This will drop the diagonal. By doing this, we ensure that the optical axis coincides with the mechanical axis. This is important for for scopes equipped with digital setting circles or computer interfaces. Now our view through the collimator shows the diagonal centered underneath the focuser. Make the primary look centered in the diagonal.
Most diagonal holders have three adjusting screws that tilt the diagonal.
Adjust these screws until the reflection of the primary looks centered in the diagonal. It helps to center dot the primary with a white notebook ring reinforcer. Now our view through the collimator shows the primary centered in the diagonal which is centered under the focuser. Collimate the primary. Almost all mirror mounts have three collimating bolts on the back of the mirror mount.