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Field RA diff. DEC diff.
["] ["]
W1 +0.014 +0.010
W2 −0.035 −0.025
W3 +0.011 +0.010
W4 −0.026 −0.023
Table 13: Mean RA and DEC differences between the external errors in each field and the mean external errors,
averaged over the four Wide fields.
external reference catalogue. This astrometric accuracy of CFHTLS stacks measures the final
absolute astrometric accuracy of each stack and their corresponding catalogues.
The astrometric calibration of the Wide survey is described in Section 3.5.
4.5.1 Astrometric calibration errors
The mean internal astrometric calibration errors are derived using the cross-identifications of sources
inside the overlapping regions of adjacent fields (see Fig. 21). All single input MegaCam Wide, Pre-
Wide (see Fig. 21) and short photometric exposures images remaining after selection are included.
The cross-identifications and the internal errors are obtained during the SCAMP calibration process. The
calibrations outputs of the Wide fields W1, W2, W3 and W4 are available from the synoptic table.
Over the whole survey, the mean internal rms error is
σ RA = 0.0279” ± 0.0049” , and , σ DEC = 0.0256 ± 0.0057” , (16)
where the reported errors are not the SCAMP internal errors, but the mean absolute difference of the
internal errors between the four Wide fields and the mean error values. Since the four fields are calibrated
independently, the errors quoted above are approximate; the internal errors for W1, W2, W3 and W4 are
given in Table 6.
For each field, the mean rms external RA and DEC errors of the astrometric solution are derived during
the SCAMP calibration process, using the cross-identification of sources inside each single CFHTLS Wide
image with the 2MASS catalogue. The mean external calibration errors over the whole survey are σ RA =
0.239” and σ DEC = 0.232”. They roughly correspond to the internal errors of the 2MASS catalogue.
The mean scatter from one Wide field to another is uncertain, because the CFHTLS is made of only four
independent fields. It can be estimated from the offsets of the mean external errors of each field separately
with respect to the mean value over the four fields. The offsets are listed in Table 13. The amplitudes
are very close to the internal astrometric calibration errors of the Wide fields, hence we conclude that the
internal astrometric accuracy for the CFHTLS Wide does not have significant field-to-field scatter.
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