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

4 Description of the CFHTLS T0007 Wide survey
4.1 Overview
The T0007 release of the Wide survey covers 171 MegaCam pointings (or tiles) divided into the W1,
W2, W3 and W4 fields and is complete in all filters. In total there are 855 u ∗ , g, r, i/y and z stacks and
171 chisquared images (2052 including the weight-maps). The positions and geometry of the four Wide
fields comprising T0007, namely W1, W2, W3 and W4, are shown on Fig. 19 and summarized in Table
3. Table 7 lists the effective area, after masking, of the four Wide patches in each of the filters.
The image selection criteria applied to the 6378 CFHTLS Validated images for the production of the
T0007 CFHTLS Wide survey are the following:
• CFHTLS L02 and L05 Wide observation period: between May 26, 2003 and November, 2008;
VIPERS-DDT images (2009BD97);
• TERAPIX selection of CFHT Validated images, with QualityFITS grade A or B (images within the
survey specifications);
• exposure time > 60 s;
• airmass < 1.7;
• few images with missing data on more than one entire CCD have been removed from the parent
sample. However, all images with missing data on only half of CCD detectors have been preserved;
• all pre-Wide survey images and all photometric short exposures. They will be included in the set
of images used for the SCAMP astrometric/photometric calibration process;
• images with seeing (FWHM 20 ) < 1.4” in u ∗ , and < 1.2” in g, r, i and z are selected for the
calibration process;
• images with seeing (FWHM) < 1.3” in u ∗ , and < 0.95” in g, r, i and z, are selected for the stack
production;
The parent sample of images after the QualityFITS selections is composed of 1000 u ∗ , 1059 g, 1728 r,
1064 i, 291 y and 1236 z-band images. 25% of the sample are short photometric or Pre-Wide exposures
that are only used for calibration but are not combined into stacks.
All stacks have the same pixel scale and cover exactly 1×1 deg 2 (19354×19354 pixels of 0.1860"). The
stacks only combine images that are part of a CFHTLS Wide observing sequence and that are within 3
arcminutes of the CFHTLS pre-defined stack center
Each stack comprises a sequence of medium exposures a few hundred seconds long each. After each
exposure the telescope is shifted by a small amount (typically a few tens of arcseconds, see the right
panel of Fig. 21) in order to fill the gaps between CCDs and enable the rejection of bad pixels and
cosmic rays during subsequent processing steps.
The adjacent fields have an overlapping region of several arcminutes width to constrain the field-to-field
astrometric and photometric calibrations. The size of the overlaps is 4 arcmin in DEC and 3 arcmin in
RA (see Fig. 21). The overlaps between fields reduce the sky coverage of the 171 CFHTLS Wide tiles
to approximately 155 deg 2 .
20 As defined in Section 4.2
27