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Figure 9: Growth curves for PSF models in the L99 r-band exposures using MAG_SNLS and IQ20 magnitudes
to estimate the total magnitudes (left and right panels respectively). Each line corresponds to the difference in
magnitude between the total flux and the flux at the stated aperture size (measured as a multiple of the FWHM) for
each L99 exposure. As expected, when using the IQ20 aperture, the scatter at 40×FWHM is considerably reduced
(right panel
the measured MAG_SNLS and the measured IQ20 magnitude. This aperture correction is plotted against
the image quality in Figure 10. It is important to note that the behavior in the regime of very good image
quality clearly departs from a constant aperture correction.
For images taken in excellent seeing, the SNLS aperture is too small to capture the same flux versus total
flux compared to poorer image quality, which makes the basic SNLS aperture photometry not reliable
for our calibration. Figure 11 shows the shape of the PSF for two different FWHM rebined to the same
size. One can clearly notice that the overall shape is different : a boxy shape at good IQ due to flux in the
spikes and a very symmetrical shape at larger IQ. A tangible explanation of the behavior of the aperture
correction at low FWHM is the flux contained in the spikes which scale differently than the bulk of the
flux of the PSF.
3.7.3 Derivation of the MAG_IQ20 magnitudes
Measuring the flux inside a large aperture of 20 × FWHM is not a trivial task in the naturally deep CFHTLS
stacks where objects are subject to crowding. Instead we chose to model the aperture correction on a per
stack basis to go from the robust, but biased, MAG_SNLS to the nearly total magnitude MAG_IQ20.
The use of a master PSF model per image is adopted to ensure a robust estimation of the IQ20 aperture
magnitudes. First a pixel-based model of the PSF is constructed using the PSFex software (Bertin, 2011)
using a large set of stars. From this PSF model (which is produced as a FITS image) the fluxes inside a
series of apertures are computed using SExtractor. The aperture correction (ApCorr) is defined as the
magnitude difference between the flux inside the SNLS and IQ20 apertures:
ApCorr = −2.5 × log 10
Flux(Ap SNLS )
Flux(Ap IQ20 )
(2)
Since photometric rescaling from L99 to the CFHTLS stacks is carried out on only 25% of the MegaCam
field of view, a unique PSF model is computed for each of the four quadrants of each L99 image. Despite
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