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Figure 7: CFHTLS T0007 absolute photometric calibration. For the Deep fields, the calibration of the photometric
zero-points can be made directly since the SNLS reference stars are inside the images. In the Wide, an intermediate
set a photometric images (L99) is needed to apply the SNLS photometry to the stacks.
filter banpasses to be used in any scientific analysis are those defined by the SNLS team and described
in section 3.7.5. The method to tie the CFHTLS to the SNLS reference system follows the method
described in Regnault et al.:
• First the fluxes of the SNLS tertiary standard stars used as calibrators are measured using the same
photometric technique used to estimate the flux for science objects;
• Secondly zero-points are derived for each image by comparing the instrumental magnitudes of the
tertiary standards with the calibrated magnitudes;
• Lastly, the local “natural” magnitudes can be obtained by applying these zero-points to the science
objects’ instrumental magnitudes.
The calibration process is different for the Wide and the Deep surveys.
The Deep stacks can be calibrated directly using SNLS tertiary standards inside the Deep fields. Zeropoints
can therefore be computed by comparing the instrumental magnitudes of the SNLS standard stars
to the calibrated magnitudes published in Regnault et al. and applying the relevant Vega to AB magnitude
offset (Table 2).
However, as there is no complete coverage of SNLS tertiary standards within the Wide fields, a special
set of observations (“L99”) were acquired. These observations comprise series of short exposures taken
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