A sophisticated mechanism was designed and built for the fast exchange and alignment of the mirrors of the optical cavity under UHV conditions. It is hosted in two twin chambers installed on either FEL exit; each chamber can host simultaneously three mirrors, and it takes few minutes to place the selected one on the optical cavity axis. Furthermore, they can be removed and replaced by others in few hours by means of a fast entry lock system, without breaking vacuum. All the motions (x,y,z and two angular motions, pitch and roll) are remote controlled and use either stepping or piezo motors, guaranteeing the necessary precision and accuracy. A water-cooling system makes it possible to keep the mirrors from overheating when exposed to synchrotron radiation. The two chambers are currently installed on the FEL exits and are fully operational.