Here's a telescope mystery we solve at the counter almost every week. Someone brings in a perfectly good reflector and says it "just isn't sharp." The Moon is soft, Saturn is a smudge, and bright stars look like tiny comets. Nine times out of ten nothing is broken. The mirrors are just a little out of line.
Lining them back up is called collimation, and it sounds scarier than it is. A Newtonian or Dobsonian has two mirrors and a handful of screws. Learn what each screw does, check it before you observe, and you'll get every bit of sharpness you paid for. This guide covers the tools, the three-step procedure, the star test, and how often you really need to do it.
The Short Answer
To collimate a Newtonian or Dobsonian, center the secondary mirror under the focuser, tilt the secondary so it aims at the center of the primary, then tilt the primary until its center mark lines up in your collimation tool, and confirm with a star test.
- Tools: a collimation cap or sight tube, a Cheshire eyepiece, or a laser collimator. A Cheshire is the most trusted tool for the final primary adjustment.
- Most of the time you'll only touch the primary mirror knobs. The secondary is mostly set-and-forget.
- How often: check it at the start of each session, especially after a car ride. Fast mirrors (low f-numbers) need more care.
- Refractors are aligned at the factory and usually need no collimation unless dropped or jarred.
1. What Collimation Is and How to Tell When You Need It
A reflector bounces light off a curved primary mirror at the bottom of the tube, up to a small flat secondary mirror, and out the side into your eyepiece. If either mirror is tilted, the image the primary forms lands slightly off the optical axis, and sharpness falls apart.
The symptoms
Look at a bright star at high power and rack the focus slightly in and out. In a collimated scope, the out-of-focus star is a set of concentric rings, like a target. If the rings are lopsided, or the in-focus star has a flare on one side, the scope is out of collimation. Astronomy magazine's guide describes exactly this check: "If the rings aren't centered, odds are good that the telescope is not collimated correctly" (Astronomy.com). BBC Sky at Night adds that miscollimated stars look like "mini-comets."
Why fast scopes are fussier
Every parabolic mirror has a small "sweet spot" on its axis where images are sharpest. According to Sky & Telescope, a perfect f/4.5 mirror is diffraction-limited only within a 2 mm circle, while an f/10 mirror's sweet spot spans 22 mm. Most beginner Dobsonians are around f/5 to f/6. The Sky-Watcher Heritage 130 Tabletop Dobsonian is a 650 mm f/5 (Sky-Watcher specs) and the Sky-Watcher Flextube 200P 8-inch GoTo Dobsonian is a 1200 mm f/5.9. These scopes reward a quick check.
Before you touch a screw
Let the scope cool to outdoor temperature. A warm mirror throws off its own heat shimmer that looks a lot like bad optics. Point the tube horizontal so nothing can fall onto the primary, and work in daylight or a lit room the first time.
2. Step-by-Step: Collimating a Newtonian or Dobsonian
Sky & Telescope breaks collimation into three steps: center the secondary under the focuser, aim it at the center of the primary, then center the primary's sweet spot in the eyepiece. Here's how that plays out on a real Dobsonian.
The tools
- Collimation cap or sight tube: a cap with a small center hole that keeps your eye centered in the focuser. Many scopes ship with one.
- Cheshire eyepiece: a sight tube with a shiny 45° face that lights up the primary's center mark. Sky & Telescope calls it the best tool for the final adjustment.
- Laser collimator: fast and handy in the dark, but the laser itself must be true (centered in its housing), or you'll collimate to its error.
- Screwdriver or hex key for the secondary, and a red flashlight for night work.
The procedure
- Check the center spot. Many reflectors ship with a small ring marking the primary's center. If yours doesn't, Sky & Telescope suggests a spot about ½ inch across. Never touch the mirror surface otherwise.
- Center the secondary. Look through the cap or sight tube. The secondary should look round and centered in the focuser. If it doesn't, adjust the secondary's center bolt and the spider vanes. This is rarely needed.
- Tilt the secondary. Loosen the three small tilt screws a little at a time, and adjust until the entire primary mirror, with all its clips, appears centered in the secondary. Re-snug the screws so the alignment holds.
- Tilt the primary. At the back of the tube, turn the primary's collimation knobs until the primary's center ring surrounds the dot of your cap or Cheshire. Loosen the locking screws first, if your scope has them, and turn only one or two knobs. Sky & Telescope recommends using just two of the three.
