| These knobs make it easy to collimate your Celestron C9¼ CPC 925 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.
This knob set works with 9¼” Celestron CPC 925 Schmidt-Cassegrain telescopes that use collimating knobs with metric threads. The CPC 925 metric secondary can be identified by 1) a factory-installed plastic cover over the collimation screws; 2) black Phillips factory collimation screws; and 3) brand information on the inside of the housing, as shown on the above photo. The secondary housing cover will still fit when Bob's Knobs are installed, but can be left off if desired. The cover is cosmetic only and will not affect operation of the scope.
Celestron 9.25 inch SCTs manufactured before 2006 have standard 6-32 collimation screws and require the #BKC9 collimation knob set with standard threads. These telescopes can be identified by 1) no writing inside the secondary housing (except for the serial number); and 2) most have silver factory collimation screws as shown in the feature image below. Do not use this metric knob set on these telescopes.
A few non-CPC 9¼” Celestron scopes manufactured in 2006 have black Phillips factory collimation screws that aren't metric. If your scope's secondary housing looks like the feature image below, it also requires the #BKC9 collimation knob set with standard threads.
The collimating screws themselves are made of black oxide steel. Their knobs are also black oxide steel, 12 mm (1/2”) 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. | 
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