How to Collimate a Newtonian Telescope: A Plain-English Walkthrough

If your reflector, the type behind most Dobsonians, shows stars as soft blobs or comet-like smears instead of sharp points, the likely culprit is collimation. That is the word for aligning the mirrors, and it is the single maintenance task reflector owners need to learn. It sounds technical. It takes about ten minutes once you know the steps, and you will do it often enough that it becomes routine.

This guide walks you through a simple method that works on most Newtonian reflectors, including the Dobsonians in our Dobsonian guide. Refractors rarely need it, and you can see why in refractor vs reflector telescopes.

What collimation actually does

A Newtonian has two mirrors. The large main mirror, called the primary, sits at the bottom of the tube. A small flat mirror, the secondary, sits near the top and sends the light sideways to the eyepiece. For a sharp image, the two mirrors have to point at each other precisely, and both have to be aligned with the eyepiece.

Over time, bumps and temperature changes can nudge the mirrors out of alignment, and the telescope is "out of collimation." The result is soft or distorted images, especially at high power. Fixing it restores sharpness.

The tools you need

A collimation cap. A small cap with a tiny hole in the center that fits in the focuser. It costs very little and is enough for a rough alignment.

A laser collimator. A tool that shines a laser down the tube. It is quick, and it needs to be accurate itself.

A Cheshire eyepiece. A special eyepiece with a sight tube and a bright reflective surface. It gives a precise alignment.

A small screwdriver or hex key. For the adjustment screws.

A flat surface to look at. A piece of white paper or a dim light helps.

For most beginners, a collimation cap and a bit of patience are enough.

Before you start

Set the telescope on a stable surface in a well-lit place, in daylight or indoors with a lamp. Point the tube at a plain wall or a piece of white paper. Take the cap off the front. Let the scope settle to room temperature if it was moved from outside.

Look at your mirrors. The main mirror has three pairs of screws on the back of the tube, one pair for each adjustment point. The secondary mirror has a center bolt and three small screws. The clips on the primary hold the mirror in place and should not be overtightened.

Take a photo or make a note of the screw positions before you start, so you can return to them if you get lost.

Step 1: Center the secondary mirror under the focuser

Remove the eyepiece and look into the focuser with the collimation cap. You will see the reflection of the secondary mirror, the primary and the cap's hole. The first job is to make the secondary appear centered under the focuser.

If the secondary mirror looks off to one side, loosen the center bolt slightly and adjust its position, or use the three small screws. Make small adjustments and check after each one.

Step 2: Aim the secondary at the primary

Next, look at the reflection of the primary mirror in the secondary. You should see the whole primary, round and centered, in the secondary's reflection. If part of the primary is cut off, the secondary is tilted the wrong way.

Use the three small screws around the secondary's center bolt to tilt it. Turn one a little, check, then another. The goal is to see the whole primary mirror centered in the secondary.

Step 3: Align the primary

Now look at the reflection of the secondary in the primary, and the reflection of your own cap or eye. You are aiming to make the reflection of the secondary sit in the center of the primary's reflection, with the cap's dot centered within.

Use the adjustment screws on the back of the primary mirror cell. Turn one screw a small amount, and watch how the reflection moves. Adjust the screws one at a time, and use the locking screws if your scope has them, so the alignment holds.

Step 4: Check with a star

Collimation is only truly confirmed on a star. On a clear night, point at a bright star, defocus until it becomes a donut, and look at the shape. The donut's inner dark circle should be centered. If it is off to one side, adjust the primary a little.

This is called a star test, and it is the final check. Fine adjustments are easier on a star than on a daytime reflection.

Tips that save frustration

Make small adjustments. A quarter turn can be a big change.

Change one thing at a time. If you adjust three screws at once, you will lose track.

Be patient. The first time takes longer, but it gets quick.

Do not touch the mirror surface. Fingerprints and scratches hurt the view.

Mark the starting points. A small pencil mark on the cell helps you return if you go wrong.

Check after you move the scope. A drive to a dark site can shift it.

Signs your scope needs collimating

• Stars look like small comets or have flares on one side.
• Defocused stars look like lopsided donuts.
• The image will not come to a sharp focus, especially at high power.
• Planets look soft.

If your scope is fine at low power but soft at high power, collimation is a likely cause.

What not to do

Do not try to clean or adjust the mirror's coating. Do not loosen the primary's clips more than needed. Do not force a screw that will not turn. If you are unsure, ask for help at a local astronomy club, since many members are glad to show you in person.

How often you will need to do it

A well-built scope holds collimation for a long time. Check it before each session with the cap, which takes a minute. Make a fix if the donut looks lopsided. Travel and temperature swings are the most common causes of drift. A Dobsonian that stays at home often needs adjustment only occasionally.

Where to go from here

If your scope still looks soft after collimation, the problem may be the air or the cooling. Our how to use a telescope guide covers cooling and other habits. If you are choosing an eyepiece to go with your scope, see telescope eyepieces explained.

Why mirrors go out of alignment

Mirrors sit in cells held by springs, clips and screws, and any shock can shift them slightly. A drive over a bumpy road, a tilt while carrying the scope or even changes in temperature can nudge the alignment. The adjustment is tiny, since a fraction of a turn on one screw is enough to change the image. That is also why a good scope holds collimation well, because the cells are firm and the tube is stiff.

Think of collimation as tuning an instrument. A guitar goes out of tune with changes in the air and the strings, and you retune it before playing. A reflector does the same, and a quick check before each session keeps the image sharp. Most people find that, after a few attempts, they can collimate a scope in a few minutes.

Working with a laser collimator

A laser collimator replaces the cap. It shines a thin beam down the tube, and the beam shows where it hits the secondary and the primary. You adjust the mirrors until the beam falls on the center mark of the primary and returns to the center of the laser. It is fast, and many observers use it as their main tool.

The catch is that the laser itself has to be collimated. Test it by rotating it in the focuser: the dot on a far wall should not move in a circle. If it does, the laser is off, and you should adjust or replace it. A cheap laser can give a false sense of accuracy, and a star test remains the final check.

Fast scopes and fine tuning

Scopes with a short focal length, a low focal ratio, are more sensitive to collimation errors. A fast Dobsonian needs a more careful alignment than a slow one to give a sharp image at the edges of the field. If your scope is fast and the stars look soft at high power, try a finer adjustment with a Cheshire eyepiece or a star test. Slow scopes are more forgiving, and a rough alignment often works for them.

If you have done everything and the image is still poor, the cause may not be collimation. Poor seeing, a scope that has not cooled and a dirty eyepiece are all common. Try a different eyepiece, give the scope more time and look at a bright star. Our how to use a telescope guide covers these habits.

How we research

This guide is based on standard practice among amateur astronomers and on manufacturer instructions. For how we work, see our about page and affiliate disclosure.