Need to know how to collimate a reflector telescope? The word alone is enough to make some beginners consider selling their reflector and buying a refractor. I get it — “aligning the mirrors of your telescope” sounds like something that requires a lab coat and a steady hand. In reality, it’s a five-minute job that gets easier every time you do it.
I’ll be honest: the first time I collimated my reflector, I made it worse. The Moon looked like it was having an identity crisis — doubled, smeared, and vaguely offended. But I adjusted, re-read the instructions, and got it right on the second attempt. You will too.
What Is Collimation and Why Does It Matter?
A reflector telescope has two mirrors: the large primary mirror at the bottom of the tube and a small secondary mirror near the top, angled at 45 degrees to redirect light to the focuser. Collimation is the process of making sure these mirrors are precisely aligned so that light reaches your eyepiece correctly.
When a reflector is well-collimated, stars look like sharp points of light. When it’s not, stars look like comets — smeared or lopsided. You’ll notice the effect most at higher magnifications and on planets, where the lack of sharpness is obvious.
Reflectors go out of collimation gradually through normal use — bumps during transport, temperature changes, even just gravity over time. It’s not a sign that something is broken. It’s routine maintenance, like tuning a guitar.
When to Collimate
Check collimation at the start of each observing session. It takes thirty seconds to check and five minutes to fix if needed. Many nights, it’ll be fine and you can get straight to observing. But after transporting the scope in a car, or if you notice soft or lopsided star images, it definitely needs attention.
What You’ll Need
You can collimate with nothing more than your eyepiece and your eye, but a few cheap tools make it much easier:
A collimation cap (£5-8): A simple cap with a central hole that fits in your focuser. It lets you look down the tube with your eye perfectly centred. This is the minimum I’d recommend buying.
A Cheshire eyepiece (£15-25): A more precise version of the collimation cap with crosshairs and a reflective surface. Makes the whole process much more intuitive.
A laser collimator (£25-50): Projects a laser beam down the tube. Quick and easy, but cheaper ones can be inaccurate and may need calibrating themselves, which rather defeats the purpose. Useful once you’re experienced enough to know if the laser itself is aligned.
Start with a collimation cap or Cheshire. They’re reliable, cheap, and teach you what you’re actually looking for.
How to Collimate a Reflector Telescope: Step-by-Step
Step 1: Check the secondary mirror. Remove any eyepiece and insert the collimation cap. Look through the peephole. You should see the small secondary mirror centred in the focuser tube. If it’s offset, adjust the secondary mirror’s central screw and three tilt screws until it appears centred. Most of the time, the secondary is fine and doesn’t need touching.
Step 2: Check the primary mirror. Still looking through the collimation cap, you should see a series of concentric reflections: the secondary mirror, the reflection of the primary mirror in the secondary, and the reflection of your eye (via the collimation cap) in the primary. All of these should be centred like a bullseye.
If the reflection of the primary mirror appears off-centre, you need to adjust the primary mirror’s collimation screws — typically three screws (or knobs) at the back of the telescope tube.
Step 3: Adjust the primary mirror. This is the bit that feels intimidating but isn’t. Loosen the locking screws slightly (if your telescope has them), then turn the collimation screws one at a time, a tiny amount each turn. Look through the collimation cap after each adjustment to see which direction things moved. The goal is to get all the concentric circles centred.
If you turn a screw and things get worse, turn it back and try a different one. There are only three screws. The process is trial and error, but you’ll develop a feel for which screw moves the reflection in which direction surprisingly quickly.
Step 4: Lock it down. Once everything looks centred, tighten any locking screws gently. Re-check through the collimation cap to make sure nothing shifted while tightening.
The Star Test
The ultimate check is a star test. Point your collimated telescope at a moderately bright star, defocus slightly (turn the focuser so the star becomes a small disc rather than a point), and look at the pattern. A well-collimated telescope shows a perfectly symmetrical ring pattern — like a bullseye. If it’s lopsided, your collimation is still slightly off.
Star testing is the fine-tuning step. For most visual observing, getting things close with a collimation cap is more than adequate.
For a video demonstration, YouTube has several excellent visual guides that complement the steps above.
How Long Does It Take?
First time: 15-20 minutes (including re-reading this guide three times and wondering if you’ve broken something). Second time: 10 minutes. Fifth time: 5 minutes. Twentieth time: 2 minutes without thinking about it, like tying your shoes.
Collimation genuinely isn’t the ordeal people make it out to be. It’s a small price to pay for the extra aperture and value that reflectors offer. And once you’ve mastered it, you’ll wonder what you were ever worried about.
Learning to collimate a reflector telescope is a rite of passage for every reflector owner. Once you can confidently collimate a reflector telescope, you’ll wonder why you were ever intimidated by it.
Clear skies.
Daniel is a self-taught astronomy hobbyist based in the north of England. He writes honest telescope guides, gear reviews, and stargazing advice — and remembers what it's like to not know a refractor from a reflector.