ESO imaging survey - Pre-FLAMES survey: Observations of selected stellar fields

This paper presents the rst set of fully calibrated images and associated stellar catalogs of the Pre- FLAMES survey being carried out by the ESO Imaging Survey (EIS) project. The primary goal of this survey is to provide the ESO community with data sets from which suitable target lists can be extracted for follow-up observations with the new VLT facility FLAMES (Fiber Large Array Multi Element Spectrograph). For this purpose, 160 stellar elds have been selected for observations in B, V and I using the 8k 8k Wide Field Imager (WFI) at the MPG/ESO 2.2 m telescope at La Silla. Out of those, over 100 elds have already been observed. The list of selected elds includes open clusters, globular clusters, regions in the Galaxy bulge, dwarf spheroidal galaxies in the vicinity of the Milky Way, contiguous regions of SMC and LMC and few nearby clusters of galaxies. The present paper discusses the results obtained for a small subset of these data, which include four open clusters (M 67, NGC 2477, NGC 2506 and Berkeley 20) and two regions of the SMC. These data have been used to assess the observing strategy adopted, a combination of short- and long-exposures, and to dene suitable reduction techniques and procedures for the preparation of input catalogs for FLAMES. In order to minimize light losses due to misplacements of FLAMES bers, the astrometric calibration of crowded stellar elds is a critical issue. The impact of dierent swarping techniques and dierent reference catalogs on the astrometric calibration of the images is evaluated and compared to those of other authors. From this comparison, one nds that both USNO 2.0 and the recently released GSC 2.2 yield comparable results with the positional dierences having a rms of about 0:15 arcsec, well within the requirements (0:2 arcsec) specied by the FLAMES science team. The internal accuracy of the astrometry is estimated to be <0.1 arcsec, primarily limited by the reference catalog used. The major dierence between these catalogs is the systematic variation of the positional residuals as a function of the apparent magnitude of the objects, with the GSC 2.2 yielding by far the best results. The astrometric calibration of the images presented here is based on the USNO 2.0 catalog because not all elds considered are covered by the current release of the GSC 2.2. Future EIS calibrations will be done using the GSC 2.2 catalog. The extraction and photometric measurements of stellar sources are carried out using a PSF tting technique. Comparison with results available in the literature shows that the photometric measurements are in good agreement, apart from possible zero-point osets, with the magnitude dierences having a scatter of0.06 mag at V = 20 mag. This demonstrates that the data allow for the selection of robust targets down to the expected spectroscopic limit of FLAMES. The combination of catalogs extracted from the short and long-exposures allows one to produce color- magnitude diagrams (CMD) spanning13 mag in V and reaching a limiting magnitude of V 22 23. These data have also been combined with data from the Two Micron All Sky Survey (2MASS) survey allowing for a better color-based object classication and target selection. The Pre-Flames (PF) survey data meet the requirements of FLAMES, and provide a good starting point for detailed studies of the examined systems. The images and catalogs presented here are publicly available and can be requested from the URL address \http://www.eso. org/eis".