Ground-based CCD astrometry with wide field imagers - I. Observations just a few years apart allow decontamination of field objects from members in two globular clusters

This paper is the first of a series of papers in which we will apply the methods we have developed for high-precision astrometry (and photometry) with the Hubble Space Telescope (HST) to the case of wide-field ground-based images. In particular, we adapt the software originally developed for WFPC2 to ground-based, wide field images from the WFI at the ESO 2.2m telescope. In this paper, we describe in details the new software, we characterize the WFI geometric distortion, discuss the adopted local transformation approach for proper-motion measurements, and apply the new technique to two-epoch archive data of the two closest Galactic globular clusters: NGC 6121 (M4) and NGC 6397. The results of this exercise are more than encouraging. We find that we can achieve a precision of similar to 7 mas (in each coordinate) in a single exposure for a well-exposed star, which allows a very good cluster-field separation in both M4, and NGC 6397, with a temporal baseline of only 2.8, and 3.1 years, respectively.