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3 Results

Optical coordinates with an accuracy of $\sim1$ arcsec have been measured for 2978 SBS objects. Rms deviation between the coordinates listed in original SBS papers and the newly measured positions is $1\hbox{$.\mkern-4mu^\prime$ }5$ arcmin in both RA and Dec. The coordinates listed for PG objects which were determined from blue plates, then converted to celestial equatorial coordinates through the method as described in Green & Morril (1978), show an rms deviation with respect to the newly measured positions of about 15'' both in RA and Dec. Case - coordinates were derived from measurements made on the IIIaJ objective-prism plates using the x, y measuring machine of Warner and Swasey Observatory. The rms difference is $\pm\ 2\hbox{$.\!\!^{\prime\prime}$ }5$ in RA and $3\hbox{$.\!\!^{\prime\prime}$ }0$ in Dec., but there is a systematic deviation of about +3'' by Dec. The comparison of our positions with the most accurate position measurements (Bowen et al. 1994; Véron-Cetty & Véron 1996) (see Table 3) shows that the accuracy of our measurements is about 1''.

Acknowledgements
The STScI/DSS was produced at the Space Telescope Science Institute under US government grant NAGW-2166. The survey images are based on photographic data obtained using the Oschin Schmidt Telescope on Palomar Mountain, and were processed into compressed digital form with the permission of Palomar Observatory. The National Geographic Society - Palomar Observatory Sky Survey (POSS-I) was made by the California Institute of Technology with grants from the National Geographic Society.

The US-based investigators wish to acknowledge the assistance of Daniel Golombek (STScI).

JS would like to thank Dr. Mark Bundy of Baltimore, MD for his hospitality during the author's two week visit measuring position coordinates at the Space Telescope Science Institute. One of us (GK) wishes to acknowledge that this research was partially supported by a grant from NASA administered by the AAS.

JAS, VHCh, LKE and VTA wish to thank V. Vlasyuk, for use of his data conversion software and H. Tovmassian for helpful discussion. We thank N. Serafimovich for her help in preparing the paper. JAS, LKE and VTA were partially supported by the research grant No. 97-02-17168 from the Russian Foundation for Basic Research and No. 1.2.2.2 grant from Russian State program "Astronomy". VHCh was supported by the CONACYT research grant No. 28499-E.

We thank the referee Dr. M.P. Véron for a number a valuable comments which helped to improve the paper.


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