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according to the fringe amplitudes measured on one hundred peak-valley pairs on each CCD. Since all
CCDs see the same sky and since the photometric zero-point is uniform across the entire field of view,
a single scaling factor is derived from the 36 CCDs. The scaled exposures are stacked, and an iterative
process similar to the one described above is carried out, with a visual control allowing the rejection
of frames containing extended astrophysical sources such as large galaxies. The fringe correction is
challenging at times in the z-band and some images get only partially corrected due to the extreme
behavior of the OH emission lines in the upper atmosphere which cause a signature too different from
the run master fringe frame. (A different observing strategy such as the one adopted for the MegaCam
Next Generation Virgo Survey would have helped but was not available during CFHTLS observations.)
Consequently the defringing recipe is unchanged for the T0007 data collection. All fringe patterns were
however re-created since they must include the signature of the new photometric B5/SNLS flat-fields.
After these detrending steps, Elixir processes all the images of the run, and derives an astrometric solution
per CCD only, at the pixel scale level (0.2 ′′ ). The goal at the Elixir level is to provide the users with a
first order astrometric solution and no global solution over the mosaic is computed; this is a task handled
by Terapix.
Following this step, all the frames containing Smith et al. (2002) photometric standards (the CFHT QSO
“Q97” program. Please refer to the description of the Observing Programs Identificators 10 ) are identified
and processed using SExtractor. A median zero-point for the entire run is derived for each filter, since
not enough observing time is available to derive enough standard star observations per night to derive
solid zero-point solutions. Again, the intention is to provide MegaCam users with a reliable photometric
scaling, offering a precision at the 4% level in absolute. But then again, since this default calibration was
clearly not precise enough for their needs, the SNLS team developed in collaboration with CFHT new
procedures to calibrate the images. This vast undertaking is the key to major improvements in T0007
compared to T0006: all knowledge acquired for the SNLS survey has been passed to the Deep and the
Wide surveys. This calibration effort is described in detail in Section 3.7.
3.3 Overview of T0007 processing at Terapix
TERAPIX processing steps from the download of Elixir pre-processed CFHT images to the final stacked
images and catalogues is illustrated in Fig. 3. The T0007 CFHTLS pre-processed images described in the
previous section were transferred from CFHT and validated against the T0007 image lists. All Queued
Service Observing validation flags archived at CFHT were also downloaded 11 .
For the T0007 release we use Elixir “B5/SNLS” pre-reduced images. The images were first ingested
with the Terapix processing pipeline YOUPI, producing weight-maps and input catalogues. We use quality
grades from previous CFHTLS releases for images which were already in the database; new images from
the “L99” photometric calibration program and the VIPERS Director’s Discretionary Time observations
(described later) were graded using the Youpi grading interface, described below.
Images were then divided into each of the four Wide and four Deep fields and processed with SCAMP
(Bertin, 2006) in order to derive the astrometric as well as the initial photometric calibrations. As explained
below, as a consequence of the new calibration scheme based on L99 images, the astrometric and
photometric calibrations were performed separately.
Once aligned astrometrically, images are co-added by SWARP (Bertin et al., 2002) using the SCAMP initial
photometric rescaling. For the Deep field, images lists are derived using seeing and photometric rescaling
10 http://www.cfht.hawaii.edu/Science/CFHTLS-DATA/cfhtlsprograms.html
11 http://www.cfht.hawaii.edu/Science/CFHTLS-DATA
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