1 Introduction

The advent of the Hipparcos Catalogue, which was constructed to coincide with the ICRF (Kovalewsky et al. 1997), and therefore can be considered an extension to the optical domain of the extragalactic radio reference frame, has provided the appropriate framework for the interpretation of subtle positional differences between the radio and optical emission centroids of CERS with star-like optical counterparts (se e.g. Costa & Loyola 1998).

This type of limited structure analysis is important for identifying the most suitable objects to fine-tune the link between the radio and optical reference frames, and also to uncover astrometric outliers that are potentially interesting sources from an astrophysical point of view. Less directly, the observation of the optical counterparts of CERS with respect to Hipparcos stars provides the means to maintain, and possibly improve, the current link between the Hipparcos reference frame and the ICRS. Although at the moment the above link procedure is less precise than other methods that contributed to the realization of the present link (see e.g. Kovalewsky et al. 1997), on account of the large number of well distributed link sources involved, it has a great potential (see e.g. Zacharias et al. 1995).

With the above objectives in mind, we have started a program to identify the optical counterparts, and determine precise positions with respect to the Hipparcos frame, of faint ($B\sim20-23$) CERS. The present survey can be considered an extension to fainter magnitudes of a two decades old CERS monitoring program carried out at the Estación Astronómica de Cerro El Roble (EACR), operated by the University of Chile (see e.g. Costa & Loyola 1997). In this paper we present our first results and a full description of the project.

Targets were selected from the NRL/USNO proposed reference frame list (Fey 1994) and the list of Jauncey et al. (1989). In order to fully evaluate the capabilities of the multi-step procedure employed to derive the positions, a variety of targets were observed, among them suspected empty fields, CERS with very faint provisional optical identification and CERS with optical counterparts showing evidence of structure.