Collimating Your Telescope: Why It Matters and How to Do It

You've got a solid telescope. You've picked a clear night. You point at Jupiter and... the image looks soft, maybe lopsided, with a strange flare on one side. Before you blame the atmosphere or your eyepiece, there's something else you should check: collimation.

Collimation is the alignment of your telescope's optical elements. When everything is lined up properly, light comes to a clean, sharp focus. When it's off, even by a small amount, your views suffer. The good news is that checking and fixing collimation takes just a few minutes once you know how.

What Is Collimation, Exactly?

Every reflecting telescope (Newtonians, Dobsonians, and most compound telescopes) has mirrors that must be precisely aimed. In a Newtonian reflector, the primary mirror at the bottom of the tube catches light and bounces it up to the secondary mirror, which redirects it out the side to your eyepiece. If either mirror is tilted even slightly off-axis, the light cone becomes distorted and your image degrades.

How Do You Know It's Off?

There are a few telltale signs that your collimation needs attention:

• Stars look like comets, bright on one side with a tail or flare extending to the other side, especially at high magnification
• Planets look soft or asymmetric, you can't quite get a crisp focus, and details appear smeared
• The defocused star test looks lopsided, when you slightly defocus a bright star, the rings should be concentric circles; if they're shifted to one side, you're out of collimation

Collimation drifts over time from transporting your telescope, temperature changes, and simply the effect of gravity on the mirror. If you have a Dobsonian, plan to check collimation at the start of every observing session. It takes two minutes and makes a real difference.

Tools You'll Need

You don't need expensive gear to collimate well. Here are the three most common approaches, from simplest to most precise:

Step-by-Step: Collimating a Newtonian Reflector

Here's the basic process. You'll adjust two things: the secondary mirror (the small diagonal mirror near the top of the tube) and the primary mirror (the big mirror at the bottom).

Step 1: Center the secondary mirror

Insert your collimation cap or Cheshire eyepiece into the focuser. Look through it and check whether the secondary mirror appears centered in the focuser tube. If it's shifted to one side, adjust the secondary mirror's position using the central bolt on the spider vane that holds it. This step is about position, not tilt.

Step 2: Aim the secondary mirror at the primary

Still looking through the collimation tool, you should see the primary mirror reflected in the secondary. Adjust the three tilt screws on the secondary mirror holder until the reflection of the primary mirror is centered within the secondary mirror's outline. You're making the secondary point squarely at the primary.

Step 3: Aim the primary mirror

Now adjust the primary mirror's tilt using the three collimation bolts at the back of the telescope (most Newtonians and Dobsonians have three push-pull pairs or three spring-loaded bolts). Tilt the primary until the reflection of the secondary, and by extension, the reflection of your own eye looking through the collimation cap, is centered. With a laser collimator, adjust until the laser dot returns to the center of the laser's target.

Step 4: Star test to confirm

Point at a moderately bright star (magnitude 2-3) and defocus slightly. The out-of-focus star should appear as concentric rings, evenly spaced and centered around the bright point. If the rings are off-center, make small adjustments to the primary mirror and recheck.

Which telescopes actually need this

Collimation is a Newtonian problem first and foremost. The design puts two mirrors at a distance from each other in an open tube, and every time the tube is carried, knocked or driven somewhere, those mirrors can shift relative to one another. A Dobsonian that lives in a car boot needs checking far more often than one that stands in a corner.

Refractors are a different case. The lens elements are fixed in a cell at the factory and there is usually nothing for the owner to adjust. If a refractor shows the symptoms described above, the cause is far more likely to be thermal, a tube that has not cooled, than optical misalignment. Schmidt-Cassegrains sit in between: they hold collimation well, and when they do drift, only the secondary is adjustable.

What the star test is really telling you

The star test is the only check that measures the whole optical path at once, which is why it comes last. Defocus a bright star slightly and you should see concentric rings with the dark shadow of the secondary exactly in the middle. If that shadow sits off to one side, something upstream is still out of alignment.

Two cautions make this test far more useful. Do it on a night of steady air, because turbulence smears the rings into something unreadable and will have you chasing an error that is in the atmosphere rather than the telescope. And let the telescope reach ambient temperature first, since a warm mirror produces its own distortion that looks convincingly like miscollimation and disappears on its own after half an hour.

How Often Should You Collimate?

It depends on your telescope and how you handle it:

• Dobsonians and truss-tube Newtonians: Check every session. These designs shift easily during transport.
• Solid-tube Newtonians: Check every few sessions or if you notice soft images.
• Schmidt-Cassegrains (SCTs): Rarely need collimation, but if views degrade, adjust the secondary mirror only (the primary is fixed).

Once you've done it a few times, collimation becomes second nature. You'll spend more time checking that it's still good (it usually is) than actually adjusting anything. And the improvement in your views, sharper stars, crisper planetary detail, tighter focus, is worth every minute.

For more on getting the best performance from your telescope, see our guides on choosing the right eyepieces and using averted vision to see faint details your eyes would otherwise miss.