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Figure 23: Distribution of exposure times over the Wide fields. The inset is a close-up view of the long exposure
tails.
4.2 Seeing and image quality
All seeing values reported in this document and in tables are computed using PSFEx (Bertin, 2011).
The seeing is measured from a two-dimensional Moffat model of the PSFm and the FWHM is defined
from the ellipticity parameters of the PSF as FHWM= √ a b, where a and b are the size of the major and
minor axes derived from the model. For single CFHTLS input images, the seeing is sampled over a CCD
(∼ 7 ′ × 14 ′ , see Fig. 24), and for T0007 stacks it is sampled over a 5 ′ × 5 ′ grid. (Note that in T0006 and
anterior releases, the seeing was measured from the radius enclosing half the object flux.)
The PSF model of each stack (Wide and Deep) is determined using stellar sources selected by PSFex.
All unsaturated and sufficiently bright stellar sources identified over the entire MegaCam field are used.
The median seeing values of the survey are given in Table 4 and Table 6.
Overall, the CFHTLS Wide is within or better than expectations, thanks to the high ranking of the
program which meant most observations were made in good-seeing conditions (i.e. close to median
conditions at CFHT). The histograms drawn in Fig. 25 show the median seeing increases from z to u ∗ .
However, more than 98% of g, r, i, y, z and more than 70% or u ∗ band stacks have seeing better than 0.9".
Note that the histograms show the distributions of seeing over the whole period of the survey. They merge
together data obtained before and after the flip of the L3 lens of the MegaPrime Wide Field Corrector
(Dec. 4, 2004). This tuning produced an unexpected and unexplained, but spectacular, improvement
of the image quality and our merging of the pre-flip and post-flip periods contribute to the large scatter
of the seeing distributions. However, the Wide survey had not made much observing progress by late
2004 compared to the SNLS/Deep survey which remained a top priority within the CFHTLS, hence
“contamination” by poorer image quality images is quite minimal in the Wide survey. In consequence,
despite this extra-scatter term, the image quality of the CFHTLS Wide survey is excellent and unique
compared to other current wide field surveys.
Figure 26 shows maps of the spatial distribution of seeings in all Wide stacks and for each filter. No bias
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