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Wide field σ u ∗ σ g σ r σ i σ z
[mag.] [mag. ] [mag. ] [mag. ] [mag. ]
W1 0.020 0.010 0.015 0.013 0.020
W2 0.020 0.010 0.012 0.011 0.023
W3 0.025 0.010 0.015 0.011 0.019
W4 0.016 0.009 0.008 0.013 0.016
Table 9: Internal field-to-field photometric errors in the T0007 CFHTLS Wide release derived from sources in
adjacent tiles. These internal errors are computed from the T0007 “merged source catalogues” (M-SC, see Section
6.3) which contains MAG_IQ20 for sources detected on the u ∗ , g, r, i/y, z and chi2 images. They are derived
from the mean of the absolute magnitude differences of source pairs, averaged over overlapping regions of neighboring
stacks. Only bright stars are used to derive a robust field-to-field offset for each contiguous tile pair. The
limiting magnitudes are u < 19, g < 22, r < 20, i < 21, z < 19 These values should be regarded as an upper
estimate of the errors as they are calculated using the stack edges which have lower signal-to-noise due to the
dithering.
of these field to field offsets is a estimator of the field to field scatter. The values for each field and filter
are listed in Table 9. To compare these errors with estimators calculated from a comparison to a reference
catalogue used in Section 4.4.3, a hypothesis has to be made on the distribution of this random variable.
If we assume that photometric measurement are Gaussian-distributed around the “true” photometry with
a dispersion of σ, then the dispersion of the field to field offsets is a Gaussian with a dispersion √ 2σ. The
values of Table 9 must be divided by √ 2 to be compared with the field to field estimations of Table 12.
The errors are larger in u ∗ and z bands for several reasons: the residuals from fringe subtractions in z,
the poorer image quality in u ∗ , especially at the edges of the stacks, and the residuals from the internal
calibration errors. Considering the factor of √ 2, these errors are completely consistent with an overall
photometric field to field scatter of 1% in g,r and i and 1.5% in u ∗ and z.
The second estimate of internal photometric errors which can be determined from the analysis of objects
observed in multiple tiles is the measurement error. Since the images are photometrically flat, the dispersion
of the magnitude offsets around the mean field-to-field shift of objects detected in multiple tiles
is dominated by statistical measurements errors. This dispersion as a function of magnitude is plotted
as the vertical error bars in Figure 35 for each of the four Wide patches and is listed in Table 10. In
this analysis, both stars and galaxies are included; magnitudes are estimated using MAG_AUTO to yield a
realistic estimate of measurement errors for both stars and galaxies. We also note the plots are broadly
similar for each of the four patches, confirming the homogeneous nature of the Wide survey. The z−band
errors are several times higher that other bands; for these longer wavelengths fringing residuals become
an true issue.
The dispersion distributions are fitted with third order polynomials. To compare these values to the
direct estimation of measurement errors in the simulations, one has to assume a gaussian distribution
(dispersion σ M ) of the measured magnitude around the true magnitude. The distribution of the magnitude
differences is then a Gaussian distribution with a dispersion equal to :
These errors are listed in Table 10.
σ diff = √ 2 × δ M (13)
Comparing these errors with those derived from the simulations shows that the errors from the overlaps
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