This locking ring, is the simply the best solution for directly connecting 2" diagonal mirrors and diagonals to SCT telescopes.
When attaching a diagonal to your SCT, the diagonal can be orientated at an awkward angle during some observing.
Using this ring will allow you orientate your diagonal to a comfortable position.
Screw this ring onto your SCT rear cell, then screw your diagonal to the point it meets the lock ring. Next slightly loosen the diagonal and position it comfortably, then slacken off the lock ring until it tightens against the diagonal and locks it into position.
A simple solution, but extremely effective!