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accurately reflect the integration time per pixel because most COSMOS images are shifted by 30 ′ and
only fill one quadrant of the CFHTLS u ∗ -band stack. The D2-u ∗ corresponds to the innermost regions of
Fig. 46, delimited by the black contours. Table 23 describes each sub-field in more detail).
5.2 Astrometric accuracy
The astrometric accuracy is derived in the same way as for the Wide survey. We refer to the beginning of
Section 4.5 for a description of the method. The Deep analysis is however simpler than the Wide because
there is only one single position per field. The astrometric calibration of the Deep fields is detailed in
Section 3.5, so we only focus on the accuracy in this section.
The results of the internal and external error analysis are given in Tables 18 and 19. The internal errors
have been measured by SCAMP, from the astrometric solutions of each Deep field, for each filter separately.
In contrast, the external errors quoted in Tables 18 and 19 are not the values derived from the
astrometric solutions. They are computed separately as well, but after the production of the Deep images,
in order to get the true astrometric errors of sources in each stack. The reference external catalogue is
2MASS, and the external errors found are very similar to the Wide survey.
On average, the mean internal astrometric rms errors in the Deep fields are
and the mean external errors
σ RA = 0.056” ± 0.011” , and , σ DEC = 0.056 ± 0.011” , (17)
σ RA = 0.283” ± 0.017” , and , σ DEC = 0.232 ± 0.014” , (18)
where, the dispersions are the rms errors. Since the four fields are calibrated independently, the values
quoted above are only rough estimates; the errors for D1, D2, D3 and D4 are given in Tables 18 and
19. The rather large scatter in the internal errors is due to the different ways each Deep field has been
calibrated (either using an internal catalogue based on the i band data, or using the external 2MASS
catalogue, see Section 3.5). The external astrometric errors are similar to the Wide survey and correspond
to the expectations, considering the astrometric errors in the 2MASS reference catalogue.
5.3 Photometric accuracy
5.3.1 Comparison with SDSS-DR8
The photometric calibration of the Deep survey fields is fully described in Section 3.7. We aim in this
section at conducting an external comparison to the SDSS survey: wherever possible, magnitude offsets
between common stars in the CFHTLS Deep and SDSS-DR8 have been measured. SDSS magnitudes are
first converted to the MegaCam AB system using the MegaCam-SDSS color transformation equations
presented in the section comparing the Wide to SDSS-DR8 (Section 4.4.3, Equation 13).
The offsets are quoted in Tables 18 and 19 and in the synoptic table. Only fields D2 and D3 have sources
in common with SDSS-DR8. The offsets are found to be similar to the means 〈∆ m=u ∗ ,g,r,i/y,z〉 (Table 11)
or 〈δ m=u ∗ ,g,r,i/y,z〉 (Table 6) found for the Wide fields.
In both the Deep and the Wide surveys, the CFHTLS to SDSS-DR8 comparisons are made using stars
with 17 < i < 21. The signal-to-noise ratio of all common stars on the Deep fields is much higher than
for the Wide, hence it is reassuring to find very similar offsets in all filters for both the Deep and the
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