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.
