4 Image alignment  

Position measurements with the precision required for the alignment of VLBI images cannot be made without extensive absolute or relative astrometry observations. In quasi-simultaneous multi-frequency VLBI observations, the phase-referencing technique (Beasley & Conway 1995) can be sufficient for the purpose of image alignment. If neither of the aforementioned techniques is available, VLBI images are usually aligned by means of the position of compact core of the source. The core is likely to be located in an optically thick environment, and its position must depend on the observing frequency, $\nu$. According to Königl (1981), the core is observed at the separation $r_{\rm core} \propto \nu^{-1/k_r}$ from the true jet origin. The term k_r is close to unity (Marcaide et al. 1985; Lobanov 1997), so that $r_{\rm core}$ depends almost inversely on the frequency. Therefore, if aligned by the position of the core, VLBI images may contain systematic position offsets undermining spectral imaging.

The frequency dependent shift of the core position can be deduced from comparison of observations made at close epochs, assuming that the superluminal features observed in the jet are optically thin and therefore should have their positions unchanged. In this case, the offsets between the component locations measured at different frequencies will reflect the respective shift of the observed position of the optically thick core. This approach has been successfully used in several studies (e.g. Biretta et al. 1986; Zensus et al. 1995; Lobanov 1997), and we deem it to be sufficient for the purpose of aligning VLBI images at different frequencies.