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The
VLT Key Project aims at measuring the variance of the convergence on scale
smaller than 5 arcmin. The goal is to obtain constrains on the cosmological
parameters Omega, sigma_8 and lambda, from weak lensing produced by large-scale
structures. The VLT project focusses on non-linear scales (below 5 arcminutes),
in contrast to the wide-field part of the project (UH8K, CFH12K, WFI and
MEGACAM).
From
the observational point of view, it consists in
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50
empty fields observed on the VLT/UT1 with FORS1 in I-band, randomly selected,
each of them separated by more than 1 degree. Regions with brights stars
or bright galaxies have been avoided;
-
each
individual field covers a field of view of 6.8'x6'8'. The total area is
0.5 degree dispached in 2 sub-areas which spread over more than 100 square-degree;
-
the
exposure time per field is 36 minutes segmented in 6 exposures of 6 minutes
each in order to ease cosmic-ray suppression and superflat. The expected
limiting magnitude per field is 24.6 which permits to reach a galaxy number
density of 30/arcmin^2.
Data
and progress report:
The
design of the survey and the strategy of the observations are based on
the ESO NTT Key project that we finally submitted as a normal programme
on the VLT, in service mode. This should in principle garantees that the
whole data set are within our specifications: seeing <0.8" . We choose
the I-band filter in order to minimized atmospheric dispersion. First exposures
taken during the Science Verification phase have shown that the I-band
images have no fringes.
The
programme was sheduled during the observing period P63 (April to September
1999; proposal 63.O-0039A) on UT1/FORS1 and was 75% successful. |
