How is the secondary mirror attached to its housing?
On most Schmidt-Cassegrain telescopes, the three collimation screws also hold the secondary mirror and its mounting plate to the secondary housing. An example is the Meade 10″ f/10. This is why it’s so important not to remove more than one collimation screw at a time when installing Bob’s Knobs. The secondary mirror mounting plate on these telescopes pivots on a tang or spring at its center, and the collimation screws, which are threaded into the plate, adjust mirror tilt. Other telescopes have a fourth screw in the center of the secondary housing. An example is a version of the Celestron C14. This central screw usually holds the secondary mirror assembly to its housing and the collimation screws, which are typically headless set screws, press against the mirror mounting plate rather than thread into it. When installing Bob’s Knobs on these scopes, it’s still important to remove only one collimation screw at a time to reduce the risk of rotating the secondary mirror and disturbing its
