This knob set works with all 5” Celestron Schmidt-Cassegrain telescopes made before 2006 (such as the original C5, the NexStar 5, etc.) with black Allen, silver Allen, or silver Phillips head collimation screws. They will also work with all 2006 and later models EXCEPT those with black Phillips factory collimation screws. The black Phillips screw models require knobs #BKC5M. The collimating screws themselves are made of black anodized aluminum alloy. Their knobs are black plastic, 13 mm (1/2”) in diameter. The factory dust cover will fit on the scope normally with Bob's Knobs installed. Some versions of the C5 have a small snap-in plastic cover over the factory collimation screws. This cover must be removed and set aside when Bob's Knobs are installed. The cover is cosmetic only and will not affect the operation of the scope.

The three collimation screws hold the secondary mirror and its mounting plate to the secondary housing. The mirror mounting plate pivots on a tang or spring at its center, and the collimation screws, which are threaded into the plate, adjust the mirror tilt (or collimation). Do not remove more than one of the original collimation screws at a time when installing Bob's Knobs, as the secondary mirror might fall off (if all three screws are removed at the same time), or the secondary can pivot inside the tube, scratching the corrector plate (if two screws are removed simultaneously). By removing only one factory collimation screw at a time, replacing it with a knob, and repeating this process for the other two collimation screws, there's no danger of dropping the secondary mirror. The instructions also recommend pointing the optical tube assembly slightly downward during the installation of the knobs for an added safety measure.