- Lock and recheck. Snug the locking screws gently and look again. Tightening can shift things slightly.
- Star test. Confirm at night (next section).
Reading the view
| What you see through the collimation cap or Cheshire | What it means | What to adjust |
|---|---|---|
| Secondary mirror looks oval or off-center in the focuser | Secondary is not centered under the focuser | Secondary center bolt (in/out) and spider vanes; rarely needed |
| Primary mirror's reflection is not centered in the secondary (clips of the primary not all visible) | Secondary tilt is off | The three small secondary tilt screws, a little at a time |
| Primary's center ring is not around the cap/Cheshire dot | Primary tilt is off | Primary collimation knobs at the back of the tube |
| Everything centered, but stars still flare or look comet-like | Fine error, or the scope hasn't cooled | Let it cool, then star test at high power |
One thing that confuses everyone: in a fast scope, the secondary often looks slightly offset in the cap even when everything is right. That's by design, and Sky & Telescope notes it's normal. Don't chase it.
3. Star Test, How Often to Collimate, and What About Refractors?
Tools get you close. A star gets you right.
The star test
Pick a bright star high in the sky (less atmosphere to look through), center it, and use a high-power eyepiece. Defocus slightly. If the ring pattern is lopsided, nudge one primary knob a small amount, recenter the star, and look again. At focus, Sky & Telescope says stars should appear as "tight, symmetric diffraction disks." On a night with steady air, you'll see the difference immediately on planets.
How often should I collimate?
Check it every session. Sky & Telescope recommends the primary adjustment at the start of each observing session, and checking occasionally through the night as temperatures change. Astronomy says reflectors need it "every time you transport" them. The secondary steps are mostly set-and-forget. Collapsible scopes like the Flextube series are designed to hold collimation well, but a 30-second check with a cap costs nothing.
Do refractors need collimation?
Usually not. Celestron says refractor alignment "is done when your scope is made at the factory and usually needs no further adjustment," though an objective can lose alignment if the scope is dropped or jarred (Celestron Knowledgebase). If a refractor's stars look lopsided after a hard knock, that's a job for the bench, not a screwdriver at the picnic table.
Common mistakes
- Turning screws before the scope cools.
- Cranking the secondary screws too tight, which can pinch or tilt the mirror.
- Trusting an un-trued laser.
- Adjusting all three primary knobs at once and getting lost. Small turns, one knob at a time.
Conclusion
Collimation is a five-minute habit, not a repair. Check the secondary once in a while, touch up the primary every session, and use a star to confirm. Do that and your reflector will show what it was built to show: crisp lunar craters, Saturn's ring gap, and stars as tight points instead of comets.
Shopping for a reflector? Browse our Dobsonian telescopes and Newtonian reflectors, read our beginner telescope guide, or bring your scope by the store and ask the telescope desk to walk you through collimating it.
Gear Mentioned in This Guide
Frequently Asked Questions
How do I know if my telescope needs collimation?
Defocus a bright star slightly at high power. If the rings are not concentric, or focused stars flare to one side like little comets, your reflector needs collimating.
How often should I collimate a Dobsonian?
Check it at the start of every observing session, especially after driving the scope somewhere. Usually only the primary mirror needs a small touch-up. The secondary rarely moves.
Do refractors need collimation?
Usually not. Refractors are aligned at the factory and normally need no adjustment unless they are dropped or jarred.
Is a laser collimator better than a Cheshire?
A laser is fast and convenient, but it must be accurately centered in its housing. Sky & Telescope recommends a Cheshire eyepiece for the final primary adjustment.
Which screws do I turn on a Dobsonian?
The large knobs at the back of the tube tilt the primary mirror. The small screws around the secondary holder tilt the secondary. If the primary has locking screws, loosen them before adjusting and snug them gently after.
Why are my stars still blurry after collimating?
The mirror may still be cooling, the air may be turbulent, or you may be using too much magnification. Let the scope cool and try a lower power.
Sources and Further Reading
- Sky & Telescope: How to collimate your Newtonian reflector
- Sky & Telescope: How to align your Newtonian reflector telescope
- Astronomy: How to collimate your telescope
- BBC Sky at Night: How to collimate a Newtonian telescope
- Celestron: How do I collimate my refractor telescope?
- Sky-Watcher: Heritage 130 Tabletop Dobsonian
- Sky-Watcher: Flextube 200P SynScan
