The Acs Survey of Globular Clusters. V. Generating a Comprehensive Star Catalog for Each Cluster

Abstract

The ACS Survey of Globular Clusters has used HST's Wide-Field Channel to obtain uniform imaging of 65 of the nearest globular clusters to provide an extensive homogeneous dataset for a broad range of scientific investigations. The survey goals required not only a uniform observing strategy, but also a uniform reduction strategy. To this end, we designed a sophisticated software program to process the cluster data in an automated way. The program identifies stars simultaneously in the multiple dithered exposures for each cluster and measures them using the best available PSF models. We describe here in detail the program's rationale, algorithms, and output. The routine was also designed to perform artificial-star tests, and we run a standard set of ∼10 5 tests for each cluster in the survey. The catalog described here will be exploited in a number of upcoming papers and will eventually be made available to the public via the world-wide web.

The HST large programme on ω Centauri – I. Multiple stellar populations at the bottom of the main sequence probed in NIR–Optical

“…We have used the method based on effective-PSF fitting (e.g. Anderson et al 2008) to derive high-precision stellar photometry and astrometry and determined stellar proper motions to separate cluster members from field stars.…”

“…Data were reduced with an adaptation to WFC3/NIR images of the software presented and described in Anderson et al (2008) for the case of the ACS/WFC camera. The software is essentially a PSF-fitting algorithm that performs simultaneous fitting of all the sources in a given patch of sky, by simultaneously using all the available images for that patch of sky, once transformed into a common reference frame.…”

“…Details for these fields were presented in King et al (2012). We independently re-reduced the two data-sets from GO-9444 and GO-10101, using the software described in Anderson et al (2008). We then averaged the stellar magnitudes observed in the two data-sets.…”

Hubble Space Telescope observations of the Kepler-field cluster NGC 6819 – I. The bottom of the white dwarf cooling sequence★

“…Finally, in all images, we masked-out by hand the ghosts of the brightest stars in the fields. 1 Photometry and relative positions were obtained with the software tools described by Anderson et al (2008). In addition to solving for positions and fluxes, we also computed two important diagnostic parameters: quality-fit and rmsSKY.…”

“…In addition to solving for positions and fluxes, we also computed two important diagnostic parameters: quality-fit and rmsSKY. The quality-fit essentially tells how well the flux distribution resembles the shape of the PSF (defined in Anderson et al 2008); this parameter can be very useful for eliminating the faint blue galaxies that plague studies of WDs (see Bedin et al 2009 for detailed discussions). The sky-smoothness parameter rmsSKY is the rms deviation from the mean, for the pixels in an annulus from 3.5 to 8 pixels from each source.…”