These Bob's Knobs fit the secondary mirror housing of the 12" Astro-Tech Ritchey-Chrétiens only. Precise collimation is essential to get the best coma-free imaging performance from a Ritchey-Chrétien, and these Bob's Knobs make it easy to tweak the collimation of your 12" R-C out in the field. You don't need any tools when using Bob's Knobs, so you never have to worry about losing a tiny Allen wrench in the grass in the dark. You adjust the collimation while looking through an eyepiece, or when using software (such as CCD Inspector) with your camera and computer, 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. The collimating screws themselves are made of stainless steel. Their knobs are easy-to-grip black plastic, 16 mm (5/8") in diameter. The factory dust cover will normally fit with the Bob's Knobs installed. The three collimation screws hold the secondary mirror and its mounting plate in the secondary housing and 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). If two screws are removed simultaneously, the secondary could tilt inside the secondary baffle tube, possibly becoming wedged in place and chipped. 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 1.75 oz. Warranty 1 year
