Collimation is the precise alignment of all optical components in a telescope.
Conversely, if the ring is slightly thicker toward the top, then it must be moved down the tube slightly. Identify the components of this multiple reflection image, and become comfortable with them. Insert the Cheshire Eyepiece into the focusser, and tighten the screw with the opening pointed towards a bright part of the sky. With the aid of an assistant at the back of the telescope, rotate each of the three bolts behind the primary, until both dots accurately co-incide. The Telescope is now almost collimated.