This interesting paper by Robert Warden refers to each mirror segment on JWST as having six actuators providing six degrees of freedom in positioning (plus one more to tweak the curvature). These degrees of freedom are described as "tip, tilt, piston, horizontal & vertical decentering and clocking". Can I assume that "piston, horizontal & vertical decentering" correspond to 3D positioning in space and "tip, tilt, and clocking" correspond to attitude? Could someone help translate these into more explicit terminology?
Also, requiring a full six degrees of freedom sounds like overkill. I imagine that three degrees of freedom would cater for almost all the required correction: motion along the z-axis and pitch and yaw with respect to the z-axis. As long as these first three corrections are available, does it matter very much where exactly the mirror is in the x-y plane or whether it's slightly rotated around its z-axis (roll)? (here the z-axis is taken to be perpendicular to the plane of the mirror segment)