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Figure 12: PSF model for the four quadrants of the CFHTLS-Wide stack CFHTLS_W_z_222054-003100_T0007.
The circles represent the apertures used for the SNLS and IQ20 magnitudes.
the PSF variations across a quarter of a MEGACam field of view, a single PSF model is computed : this
approximation is motivated by the small variation of the PSF shape and size across the MegaCam field
of view compared to the (much larger) variations between the L99 images and the stacks.
The CFHTLS T0007 catalogues offer the MAG_SNLS magnitude since it is the measured value for each
source on the image, but it is the aperture corrected magnitude based on the four PSF models for each
stacks (and the corresponding four aperture corrections - ApCorr), MAG_IQ20, that ought to be used for
any calibration or scientific purposes on point sources.
MAG_IQ20 = MAG_SNLS + ApCorr (3)
3.7.4 Quantifying the improvements using MAG_IQ20 compared to MAG_SNLS
Deep fields observations bracket L99 observations. Since L99 observations were taken in photometric
conditions, the zero-points derived from the comparison of the instrumental magnitudes and the SNLS
tertiary standards catalogues should not change. By comparing the zero-points in these pairs of Deep
observations (before and after L99 observations) in Table 1, on can see that the MAG_IQ20 is a more
stable estimator than MAG_SNLS. stability improves even for Deep images with identical seeing. This
result indicates that the scatter in the aperture correction at a given seeing seen in Figure 10 captures real
information on the varying PSF shape of identical size.
In conclusion, the variation of the aperture correction with the seeing shown in Figure 10 provides indication
that the SNLS flux is not a truly valid proxy of the total flux (measured by the IQ20 aperture
photometry) when the images have a large spread in image quality. Our current approach circumvent this
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