Color transformation between the Landolt system and the reference MegaCam system. Open black circles show the MegaCam and Landolt measurements of the Landolt secondary standards. The overall scale is set by the magnitudes of the primary spectrophotometric standard star BD +17 4708 (solid red square). Measurements of BD +17 4708 magnitudes in the Landolt system are provided by Landolt & Uomoto (2007). Since BD +17 4708 cannot be directly observed with MegaCam, its MegaCam magnitudes are inferred from the average color transformation (black line) determined from the secondary standard measurements. Potential deviations from this average color transformation are examined in R09 (see Sect. 5.1). Other primary standards from Landolt & Uomoto (2007) are displayed as red crosses; they are not close enough to the bulk of secondary standards to be reliably transformed with the average transformation. Observations of the Landolt standards in i2_M were not numerous enough to deliver a robust measurement of the i2_|x0 − I transformation (refer to Sect. 5.3 for the i2_M calibration).