If you own a Newtonian reflector and your stars look like fuzzy little commas instead of sharp points of light, collimation is almost always the culprit. It sounds intimidating the first time someone says it out loud, but collimation is really just making sure the mirrors inside your telescope are pointed at each other correctly. Nudge a few screws, check the view, nudge again. That’s it. Most people spend more time worrying about it than actually doing it.
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Refractors (the lens-based scopes) mostly hold their alignment for life because there’s nothing to shift — light just travels in a straight line through glass. Reflectors are a different story. A Newtonian bounces light off a large primary mirror at the back of the tube, up to a small angled secondary mirror near the focuser, and out into your eyepiece. Two mirrors, two mounting brackets, and enough travel, shipping, and temperature swings over time that they will eventually drift out of alignment. This isn’t a defect. It’s just how the design works, and it’s the tradeoff you make for getting a lot of aperture per dollar, which we talk about in our beginner telescope guide.
The good news is that collimation isn’t a repair — it’s routine maintenance, like checking tire pressure. Once you’ve done it a couple of times, it takes less than five minutes.
Before you touch anything, confirm collimation is really the problem. The classic symptom is a bright star that won’t come to a clean point no matter how you focus — instead it looks like a comet or has a flare on one side. A quick daytime test: point the scope at a distant object, rack the focuser all the way in, and look through the empty focuser tube (no eyepiece). You should see a set of concentric circles — the edge of the focuser tube, the secondary mirror, the clips holding the secondary, and the reflection of the primary mirror, all centered inside each other like a bullseye. If they look lopsided or off-center, it’s time to collimate.
You have three common options, in order of what a beginner should actually reach for:
Collimation cap (sight tube): A simple plastic or metal cap with a small hole in the center that fits where your eyepiece goes. It costs almost nothing and forces your eye into the correct viewing position. This is the right starting point — it’s very hard to make things worse with one.
Cheshire eyepiece: A step up from the cap, with a small internal reflector that makes it much easier to see whether the secondary mirror is centered and square. Most experienced owners consider this the best all-around tool for a Newtonian.
Laser collimator: Plugs into the focuser and shines a beam down the tube that bounces off both mirrors and back onto a target. It’s fast and satisfying to use, but in inexperienced hands it can actually collimate you into a worse position than you started, because the laser itself needs to be well-centered in the focuser to be trustworthy. Save it for once you understand what “correct” looks like with a cap or Cheshire.
Do this part indoors, in daylight, pointed at a plain wall or the sky (never the sun).
1. Remove the eyepiece and rack the focuser all the way in.
2. Insert your collimation cap or Cheshire and look through it.
3. You should see the primary mirror reflected inside the secondary mirror. If the primary’s outline isn’t centered in that reflection, the secondary needs adjusting.
4. Most secondary mirrors have three small adjustment screws (sometimes with a locking screw too). Loosen the lock if there is one, then make small alternating adjustments to the three screws until the primary mirror’s edge appears centered in your view.
5. Don’t touch the single center screw that holds the secondary’s support strut — that one sets distance, not angle, and rarely needs adjustment.
1. Now look for the small dot or ring marker in the center of your primary mirror (most modern mirrors ship with one factory-applied; if yours doesn’t have one, it’s worth adding a small collimation dot before you start).
2. Using the three larger screws at the back of the tube — usually a pair of push/pull screws per position — adjust until the reflected image of the secondary mirror (and the crosshair or marker in your Cheshire) lines up concentrically with that center dot.
3. Work in small increments and re-check often. Big turns overshoot easily.
4. Once everything lines up — the focuser edge, secondary outline, primary outline, and the reflection of your own eye — you’re collimated.
Daytime collimation gets you close, but the definitive test happens at night. Center a moderately bright star, rack it slightly out of focus, and look at the diffraction pattern — the rings of light around the star’s disk. On a well-collimated scope those rings appear as even, concentric circles around the center dot. If they look bunched to one side, your collimation still needs fine-tuning. This is also a great way to judge atmospheric conditions, something we touch on in our stargazing hub.
If your telescope lives at home and doesn’t travel much, check collimation every few months, or any time you notice image quality dropping. If you regularly pack it into a car for dark-sky trips, check it every session — it takes two minutes once you’re comfortable with the process and it’s the difference between a night of sharp planetary detail and one of frustrating mush. Pairing good collimation with a properly matched eyepiece and a stable mount is what actually determines whether you can split a tight double star or resolve Cassini’s Division in Saturn’s rings.
For a deeper technical walkthrough with diagrams, Celestron’s own knowledge base has a solid reference guide worth bookmarking: How do I collimate my Newtonian?
Not exactly — it drifts. Normal handling, temperature changes, and vibration from transport gradually shift the mirrors a fraction of a degree. That’s enough to soften your views without anything being damaged. Think of it as needing a tune-up, not a repair.
The concept is similar (aligning mirrors), but SCTs and Maks typically only require adjusting the secondary mirror via screws on the front corrector plate, since the primary is fixed at the factory. The tools and exact steps differ enough that it’s worth following a guide specific to your scope’s design rather than applying Newtonian steps directly.
Often, yes — even a well-packed reflector can shift slightly in shipping. It’s a good habit to run through the daytime check the first time you set up any new Newtonian, before you assume something’s wrong with the optics themselves.
Once collimation stops feeling mysterious, it becomes one of those five-minute habits — like polar-aligning a mount or checking your finder scope — that quietly makes every session better. If you’re still building out your kit, our accessories buying guide covers the collimation tools alongside the other small additions that make the biggest difference for beginners.
