Documentation [PDF] - Canada France Hawaii Telescope ...

Wide Field Magnitude range u ∗ g r i z
[MegaCam AB] [mag.] [mag.] [mag.] [mag.] [mag.]
W1 [19.0 − 20.0] 0.013 0.007 0.009 0.010 0.025
[20.0 − 21.0] 0.016 0.008 0.013 0.016 0.054
[21.0 − 22.0] 0.024 0.014 0.023 0.031 0.112
[22.0 − 23.0] 0.039 0.026 0.043 0.062 0.210
[23.0 − 24.0] 0.065 0.046 0.078 0.114 0.359
W2 [19.0 − 20.0] 0.010 0.006 0.010 0.009 0.020
[20.0 − 21.0] 0.014 0.009 0.015 0.015 0.045
[21.0 − 22.0] 0.024 0.016 0.025 0.030 0.100
[22.0 − 23.0] 0.043 0.027 0.047 0.059 0.195
[23.0 − 24.0] 0.074 0.044 0.083 0.108 0.344
W3 [19.0 − 20.0] 0.011 0.007 0.009 0.010 0.023
[20.0 − 21.0] 0.016 0.009 0.013 0.016 0.049
[21.0 − 22.0] 0.025 0.014 0.022 0.033 0.101
[22.0 − 23.0] 0.040 0.023 0.043 0.066 0.189
[23.0 − 24.0] 0.062 0.036 0.079 0.123 0.324
W4 [19.0 − 20.0] 0.008 0.006 0.008 0.010 0.019
[20.0 − 21.0] 0.012 0.008 0.013 0.016 0.044
[21.0 − 22.0] 0.023 0.013 0.024 0.031 0.097
[22.0 − 23.0] 0.044 0.022 0.046 0.061 0.189
[23.0 − 24.0] 0.079 0.036 0.083 0.111 0.332
Table 10: Internal photometric errors per magnitude bins derived for each CFHTLS Wide field. The magnitudes of
the objects detected in two stacks are compared. For each pair of stacks, a mean offset and dispersion is calculated
from the magnitude differences of each pair of objects. Assuming a gaussian distribution of errors, the listed values
are estimated as the dispersion divided by √ 2.
are larger. This is to be expected and can be explained by several reasons: the statistics on the objects
in the overlaps rely on sources detected at field edges where image quality and signal-to-noise is lowest
due to dithering and PSF degradation at the edge of the field. Furthermore, the flatness of the photometry
across the field of view show a departure from the reference central photometry on the very edges.
This effect is slightly non-symmetric and affects the reliability of the magnitude measurements at the
field edges. Therefore these error estimates should be considered as upper limits and contain residual
systematic effects which add to the pure statistical flux measurement errors.
4.4.3 Comparing CFHTLS-T0007 with SDSS-DR8
As a further check of our reduction and calibration procedures we wish to compare photometric measurements
on the CFHTLS Wide with an external and well-characterized survey. Our aim is to check
both the relative tile-to-tile calibration and the absolute photometric calibration. At the present time, the
best candidates are the surveys conducted using the Sloan telescope at Apache Point Observatory. The
latest release of the main Sloan survey, SDSS-DR8, overlaps at least partially with all four Wide patches
of the CFHTLS.
50