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een preferable because of the higher surface density of sources in SDSS and the similarity of SDSS and
CFHTLS filter sets; however the SDSS does not cover the whole CFHTLS survey; only W3 and D2 and
D3 fields have complete coverage.)
To match CFHTLS and 2MASS catalogues we choose a source matching radius (CROSSID_RADIUS) of
2.0 arcsec for all Wide and Deep fields. A 3 rd order polynomial distortion model is then derived by
minimizing a weighted quadratic sum of differences in positions between the 2MASS and the QFITSin
matched sources, and, internally, between different QFITS-in catalogues with overlapping regions of
MegaCam images (see Fig. 21). Scamp then computes the astrometric internal errors from the differences
of astrometric positions of sources inside the overlapping regions and the external errors from the
comparisons of astrometric positions of MegaCam sources with the 2MASS catalogue.
Images are calibrated separately for each of the four Wide and four Deep fields. However, in order to
take full advantage of L99 photometric images, processing is done in two steps:
• Astrometric calibration – this includes all types of images: science images, short and L99 photometric
calibration images, r-band astrometric anchor pre-survey exposures and supplementary
VIPERS-DDT (discretionary time) exposures for filling missing half-CCDs in W1.
• Initial photometric rescaling – this includes only science images and photometric bootstrapping
(Q98 - Observing Programs Identificators 15 ) images and it uses the astrometric solution from the
previous step. The primary goal of this step is to compute the relative rescaling of the individual
images in each pointing before stacking. The absolute photometry is then anchored to the L99
exposures (see the procedure described in Section 3.7).
In addition, due to the large number of exposures in some of the fields the astrometric calibrations cannot
be solved globally (due to memory requirements) and are additionally split by filter. For those fields
the large number of observing runs produces too many astrometric contexts which cannot be handled
by Scamp using current Terapix computing resources. For this reason, the astrometric calibration is
computed in a field-dependent way.
For the W1 field, for which the astrometric solution is derived with respect to a reference catalogue we
use this procedure:
• We first combine (r, i/y) .ldac catalogues of the science exposures as well as the r-band Wide presurvey
astrometric anchoring images and find an astrometric solution using the 2MASS reference
catalogue and use this to produce an internal (r, i/y) astrometric reference catalogue, calibrated
with 2MASS;
• We then run Scamp on all W1 exposures using this internal reference catalogue on the (u, g), (r, z),
and (i/y) samples separately. The homogeneity and consistency of the calibrations are ensured by
the common r-band Wide pre-survey data used for all samples.
For W2, W3 and W4 fields, the astrometric solution is calculated once for each field by considering all
selected images simultaneously, regardless the filter, exposure type or epoch, using 2MASS. In this way
all images of a given Wide field are calibrated globally and in a homogeneous way.
All Deep fields are calibrated with SCAMP in a similar way as W1. However, the image selection used to
construct the internal reference catalogue is done differently:
15 http://www.cfht.hawaii.edu/Science/CFHTLS-DATA/cfhtlsprograms.html
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