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

This paper presents new positions for 689 FK5 stars determined directl y in the extragalactic reference system from CCD scan observations mad e with the Flagstaff Astrometric Scanning Transit Telescope (FASTT), A ll the observations were made in 1994-1996 with an accuracy of similar to+/-40 mas in each coordinate. When these positions are compared wit h 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 th e FK5 were found, Moreover, the true error in FK5 star positions is de termined to be similar to+/-80 mas in both coordinates or two times th e expected catalog positional errors. Then is excellent agreement betw een the systematic errors in the FK5 found in this paper and those pre sented in Lindegren et al. (1995, A&A, 303, 34) comparing FK5 and Hipp arcos 30-month star positions. By comparing corresponding FASTT and FK 5 star positions. the link between the optical and extragalactic refer ence frames was determined and is given by the following rotations (om ega(s),omega(y),omega(z))=(3+/-5,25+/-5,16+/-4) (s.e.) mast which agre e well with previous determinations made by other investigators. Two o f the rotations (omega(y),omega(z)) are significantly larger than thei r standard errors and, therefore, presumably real. Finally, since thes e rotations are small (less than or equal to 25 mas), the FK5 and extr agalactic reference flames must be closely aligned to one another.