This afternoon I installed the picomotor and the translation stage that will be used to move the retaining rings up and down. No partciular problem: I only had to add some small aluminum foil shims between the ear of some rings and the square plate, to make the rings as horizontal as possible.
I tested the motion: with 300000 steps it's possible to move the rings all the way from the parked (down) position, to the up position. I also checked that when the rings are up, I can place four substarates and they fall properly into the alignment groove. Since the maximum speed of the picomotor is 2000 steps/s, it takes 150 seconds to move up and down the ring.
Finally, positive steps means that the rings are moving up, negative that they're moving down.
I raeligned the optical levers to the position I obtained by centering the samples with the rings. I haven't tested the repeatability yet.