A simplified diagram of an AGN at milliarcsecond scales in the plane of the sky. The “BH” cross marks a position of the supermassive black hole as well as the accretion disk. The arrow represents the jet (j vector) while the counter-jet goes in the opposite direction. The apparent base of the jet in radio is shown by “C_r”, in the optical band it is expected to be closer to the central engine “BH” (Kovalev et al. 2008) and is shown as “C_o”. The absolute radio VLBI “P_V” and optical Gaia “P_G” reference points are shown by blue and green dots, respectively. The red offset vector VG connects these points. The host galaxy can be relatively bright in optical band and shift the optical centroid in any direction, the galaxy center is marked by a star.