Hack40.Star-Collimate Your Scope

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.

Don't confuse star-collimating with star-testing. The purpose of star-collimating is to get the scope's optics aligned precisely. The purpose of star-testing is to check the quality of the optics. Star-testing requires perfect seeing (atmospheric stability) if it is to provide meaningful results. Star-collimating can be done even under mediocre seeing conditions.

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:

1. Point the scope at a 0th or 1st magnitude star. We generally use Polaris. It's bright enough for this step, and it has very little apparent motion. (It's critical to keep the star centered in the field of view during these tests.)

2. Use an eyepiece or eyepiece/Barlow combination that yields a magnification equal to the aperture in millimeters, plus or minus 20%. For example, for a 10" (250mm) scope, 250X magnification is ideal, but anything in the 200X to 300X range is acceptable.

3. Defocus the star until it becomes a huge white blob that fills the field of view. There will be a dark circle at or near the center of the field of view, which is the shadow of the secondary mirror. That shadow should be perfectly circular and perfectly centered in the field of view. If it is not, your rough collimation was rougher than you thought.

4. Use the primary mirror collimationscrews to center the secondary shadow. Having a helper adjust the collimation screws while you observe the image makes this step much faster and easier. During this process, you'll have to refocus the star frequently, ensure it's centered in the field of view, and then defocus quickly to check the centering of the secondary shadow.

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:

1. Point the scope at a 2nd or 3rd magnitude star. (Once again, it's important to keep the star centered in the field of view during these tests.)

2. Use an eyepiece or eyepiece/Barlow combination that yields a magnification equal to two or three times the aperture in millimeters. For example, for a 10" (250mm) scope, we use 500X to 750X for tweaking the collimation.

3. Defocus the star slightly until it shows a complex diffraction pattern similar to that shown in Figure 3-25. The center image shows the point of exact focus and the images to either side show the patterns that result as you move the focuser increasingly far inside and outside focus. The exact pattern is not important. That will vary with differences in optical quality, magnification, and many other factors. What is important is that the diffraction rings appear circular and concentric. If the rings are oval and the center bright spot is off-center, tweak the primary mirror collimation screws until the pattern is as circular and concentric as you can get it. Check both sides of focus.

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.