3 Reductions

The X, Y coordinates of the reference stars on the photographic plates were measured to $1\mu$ ($0.1\hbox{$^{\prime\prime}$}$ at the plate scale) with a digital Zeiss-Jena Ascorecord measuring machine. Only one setting was made on the catalogue, secondary and tertiary reference stars. In the few cases in which the optical counterpart of the CERS was measurable directly on the long exposure plate, at least 4 settings were made on it. The photographic position thus determined was used to check for consistency. Plates were measured both in direct and reverse positions in an effort to cancel possible systematic errors in the X, Y values. Six term (const., X, Y, XY, X², Y²) quadratic relations were used in the reductions. Third order terms were not included because the flat field of the Maksutov Astrograph is almost free of distortions. Also, since all plates were taken near culmination, refraction third order terms were not important. The standard deviation of the differences between calculated and catalogue values of the Hipparcos stars turned out to be $0.23\hbox{$^{\prime\prime}$}$ in X and $0.22\hbox{$^{\prime\prime}$}$ in Y.

All raw CCD frames were first calibrated (overscan, zero level and flat field corrected) using standard IRAF (version 2.10, NOAO, University of Arizona) tasks. The four CCD images taken of each CERS were then registered and combined using algorithms available in the IRAF IMAGES package, to produce the final image from which the X, Y coordinates of the PSF centroids of the CERS and the tertiary stars were extracted. This latter step was carried out by means of the PEAK task within the DAOPHOT package (Stetson 1987). Although this package was originally designed for photometric programs, it has proven to be an excellent tool to compute image positions (Monet 1992; Anguita & Loyola 1995).

  

Table 2: Optical positions in the ICRS system of 15 compact extragalactic radio sources

   CT stands for Cerro Tololo,
   LC stands for Las Campanas,
   Phot. indicates a position derived from direct photography.