richard

Cool (Arizona) report:

The CFA Supernova Group obtained UBVRr'i' photometry of 9 comparison stars

in the field of GRB060218/SN2006aj on March 4, 2006 UT.  Data was taken

using the FLWO 1.2m telescope at Mt. Hopkins, Arizona.  We present our

V-band light curve in Modjaz, et. al. (2006), astro-ph/0603377, submitted

to ApJL.  A finding chart, coordinates and photometry can be found at the

following website:

It should be noted that our V magnitudes are approximately 0.27 mag

fainter than those derived in GCN 4777 by Cool, et. al. at

http://mizar.as.arizona.edu/~grb/public/GRB060218.

There is also an offset in the other bands.  In investigating this

photometric offset, Richard Cool has discovered calibration offsets

between the two SDSS photometric reductions of this field indicating

possible non-photometric conditions or other calibration problems in the

SDSS photometry. The problem is being investigated to ensure data with

suspicious photometric quality will be flagged as such in future SDSS GRB

releases.