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Figure 25: Distribution of the median seeing over the Wide tiles. The seeing is the FWHM= √ a b derived with
PSFEX, from the fitting of a 2-dimension Moffat model of the PSF. The mean seeing value over the MegaCam field
of each stack is available at http://terapix.iap.fr/cplt/T0007/table_syn_T0007.html.
is observed, the median seeing of each stack appears randomly distributed over the four Wide fields.
Figure 27 shows the seeing fluctuations on MegaCam. The seeing is averaged over a CCD field of view
and compared to the mean seeing over the whole MegaCam field. The statistics is based on 6378 input
images used for the CFHTLS Wide T0007 release. For CCDs at the edges of the detector, the differences
are important but never exceed 0.065", and the maximum peak-to-peak amplitude is less than 0.1" . This
upper limit is acceptable. It still preserves the mean seeing over the whole MegaCam field of view below
one arcsecond for all input images that will be combined into stacks. The legend of Figure 27 elaborates
on the extreme corners of the MegaCam field of view which exhibit a rapid image quality degradation,
but represent an area limited to just 3% of each image.
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