Hack 40. Star-Collimate Your Scope
Align your scope perfectly using the properties of light.
Collimation is the process of aligning a telescope so that all of the mirrors and lenses share a common optical axis. In order to provide the best possible images, a scope must be collimated perfectly. Not just "collimated pretty well" or even "almost-perfectly collimated." An almost-perfectly collimated scope may show you 50% or less of the detail that is visible when collimation is dead-on.
Just as no pianist would play an untuned piano, no astronomer should observe with an uncollimated scope. And yet, very few astronomers collimate sufficiently often or sufficiently well to get the best performance possible from their scopes. Newbies never collimate. They're afraid they'll muck things up beyond repair. SCT and refractor owners seldom collimate. They're convinced their instruments don't require frequent collimation. Newtonian owners generally collimate fairly frequently, but most simply use a sight-tube/Cheshire and/or a laser collimator to get their scopes roughly collimated and then declare the job Good Enough. They're all wrong.
No physical collimation tool can ensure anything more than a rough collimation, and that's simply not good enough. The only way to collimate a scope properly is to defocus a star and observe the patterns that exist on both sides of proper focus. That's true for two reasons. First, physical collimation tools are accurate to only a few tenths of a millimeter, while starcollimation uses diffracted light patterns that are several orders of magnitude more precise. Second, the physical center of a lens or mirror does not always correspond to its optical center. In real-world telescopes, the difference may be only a fraction of a millimeter, but that is sufficient to prevent perfect collimation using only physical methods.
Fortunately, it's pretty easy to star-collimate a scope. To begin, get your scope roughly collimated using your sight-tube/Cheshire, laser collimator, or other physical collimation tools [Hack #37][Hack #38]. In a Newtonian reflector, it's important at this stage to get the secondary mirror collimated as closely as possible. Once the scope is roughly collimated, you're ready to start star-collimating. Take the following steps:
Once you complete these steps, the scope is roughly collimated and you're ready to fine-tune the collimation. (Most scopes, once collimated, hold collimation fairly well, so it shouldn't be necessary to repeat the above steps each time you collimate the scope.) To fine-tune the collimation, take the following steps:
Figure 3-25. Stellar diffraction patterns from inside focus to focused (center image) to outside focus
We generated the patterns shown in Figure 325 with the free software utility Aberrator (http://aberrator.astronomy.net). These patterns are only examples. They assume an optically perfect 250mm f/5 scope with a 25% central obstruction and perfect seeing conditions. The patterns you see when you collimate your scope will certainly be different.
Atmospheric turbulence may break the patterns up or cause jaggies or wavering. With even moderate turbulence, you'll still be able to judge when the patterns are close to being circular and concentric. If the atmosphere is too turbulent to allow that, critical collimation doesn't matter because you won't be able to see fine detail anyway.
If the patterns are concentric but different on opposite sides of focus, that means your mirror or objective lens is not optically perfect. Don't worry too much about that. Even the best-quality telescopes don't necessarily show identical patterns inside and outside focus.