Radio-Optical Reference Frame Link Using the US Naval Observatory
  Astrograph and Deep CCD Imaging
  Between 1997 and 2004 several observing runs were conducted mainly with the
CTIO 0.9 m to image ICRF counterparts (mostly QSOs) in order to determine
accurate optical positions. Contemporary to these deep CCD images the same
fields were observed with the US Naval Observatory (USNO) astrograph in the
same bandpass. They provide accurate positions on the Hipparcos/Tycho-2 system
for stars in the 10 to 16 magnitude range used as reference stars for the deep
CCD imaging data. Here we present final optical position results of 413 sources
based on reference stars obtained by dedicated astrograph observations which
were reduced following 2 different procedures. These optical positions are
compared to radio VLBI positions. The current optical system is not perfectly
aligned 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 exceed
the total, combined, known errors in the observations. Systematic errors in the
optical reference star positions as well as physical offsets between the
centers of optical and radio emissions are both identified as likely causes. A
detrimental, astrophysical, random noise (DARN) component is postulated to be
on about the 10 mas level. If confirmed by future observations, this could
severely limit the Gaia to ICRF reference frame alignment accuracy to an error
of about 0.5 mas per coordinate axis with the current number of sources
envisioned to provide the link. A list of 36 ICRF sources without the detection
of an optical counterpart to a limiting magnitude of about R=22 is provided as
well.
