These knobs make it easy to collimate your Celestron C11 for optimum image quality. You don't need any tools, so you never have to worry about losing a tiny Allen wrench or screwdriver in the grass in the dark. While it may be a long reach for some people, you can usually adjust the collimation by yourself while looking through the eyepiece, so there's instant feedback about how your optics are performing. Installation of the knobs is a snap with the included instructions, and you can undo the modification at any time. This knob set works with all 11" Celestron Schmidt-Cassegrain telescopes that have a flat secondary housing. This includes most scopes with an aluminum optical tube, with the exception of the CPC1100 (which uses metric collimation screws and requires the #BKC11M knob set). Some recent aluminum tube versions and most carbon fiber tube models require the #BKC11CF knobs designed for the "Trifid" rotary secondary housing (which has a cover with three raised portions that rotate to cover and uncover the recessed collimation screws). The collimating screws themselves are made of black anodized aluminum alloy. Their knobs are black plastic, 16 mm (5/8") in diameter. The factory dust cover will fit on the scope normally with Bob's Knobs installed. 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. Tech Details: Weight 1.6 oz. Warranty 1 year
