Deep FORS1 observations of the double main sequence of ω Centauri

We present the results of a deep photometric survey performed with the VLT FORS1 aimed at investigating the complex main-sequence structure of the stellar system ! Centauri. We confirm the presence of a double main sequence and identify its blue component (bMS) over a large field of view up to 260 from the cluster center. We found that bMS stars are significantly more concentrated toward the cluster center than the other normal'' MS stars. The bMS morphology and its position in the CMD have been used to constrain the helium overabundance required to explain the observed MS morphology.