The jet collimation in Cygnus A
The powerful radio galaxy Cygnus A is one of the very few targets where the jet acceleration and collimation region can be probed directly through VLBI imaging. Through a rich, multi-wavelength VLBI data set, we were able to reconstruct the two-sided collimation profile of the outflow, and to follow its evolution from scales of a few hundreds to millions Schwarzschild radii. Preliminary results suggest that important transitions occur over the examined scales. These are likely to mark changes in the physical conditions of the plasma or of the ambient medium.