It does not directly measure where the tube is pointing because there is no encoder on either shaft. When a CCD guider is added, the situation is quite different. In this case, the CCD guider looks at a star and tells the computer how it needs to move the tube to re-center the star on the guider chip. In this case, the tube is in the feedback loop so that control can be made as accurate as the CCD chip can manage. One of the problems in the system is that of lash in the mechanical parts of the system.
Mechanical lash is a dead zone in the feedback loop.
Control about the dead zone can be unstable. This causes jumps and jiggles in the operation. A tight system, one with very little mechanical lash, will behave more smoothly. The idea of my new design is to tighten the system mechanically as much as possible. That includes reducing the mechanical lash to a very small amount.