Imagine two plates, one moving and one stationary. Take and imagine taking 3 hinges (make 3 of these assemblies) and tying them together in series so that you can move the two end pieces of the hinge assembly in one direction (in and out from each other) without having to move them right and left as the center hinge moves left and right. Put the 3 assemblies around the two plates so that they are evenly spaced (120deg. apart) and you will find that the upper plate will move in and out from each other but won't move in any other way. Place a spring to pull them together and a threaded rod to push them apart. Voila, a focuser that moves the primary without any image shift!