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Figure 14: Photometric calibration process of the CFHTLS-Deep stacks and Deep calibration images used for
L99.
MU_MAX (magnitude of the brightest pixel) against MAG diagram (left plot). The saturation level is identified
in the MU_MAX axis from the broadening of the distribution along the MAG direction. The resulting
saturation level is also plotted in the FLUX_RADIUS versus MAG scatter plot. This estimator is quite robust
but can still fail for 1% of the fields for several reasons (such as heterogeneous seeing due to the stacking
of images with different image quality or higher number of objects close to the saturation level which
blur the determination of the saturation limit). For this reason all plots are manually inspected. Finally,
to ensure the cleanest possible calibration sample, an additional 0.5 magnitude margin is subtracted from
the MU_MAX magnitude saturation limit.
Sources in the Deep stacks are matched with the SNLS standards using a nearest neighbor procedure
with a 2 ′′ matching radius. All stars with a non-zero SExtractor flag are discarded (to avoid blended
objects with potentially corrupted photometry). Figure 16 shows the magnitude difference distribution
as a function of reference magnitude after matching. The scatter is low, around 1% in average. The 2-σ
clipping applied only removes a small fraction of objects (as can be seen in the in the right panel).
The final estimator is the median offset after an iterative two-pass 2σ clipping. A linear regression
is used to verify that the estimator does not depend strongly on the magnitude of the selected objects
(which would be a sign of systematic biases in our magnitude measurement scheme).
The list of rescaling factors applied to the stacks are given in the Appendix in Table 33. Images are
rescaled to keep a final zero-point of Vega 30.0. In a last step, these pixels are scaled to a zero-point of
30.0 in AB. Given a magnitude offset of δ SNLS versus the published total magnitude (CAT_SNLS), we get:
δ SNLS = Median_Clipped(MAG_IQ20 i − CAT_SNLS i ) (4)
And we can compute a pixel scaling factor of F SNLS as follows:
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