Improved Photometric Calibration of the SNLS and the SDSS Supernova Surveys???

Context. We present a combined photometric calibration of the Supernova Legacy Survey (SNLS) and the SDSS supernova survey, which results from a joint effort of the SDSS and the SNLS collaborations. Aims. Our primary motivation is to eventually sharpen cosmological constraints derived from type Ia supernova measurements by improving the accuracy of the photometric calibration. We deliver fluxes calibrated to the HST spectrophotometric star network for large sets of tertiary stars that cover the science fields of both surveys in all photometric bands. We also cross-calibrate directly the two surveys and demonstrate their consistency. Methods. For each survey the flat-fielding is revised based on the analysis of dithered star observations. The calibration transfer from the HST spectrophotometric standard stars to the multi-epoch tertiary standard star catalogs in the science fields follows three different paths: observations of primary standard stars with the SDSS PT telescope; observations of Landolt secondary standard stars with SNLS MegaCam instrument at CFHT; and direct observation of faint HST standard stars with MegaCam. In addition, the tertiary stars for the two surveys are cross-calibrated using dedicated MegaCam observations of stripe 82. This overlap enables the compariso