2 Observations and reductions

References to the optical identifications, finding charts, and other relevant data for the sources can be found in Jauncey et al. (1996). Notes on individual objects can be found in Sect. 5.

The two-step observational and reduction procedure was the same one described in detail in Costa & Loyola (1992, hereafter Paper I), with the exception that on this occasion the Hipparcos Catalogue was used as primary reference frame. The number of Hipparcos stars that were identified in each $5^\circ \times 5^\circ$ field around the CERS varied between 48 and 129, which gave an average of roughly 60 measurable stars (excluding close pairs, stars near the edges of the plates and bright stars); with lower densities found towards the equatorial zone. In comparison to the IRS system used in our previous work, the Hipparcos Catalogue not only provides a more precise primary reference frame, but also a denser and more homogeneous grid of reference objects in the field of the CERS. This latter characteristic is a major improvement for our application; extensive experience with the IRS and AGK3R/Perth 70 catalogues, which provided an average of less than 20 measurable stars in the fields of the CERS, has shown that the solutions for cases with low density of catalogue stars are critically dependent on their distribution.