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Figure 17: Photometric calibration process of the CFHTLS Wide stacks.
3.7.7 Photometric calibration of the Wide survey
The flowchart of the photometric calibration procedure for the CFHTLS Wide stacks is presented in
Figure 17. The key aspect of this procedure is the use of the L99 calibration observations to “transfer” the
photometry from the SNLS standard stars to the Wide stacks. The L99 fields are observations “bracketed”
(proceeded and followed) by an exposure on the Deep survey field closest on the sky to the observed Wide
patch. The zero-point of the Deep exposures are calculated in the exact same procedure than described in
the previous section. Assuming a perfect stability of the photometric conditions during the series of L99
and Deep observations, the Deep data zero-points are used to calculate the corresponding zero points for
the L99 images (taking into account airmass differences).
The zero-point applying to all L99 image taken in the sequence is computed from the mean value of
the bracketing Deep fields. In the next step, catalogues with MAG_IQ20 are extracted from both the
L99 calibrated images and the Wide stacks. As before, saturated objects are carefully flagged and only
stellar objects for which MAG_IQ20 can be reliably measured are selected. The catalogues of overlapping
L99-Wide pairs are then matched with the following object selection:
• Magnitude ranges (u:15.0-20.5, g:15.0-21.5, r:15.5-21.5, i:15.5-20.5, y:15.5-20.5, z:15.0-20.0)
• Stellar objects: SExtractor CLASS_STAR > 0.95;
• Unblended objects: SExtractor FLAGS==0;
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