Photometric Calibration of the Supernova Legacy Survey Fields
  We present the photometric calibration of the Supernova Legacy Survey (SNLS)
fields. The SNLS aims at measuring the distances to SNe Ia at (0.3<z<1) using
MegaCam, the 1 deg^2 imager on the Canada-France-Hawaii Telescope (CFHT). The
uncertainty affecting the photometric calibration of the survey dominates the
systematic uncertainty of the key measurement of the survey, namely the dark
energy equation of state. The photometric calibration of the SNLS requires
obtaining a uniform response across the imager, calibrating the science field
stars in each survey band (SDSS-like ugriz bands) with respect to standards
with known flux in the same bands, and binding the calibration to the UBVRI
Landolt standards used to calibrate the nearby SNe from the literature
necessary to produce cosmological constraints. The spatial non-uniformities of
the imager photometric response are mapped using dithered observations of dense
stellar fields. Photometric zero-points against Landolt standards are obtained.
The linearity of the instrument is studied. We show that the imager filters and
photometric response are not uniform and publish correction maps. We present
models of the effective passbands of the instrument as a function of the
position on the focal plane. We define a natural magnitude system for MegaCam.
We show that the systematics affecting the magnitude-to-flux relations can be
reduced if we use the spectrophotometric standard star BD +17 4708 instead of
Vega as a fundamental flux standard. We publish ugriz catalogs of tertiary
standards for all the SNLS fields.
