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Each tile is centered at a well-defined position. The coordinates of a stack center position are identical
for all filters. W1, W2, W3 and W4 are therefore composed of a complete set of u ∗ , g, r, i, y and z adjacent
square tiles of 1 deg 2 each.
The integer part of the center position of each tile is used to name the final stacks and other data products.
This naming convention was agreed by CADC, Terapix and the CFHTLS Steering Group and is valid for
all CFHTLS releases.
In addition to the official astronomical CFHTLS naming convention, Terapix defined in 2002 a shorter
naming convention based on a Cartesian grid coordinate system, where the increment unit is a MegaCamsize
field and the center positions of the grid points are the center positions of the tiles 21 .
The reference center of each Wide field of view is defined as the reference W[1, 2, 3, 4](0,0) and the
Cartesian field names increase toward the East and North. The tiling and field naming conventions are
drawn on Fig. 20 and the complete list of the T0007 Wide stacks is given in Tables 31.
The total exposure time per filter is approximately the same for each tile (see Table 31 and the synoptic
table). For some tiles, the exposure time is longer than the nominal value, because exposures that were
considered out of spec by QSO and hence immediately reobserved have been later on salvaged at the post
processing time. It produces very sharp exposure time distributions for all filters (see Fig. 23). Some
observations have been adjusted to take into account unexpected events (like absorption by clouds/cirrus
or technical problems). There are a few fields that have deeper u ∗ -band and z-band exposures which
explains the small tails shown in the inset of Fig. 23. These longer exposures were taken before it was
decided to cut the u ∗ and z exposure times by a factor of two at the survey mid-term review (2005).
In addition to the original CFHTLS exposures, the VIPERS consortium took complementary exposures
to fill in the holes in W1 which were caused by malfunctioning CCDs later in 2003. These images
(distributed in 6 pointings and three filters u,g and y) have been added to T0007. As a result, these
pointing are deeper than the rest of the survey. A detailed description of the added VIPERS-DDT images
can be found in Table 5. Their position on the W1 patch are shown in Figure 22.
Wide position Filter Exposure time (per image) Number of exposures
[s]
W1+2+2 u.MP9301 600 6
W1+4+2 g.MP9401 290 7
W1+3+1 g.MP9401 290 5
W1+3+1 g.MP9401 290 5
W1+4+1 g.MP9401 290 5
W1+2+1 i.MP9702 300.096 7
W1+3+2 i.MP9702 300.092 5
Table 5: Description of the 40 additional VIPERS-DDT images.
All r-band stacks combine data taken during two epochs separated by at least two years and totalling
21 http://terapix.iap.fr/cplt/oldSite/Descart/summarycfhtlswide.html
32