One of the hardest things about SCT collimationis the fact that, unless you have really long arms, you may not be able to turn the collimation screws on a larger than 5-inch CAT's secondary mirror mount without taking your eye away from the eyepiece. Adjustment is much easier if you're able to watch the image of a star while moving the screws. A clever solution for Meade and older Celestron telescopes is to make an extension handle for the Allen wrench used to turn the screws. A length of wooden dowel or a piece of coat hanger wire can be taped to a wrench and will allow collima-tion to be adjusted while watching the star image.
Some amateurs make collimation adjustments easier still by making three wrench/ handle combinations, inserting one wrench in each collimation screw before beginning an adjustment session. If the Allen wrenches are the proper size for the telescope's screws, they should stay in place in the screw heads by themselves, even with the added weight of a handle. With a wrench in place in each screw, collimation goes quickly, as there's no need to remove a wrench from one screw and fumble it into another in the dark. Sadly, Celestron SCTs now use Phillips rather than Allen screws, which do not lend themselves to handles. Owners of Celestron telescopes are probably best off replacing the screws, with Bob's Knobs, knob-headed screws designed to ease collimation.
Video Collimation All is not lost for Celestron owners—or anyone who wants a super-precise easy-to-use collimation method. A small, sensitive video camera can be used in place of an eyepiece to display a star's diffraction rings on a monitor screen. Use a Barlow ahead of the camera, and a large diffraction pattern can be achieved. Place the monitor near the corrector end of the telescope and collimate with ease. Video cameras suitable for collimation use can be obtained from the security video dealer, Supercircuits (Appendix 1). Their PC33C camera, which costs $100, is great for collimating and also can deliver excellent (color) images of the Moon and planets.
A webcam is even better than a video camera for collimating. See the discussion on these devices in Chapter 11 for a rundown on choosing an astronomy-appropriate webcam. Why are webcams better? They are lightweight, feature small chips that provide high magnification "factors," and, most of all, they are digital devices that can be used with software. What makes the webcam a real winner for CAT collimation is a piece of freeware called MetaGuide. What MetaGuide does that is special is that it creates a very clear diffraction pattern from an incoming webcam video of the defocused star, and does that even when seeing is relatively poor. It also provides an on-screen indication of the screw or screws that should be turned to produce a perfect collimation bull's-eye. Finally, if MetaGuide is connected to a scope via an autoguide cable, it will re-center the star automatically, following collimation adjustments. See Appendix 2 for information on how to get this handy astro-soft.
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