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Figure 38: Comparison of one CFHTLS field with the Pickles stellar library (lower panel) and the Basel 3.1 stellar
library using [Fe/H]=-1.0 (upper panel).
and 0.008 respectively; after the application of the SLR regression, this becomes 0.014, 0.011, 0.010 and
0.008.
The use of SLR clearly makes no improvement to our W3 photometric calibration, and demonstrates in
our case the SLR calibration is limited to a precision larger than 1%. Without a detailed knowledge of
the stellar population variations across the Wide patches, it is difficult to see how this could be improved
further.
Considering the lack of improvement in the recalibration of W3 using the SLR, we chose not to use this
method in T0007.
4.5 Astrometric accuracy
The astrometric accuracy is measured in two different ways.
• Firstly, at the end of the global astrometric calibration process, by comparing the astrometric positions
of sources in all single CFHTLS images with sources in an internal and an external reference
catalogue. This astrometric calibration error represents the upper limit for the astrometric accuracy
of the survey.
• Secondly, by comparing position of sources inside each CFHTLS Wide stack with respect to an
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