| These Bob’s Knobs make it easy to collimate your Celestron C9¼ Schmidt-Cassegrain with a “Trifid" Fastar secondary mirror housing 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. You adjust the collimation 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.
These knobs fit any Celestron 9.25" f/10 SCT with the “Trifid" Fastar secondary mirror housing, including the EdgeHD version. The “Trifid" housing has a three-leg cover, as seen in the image above. The covers rotates to hide the collimation screws. The housing has black Phillips-head factory collimation screws that are recessed into the secondary housing that are revealed when the cover is rotated. The rotary secondary cover must remain open when the Bob’s Knobs are installed. The cover is cosmetic only and will not affect operation of the telescope.
The Bob’s Knobs collimating screws themselves are made of stainless steel. Their knobs are black plastic, 9.5 mm (3/8") in diameter. The knobs include a set of long spacers so that the heads of the knobs clear the secondary housing lip. 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.
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