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PI :
 Y. Mellier

Co-I :
 F. Bernardeau
 M. Dantel-Fort
 T. Erben
 B. Fort
 B. Jain
 R. Maoli
 E. Semboloni
 P. Schneider
 L. van Waerbeke

The VLT-DESCART ESO Large Programme aims at measuring the variance of the gravitational shear on angular scale smaller than 5 arcmin. The goal is to analyse the evolution of the cosmic shear variance as function of angular scale in order to derive constrains on the cosmological parameters Omega, sigma_8 and lambda. The signal is produced by gravitational weak lensing effects of light beams that cross large-scale structures of the universe on cosmological scales.

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) that can probe much larger angular scales. However, the VLT-DESCART survey can benefit from its large number of uncorrelated fields to handle cosmic variance.

From the observational point of view, it consists in :
 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/arcmin².

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 program on the VLT, in service mode. This should in principle guarantees 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 program was scheduled during the observing period P63 (April to September 1999; proposal 63.O-0039A) on UT1/FORS1 and was 75% successful.

VLT-Descart Scientific results

At ESO-Garching :
 ESO/VLT Press Release
 ESO Messenger : page 10

1. A&A 368, 766 (2001) :"Cosmic Shear Analysis in 50 Uncorrelated VLT Fields. Implications for Omega_0, sigma_8" by R. Maoli (IAP, Obs. de Paris, La Sapienza, Rome), L. van Waerbeke (IAP), Y. Mellier (IAP, Obs de Paris), P. Schneider (Univ. Bonn), B. Jain (JHU), T. Erben (IAP, MPA), B. Fort (IAP)