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The stellar color-color tracks of the four fields are remarkably similar. No color offsets can be measured
between D1, D2, D3 and D4. This is illustrated on Fig. 43. It shows a superimposition of the (u ∗ −
g)/(g − r) and (g − r)/(r − i) color-color plots for the four D-85 stacks together.
5.3.2 Internal photometric errors
The internal errors have been measured by comparing the photometry of common sources in the D-25
and D-85 stacks. The D-25 and D-85 parent samples comprise all source pairs listed in the merged
catalogues of the four Deep fields. The selected sources must have magnitudes measured in all filters
(no objects with magnitude/mag_err = 99.0 in any u ∗ , g, r, i, y and z filters, for both the D-25 and D-85
catalogues), with a SExtractor extraction flag FLAGS=0 (objects with good photometry) and a Terapix
object flag=1 or 0 (i.e. stellar or extended sources, only non-saturated objects in non-masked regions).
Only sources fainter than u ∗ , g=20.5 and r, i, y, z=19.5, and brighter than the D-25 80% completeness
limit of extended sources are selected. This cut ensures the homogeneity and completeness of the D-25
and D-85 populations.
The internal photometric errors are derived from the distributions of magnitude differences of source
pairs between the D-25 and D-85 catalogues, D25–D85, as a function of D-25 magnitudes. The analysis
are carried out in three magnitude ranges, for all filters, and after a 3-σ clipping over the distributions of
magnitude differences. The statistics use the MAG_AUTO magnitudes to derive first the median and mean
systematic magnitude offsets between D-25 and D-85, then the mean scatter, based on the rms. We
verified that the median systematic offset is randomly distributed around zero and never exceeds 0.013
mag., for all sub-samples.
The distributions are then fitted by a Gaussian, which gives the FWHM of the mean magnitude difference
of cross-identified sources in D-25 and D-85. The mean internal photometric error is then defined as
σ D25−D85 =FWHM/2.35. We checked that the error estimate based on the Gaussian fitting is very close
to the rms errors. The results are summarized in Table 22. Overall, they look very similar to the Wide
survey, when sources with same signal-to-noise ratio are compared.
We use the results quoted in Table 22 to compute the photometric errors listed in the summary Tables 18
and 19. The internal photometric errors are the mean values of the three magnitude ranges. This is a
reasonable but probably optimistic estimate because it does not take into account there are many more
faint than bright objects. Note that these results are valid for both stellar and extended sources.
In contrast with the Wide survey, we do not have many sources in common between CFHTLS and SDSS
in D2 and D3 to estimate accurately external errors. In addition, we do not have any common sources at
all for D1 and D4. However taking into account the statistics for the internal errors and the remarkable
consistency of the results with the Wide survey, conclusions for the Wide survey apply equally: we
consider the following errors are reasonably accurate and conservative estimates of the rms photometric
errors over a rather broad magnitude range of the CFHTLS Deep survey down to the 80% completeness
limit (details in Table 22):
• 5% in u ∗ ; 3% in g, r and i/y, 4% in z .
For faint sources beyond the 80% completeness limit, photometric errors rise by a factor of ∼ 2 in all
bands with respect to the magnitude range quoted above and in Tables 18 and 19.
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