THE ASTRONOMICAL JOURNAL    VOLUME 114, NUMBER 2, PAGE 850        AUGUST 1997

            SYSTEMATIC ERRORS IN THE FK5 CATALOG AS DETERMINED FROM
             CCD OBSERVATIONS IN THE EXTRAGALACTIC REFERENCE FRAME

                                RONALD C. STONE

                  U.S. Naval Observatory, Flagstaff Station,
                    P.O. Box 1149, Flagstaff, Arizona 86002


                                   ABSTRACT

     This paper presents new positions for 689 FK5 stars determined
     directly in the extragalactic reference system from CCD scan
     observations made with the Flagstaff Astrometric Scanning Transit
     Telescope (FASTT). All the observations were made in 1994-1996 with an
     accuracy of ~ +/- 40 mas in each coordinate. When these positions are
     compared with their counterparts in the FK5 catalog, systematic errors
     in the FK5 are found that vary in both right ascension and declination
     and can be as large as 100 mas in magnitude. No magnitude-dependent
     errors in the FK5 were found. Moreover, the true error in FK5 star
     positions is determined to be ~ +/- 80 mas in both coordinates or two
     times the expected catalog positional errors. There is excellent
     agreement between the systematic errors in the FK5 found in this paper
     and those presented in Lindegren et al. (1995, A&A, 304, 44) comparing
     FK5 and Hipparcos 30-month star positions. By comparing corresponding
     FASTT and FK5 star positions, the link between the optical and
     extragalactic reference frames was determined and is given by the
     following rotations (Omega_x,Omega_y,Omega_z) = (3+/-5, 25+/-5,
     16+/-4) (s.e.) mas, which agree well with previous determinations made
     by other investigators. Two of the rotations (Omega_y,Omega_z) are
     significantly larger than their standard errors and, therefore,
     presumably real. Finally, since these rotations are small (<= 25 mas),
     the FK5 and extragalactic reference frames must be closely aligned to
     one another.