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Between 1997 and 2004 several observing runs were conducted mainly with theCTIO 0.9 m to image ICRF counterparts (mostly QSOs) in order to determineaccurate optical positions. Contemporary to these deep CCD images the samefields were observed with the US Naval Observatory (USNO) astrograph in thesame bandpass. They provide accurate positions on the Hipparcos/Tycho-2 systemfor stars in the 10 to 16 magnitude range used as reference stars for the deepCCD imaging data. Here we present final optical position results of 413 sourcesbased on reference stars obtained by dedicated astrograph observations whichwere reduced following 2 different procedures. These optical positions arecompared to radio VLBI positions. The current optical system is not perfectlyaligned to the ICRF radio system with rigid body rotation angles of 3 to 5 mas(= 3 sigma level) found between them for all 3 axes. Furthermore,statistically, the optical minus radio position differences are found to exceedthe total, combined, known errors in the observations. Systematic errors in theoptical reference star positions as well as physical offsets between thecenters of optical and radio emissions are both identified as likely causes. Adetrimental, astrophysical, random noise (DARN) component is postulated to beon about the 10 mas level. If confirmed by future observations, this couldseverely limit the Gaia to ICRF reference frame alignment accuracy to an errorof about 0.5 mas per coordinate axis with the current number of sourcesenvisioned to provide the link. A list of 36 ICRF sources without the detectionof an optical counterpart to a limiting magnitude of about R=22 is provided aswell.

Radio-Optical Reference Frame Link Using the US Naval Observatory Astrograph and Deep CCD Imaging