How does the ICRF differ from the FK5 frame?

ICRF objects are extragalactic and the measurements are derived from interferometric observations at microwave frequencies. FK5 objects are galactic stars measured optically. ICRF objects have no measurable proper motion. The z-axis and right ascension origin of the ICRS are defined by the positions of the ICRF defining sources. The mean pole and right ascension origin of FK5 are nominally aligned with the equator and ecliptic of J2000.0. The two frames are oriented within 50 mas in pole position and 80 mas in right ascension origin (at J2000); FK5 rotates with respect to ICRS about 3 mas/y.