There's only one way to successfully collimate an SCT, and that involves looking at the diffraction rings of a star, just as is done in the star test. That's a good thing. Unlike the Newtonian owner, the SCT user doesn't need to buy collimation tools. All that's required is an eye, a medium-high powered eyepiece, and a bright star. Magnitude 2.0 Polaris is just about perfect, since it doesn't move much. What if Polaris isn't conveniently placed? Use another star of similar brightness. If a larger than 8-inch scope is to
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DOI: 10.1007/978-0-387-09772-5_9, © Springer Science + Business Media, LLC 2009
be collimated, a star dimmer than 2nd magnitude may show its diffraction rings more clearly. Believe it or not, Polaris becomes too bright in a large aperture CAT.
Collimation can also be done in the daytime with an artificial star, if that's more convenient. The time honored way to "make" a star is to point the scope at the reflection of the Sun off a distant power pole insulator. That works fine, though daytime seeing effects may make it hard to see diffraction rings clearly. If there isn't a suitable power pole insulator in the vicinity, climb into the attic and retrieve a shiny round Christmas tree ornament. Place it in the Sun so it provides a good reflection. Be careful not to point the scope at the actual Sun, of course. See Chapter 12 for some further ideas for artificial stars.
SCT collimation is a three-step process. First is a rough collimation to get the secondary mirror "in the neighborhood." Next is fine tuning, where the secondary is tweaked while observing the star's diffraction pattern. If conditions permit, a final check and an even more precise adjustment can be made by observing an in-focus star's diffraction pattern. As mentioned earlier, the only collimation adjustment that can be made by SCT users is to the secondary mirror via three screws.
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