The T0007 documentation describes the 7th and final release of the
Canada-France-Hawaii Telescope Legacy Survey produced by Terapix based on
a data set collected with MegaCam on the CFHT. CFHTLS-T0007 is a deep
sub-arcsecond (0.8") wide-field (157 square degrees 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 arcsec). This final release is directly public, open to the
CFHTLS-T0007 has two components: 1) the "CFHTLS Deep", four
independent single 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 square degrees 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
here. 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 the 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 Elixir. In
addition, this release contains 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 square degree with 0.186 arcsec pixels.
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 the 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.
Tables in the T0007 documentation 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 Terapix T0007 synoptic table.
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 of the T0007 documentation.
In addition, a list of all images used in each Wide stack is provided.
Weight maps and astrometric solution files are also provided for each
All T0007 data products are immediately available worldwide along with
the photometric redshifts which are released simultaneously.
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). Terapix has painstakingly curated the data up to this final
achievement. Data have been archived and distributed by CADC.
Overall survey integrity has been ensured by the CFHTLS Steering
Group and oversight by the CFHT Science Advisory Committee. 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/IRFU, at the Canada-France-Hawaii Telescope (CFHT)
which is operated by the National Research Council (NRC) of Canada, the
Institut National des Science 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 available at the
Canadian Astronomy Data Centre as part of the Canada-France-Hawaii Telescope Legacy
Survey, a collaborative project of NRC and CNRS.