This document describes T0007, the 7th and final release of the Canada-France-Hawaii Telescope Legacy Survey CFHTLS1 , produced by Terapix2 based on a data set collected with MegaCam3 on the CFHT. CFHTLS-T0007 is a deep sub-arcsecond (0.8′′) wide-field (157 deg2 total) optical survey (u*,g,r,i,z bands) providing a high quality and homogeneous data set precisely calibrated photometrically (1.0%) and astrometrically (0.028′′). This final release is directly public, open to the worldwide community.
CFHTLS-T0007 has two components: 1) the “CFHTLS Deep”, four independent 1 deg2 MegaCam ultra deep pointings, reaching a 80% completeness limit in AB of u*=26.3, g=26.0, r=25.6, i=25.4, z=25.0 for point sources, and 2) the “CFHTLS Wide” made of 171 MegaCam deep pointings which, due to overlaps between adjacent fields consists of a total of ~ 155 deg2 in four independent contiguous patches, reaching a 80% completeness limit in AB of u*=25.2, g=25.5, r=25.0, i=24.8, z=23.9 for point sources. The sky location of these fields is shown in Figure 1. This final release of the CFHTLS greatly benefits from vastly improved flat-fielding and photometric calibration techniques developed by the Supernova Legacy Survey (SNLS) team in collaboration with the CFHT. These new recipes significantly improve the precision of our photometric calibration compared to previous releases.
T0007 is derived from a parent sample comprising all validated images taken during the survey between May 26, 2003 and Feb. 02, 2009 and delivered by the CFHT pre-processing pipeline Elixir4 . In addition, this release contains VIPERS-DDT (CFHT Director’s Discretionary Time) observations targeted to fill missing half-CCDs caused by malfunctioning detectors over the course of five months in 2003. All science stacks have been visually inspected; quality control information is available for each of them.
Observations were made in u*,g,r,z and either i or y filters (y is the Terapix designation for the replacement i-band filter, also known as i2 at CFHT). Stacks are provided as MegaCam-size FITS images each covering 1 deg2 with 0.186′′pixels. Each tile is located at each pre-defined center position listed in Table 30 for the Wide survey and Table 16 for the Deep survey. In addition, the Deep survey contains two sets of stacks, those comprising the “85%” best-seeing images (D85) and those comprising the “25%” best seeing images (D25). In total there are 48 Deep stacks corresponding to u*,g,r,i,y and z bands, 12 for each field. There are 855 Wide stacks, in u*,g,r,i,y and z bands or 360, 125, 245, 125 for W1-4. Over the 171 fields of the Wide, 141 are covered in i-band and 30 in y-band. The total volume of the T0007 release (stacks and catalogs) amounts to nearly 9 terabytes.
The internal astrometric errors of the stacks are between 1/15 and 1/3 of pixel, in x and y. The external astrometric errors are between 0.20′′ and 0.27′′ and are limited by the reference catalogue accuracy. The astrometric offsets between the CFHTLS and the astrometric reference catalogue are negligible. They are smaller than ±1/10 of pixel in both axes and for all fields. Photometric errors have been estimated for all stacks using several independent methods. Based on external photometric comparisons with the SDSS, we find that our absolute photometric accuracy in the AB standard as well as the internal photometric homogeneity over the entire survey is at the 1.0% level or better in the g,r,i bands, 1.5% level in the z band, and 2% level in the u* band.
Catalogues have been produced for each stack using a g-r-i selected “chi2” as a detection image and an associated mask. These data sets are complemented by a series of merged catalogues that combine information for each source in all filters into one file in addition to many different types of quality control data (tables, plots, figures). Merged catalogues for each of the four Deep fields and each of the four Wide patches are provided. Sources from overlapping tiles have been dealt with correctly and the parent tile information is recorded for each source. Aperture magnitudes, total magnitudes, and variable-aperture magnitudes (based on the image quality for each tile) are provided for each source.
For a concise overview of the survey, the reader should consult Tables ??, 17 and 18. These tables provide all basic data on the survey coverage, seeing, exposure times, depth and completeness, astrometric and photometric errors. This release document is complemented by practical information at the Terapix T0007 release page and the T0007 “synoptic table”5 . All data sets are available worldwide directly from both Terapix and the Canadian Astronomy Data Centre (CADC). In addition, searchable catalogues are made available at the Centre de Données astronomiques de Strasbourg (CDS) through VizieR while the entire survey can now be explored visually through the Aladin sky atlas in the HEALPix format.
For users who prefer to produce their own stacks using the individual Elixir images, which are available at CADC along with their new weight-map produced by Terapix, a complete set of T0007 configuration files are listed in the Appendix A.2. In addition, a list of all images used in each Wide stack is given in Table 31. Weight maps and astrometric solution files are also provided for each individual image.
All T0007 data products are immediately available worldwide along with the photometric redshifts which are released simultaneously (Ilbert et al. 2012). All the products can be downloaded from the CFHTLS T0007 release page at TERAPIX: http://terapix.iap.fr/rubrique.php?id_rubrique=2666
For further information, the Terapix team can be contacted at email@example.com.
CFHT, with support from CEA, has provided outstanding MegaPrime/MegaCam support, ensuring the steady collection of data of uniform quality through CFHT’s Queued Service Observing. CFHT provided data pre-processing and calibration (Elixir) and distribution (DADS). Overall survey integrity has been ensured by the CFHTLS Steering Group and oversight by the CFHT Science Advisory Committee. Data have been archived and distributed by CADC. It is important to acknowledge this collective effort in all works using the CFHTLS data. Following recommendations of the CFHT Executive Director and the CFHT Board of Directors, the following acknowledgement should be part of any publication using CFHTLS data:
”Based on observations obtained with MegaPrime/MegaCam, a joint project of CFHT and CEA/DAPNIA, at the Canada-France-Hawaii Telescope (CFHT) which is operated by the National Research Council (NRC) of Canada, the Institut National des Sciences de l’Univers of the Centre National de la Recherche Scientifique (CNRS) of France, and the University of Hawaii. This work is based in part on data products produced at Terapix and the Canadian Astronomy Data Centre as part of the Canada-France-Hawaii Telescope Legacy Survey, a collaborative project of NRC and CNRS.”7