The VIRMOS deep imaging survey : III . ESO / WFI deep U - band imaging of the 0226 - 04 deep field . ⋆

In this paper we describe the U−band imaging of the F02 deep field, one of the fields in the VIRMOS Deep Imaging Survey. The observations were done at the ESO/MPG 2.2m telescope at La Silla (Chile) using the 8k × 8k Wide-Field Imager (WFI). The field is centered at α(J2000)=02 h 26 m 00 s and δ(J2000)=-04 • 30 ′ 00 ′′ , the total covered area is 0.9 deg 2 and the limiting magnitude (50% completeness) is U AB ∼ 25.4 mag. Reduction steps, including astrometry, photometry and catalogue extraction, are first discussed. The achieved astrometric accuracy (RMS) is ∼ 0.2 ′′ with reference to the I−band catalog and ∼ 0.07 ′′ internally (estimated from overlapping sources in different exposures). The photometric accuracy including uncertainties from photometric calibration, is < 0.1 mag. Various tests are then performed as a quality assessment of the data. They include: (i) the color distribution of stars and galaxies in the field, done together with the BVRI data available from the VIMOS survey; (ii) the comparison with previous published results of U−band magnitude-number counts of galaxies.