Telescope Collimation Guide: How to Align Your Reflector for Sharp Images

The Five-Minute Task That Doubles Your Image Quality

If you own a reflector telescope — a Newtonian Dobsonian, an imaging Newtonian, or any scope that uses a primary and secondary mirror — there is a good chance your optics are not perfectly aligned. And if they are not aligned, you are not getting the full performance you paid for. Collimation is the process of adjusting the mirrors so they work together as a single, precisely aimed optical system. It sounds intimidating, but once you understand what you are doing, it takes about five minutes and makes an immediately visible difference.Plenty of telescope owners avoid collimation because they are afraid of breaking something or making things worse. Neither of those things is going to happen. The adjustment screws move the mirrors in tiny increments. If you overshoot, you simply nudge them back. There is no way to damage the optics by adjusting collimation.

Why Does Collimation Matter?

A telescope’s mirrors need to be precisely aligned to bring light to a single, sharp focus. When the mirrors are out of alignment — even slightly — stars appear as distorted blobs instead of tight points, planetary detail gets smeared, and contrast drops across the board. A perfectly collimated 6-inch Newtonian will outperform a poorly collimated 10-inch scope. That is not an exaggeration.The effect is most noticeable at higher magnifications. Low-power, wide-field views are more forgiving of minor misalignment. But the moment you push to 150x or higher — which is where planetary detail and tight double stars live — any collimation error becomes obvious.

Signs Your Scope Needs Collimation

• Stars look like comets or little triangles instead of round points, especially toward the center of the field of view. (Stars at the edges being slightly distorted is normal for Newtonians — that is coma, a separate optical property.)
• You cannot reach sharp focus. You rack the focuser in and out and the image never quite snaps to a crisp point.
• Planetary detail is mushy on nights when other observers nearby are getting sharp views.
• You just transported the scope. Newtonian reflectors can lose collimation during car rides. Many observers check and touch up collimation at the start of every session — it takes 30 seconds once you are practiced.

Tools You Will Need

You can collimate with nothing more than your eye, but a couple of inexpensive tools make it faster and more precise:

• A collimation cap (or sight tube): This is simply a cap that fits in your focuser with a small central hole. Looking through it, you can see the reflections of the secondary mirror, primary mirror, and the collimation cap itself, all of which should appear concentric when collimation is correct. You can make one from a 35mm film canister or buy one for about $10.
• A Cheshire eyepiece: A step up from a collimation cap. The Cheshire has a crosshair and an angled reflective surface that creates a bright reference ring, making it much easier to center everything precisely. About $25-40.
• A laser collimator: Projects a laser beam down the optical path. When the beam hits the center of the primary mirror and the reflected beam returns to the center of the laser’s target, the scope is collimated. Fast and satisfying to use, but cheaper models can themselves be out of alignment, which defeats the purpose. Stick with quality units from brands like Howie Glatter, Farpoint, or Baader. $50-$120.

If you are just starting out, a Cheshire eyepiece is the best value. It requires no batteries, cannot go out of alignment itself, and teaches you what correct collimation actually looks like.

Step-by-Step Newtonian Collimation

Collimation has two parts: aligning the secondary mirror, then aligning the primary mirror. Always do them in this order.

Step 1: Center the Secondary Mirror Under the Focuser

1. Insert your collimation cap or Cheshire into the focuser.
2. Look through it. You should see the secondary mirror (the small, flat, angled mirror near the top of the tube) centered in the focuser tube. If it is offset, use the three adjustment screws on the secondary mirror holder (the spider vanes) and the central bolt to shift the secondary until it appears centered.
3. You should also see the reflection of the primary mirror in the secondary. At this stage, it may not be centered — that is fine. You just want the secondary itself sitting in the middle of the view.

Step 2: Aim the Secondary at the Primary

1. Still looking through the collimation tool, tilt the secondary mirror using its three adjustment screws until the reflection of the primary mirror (the large circle you see) is centered within the outline of the secondary mirror.
2. If you are using a Cheshire, the crosshair should sit in the center of the primary mirror’s reflection, and you should see the primary’s center mark (a reinforcement ring or donut sticker — if your primary does not have one, adding a self-adhesive donut from an office supply store is a good idea).

The swirling detail in Jupiter’s atmosphere, captured by Juno. A properly collimated telescope reveals dramatically more planetary detail than a misaligned one. Credit: NASA/JPL-Caltech/SwRI/MSSS

Step 3: Align the Primary Mirror

1. Now adjust the primary mirror’s three collimation bolts (located at the back of the telescope tube, often accessible through holes in the mirror cell cover). Most Newtonians have three push-pull bolt pairs or three spring-loaded bolts with locking nuts.
2. Looking through the Cheshire, turn the primary mirror’s adjustment bolts until the reflection of the Cheshire’s crosshair (or the collimation cap’s peephole) sits precisely at the center of the primary mirror’s center mark.
3. When everything is concentric — the focuser tube, secondary mirror outline, primary mirror reflection, center mark, and collimation tool reflection all forming neat concentric circles — you are done.

The whole process takes five minutes or less once you have done it a few times. The first time might take 15-20 minutes as you figure out which screws do what.

The Star Test: The Final Check

The ultimate test of collimation is what you see through the eyepiece. Point your telescope at a moderately bright star, use a high-power eyepiece (150x or more), and carefully defocus slightly in both directions:

• Inside focus (racking the focuser inward): The star should expand into a set of concentric rings (diffraction rings) with the secondary mirror’s shadow as a dark circle in the center.
• Outside focus (racking outward): You should see a similar concentric ring pattern.

If the rings are concentric and symmetric on both sides of focus, your collimation is spot-on. If the rings are lopsided — bunched up on one side — the optics are still slightly out of alignment. Go back to the primary mirror adjustment and nudge the bolts until the star test shows symmetric rings.On a night of excellent seeing, a perfectly collimated scope will show a textbook Airy disk at focus — a tiny bright central dot surrounded by one or more faint, perfectly round diffraction rings. When you see that, you know your optics are performing at their theoretical limit.

How Often Should You Collimate?

It depends on how you use your scope:

• Dobsonians and portable Newtonians: Check at the beginning of every observing session. Car rides knock collimation out. A quick Cheshire check takes 30 seconds.
• Permanently mounted scopes: Collimation tends to hold well if the scope stays in one place. Check once a month or whenever views seem off.
• Before any astrophotography session: Always. Miscollimation shows up in photos as misshapen stars and reduced contrast that no amount of processing can fix.

A Quick Note on SCT Collimation

Schmidt-Cassegrain telescopes (SCTs) also require occasional collimation, but the process is different. Only the secondary mirror is user-adjustable, via three small screws on the front face of the secondary holder. The adjustment is done entirely using the star test — there is no Cheshire method for SCTs. Because the screws are extremely sensitive (a tiny turn moves the image a lot), SCT collimation requires patience and a very steady hand. But the principle is the same: adjust until the defocused star shows concentric rings.

Stop Avoiding It — Start Collimating

Collimation is one of those skills that seems scary until you do it once. After that, it becomes a routine part of setting up — no different from focusing. And the payoff is immediate. The first time you look at Jupiter through a freshly collimated scope and see cloud band detail you have never noticed before, you will wonder why you waited so long.