How to Collimate a Newtonian Telescope

Collimation is the process of aligning a Newtonian reflector's two mirrors, the large primary at the back of the tube and the small angled secondary near the focuser, so that light travelling down the tube reaches the eyepiece along the axis the optics were designed for. Out of collimation, stars look bloated, one-sided, or comet-shaped instead of sharp points, and the effect gets worse the higher the magnification.

This applies specifically to Newtonian reflectors. Refractors and Maksutovs effectively do not need this: both are sealed, factory-aligned designs with no simple owner adjustment, which is one of the real trade-offs against their higher cost per inch of aperture. If you own a Newtonian Dobsonian or any other open-tube reflector, this maintenance is yours to learn, and it is genuinely a five-minute job once you know the sequence.

What tools do I need to collimate a Newtonian?

Three tools cover the whole job, and they are not interchangeable, because each one answers a different question.

A collimation cap is the simplest: a cap with a small centre peephole that drops into the focuser in place of an eyepiece, letting you sight straight down the tube. It is cheap and good for a rough first pass, but the small unlit peephole makes fine judgment difficult. A Cheshire collimating eyepiece improves on this directly: it adds a small angled surface that reflects light onto the primary mirror, illuminating the mirror's edge and centre spot clearly, along with a crosshair to judge centring against. This is the tool for the actual visual adjustment steps below. A laser collimator projects a beam down the tube that bounces off both mirrors and lands back on a target near the laser itself, which makes primary mirror tilt fast and repeatable, but as covered further down, it is the wrong tool for the secondary mirror step.

Step one: how do I centre and rotate the secondary mirror?

Do this step first, before touching the primary mirror at all, because every later step is judged relative to the secondary's position. Adjusting the primary before the secondary is correctly placed just chases a target that keeps moving.

Remove the eyepiece and drop in a collimation cap or Cheshire. Looking down the focuser, you should see the secondary mirror, held on a spider, a set of thin metal vanes, roughly centred and appearing as an even oval under the focuser opening. Two separate adjustments matter here: centring, moving the whole secondary mirror assembly slightly along the tube's axis using the central screw that holds it to the spider, so it sits directly under the focuser; and rotation, turning the secondary mirror's face so it points squarely back up toward the focuser rather than off to one side. Most secondary mirrors rarely need this step touched after the original factory setup unless the spider assembly has been disturbed, but it is always worth checking first, because if it is wrong, nothing downstream will look right no matter how carefully you adjust the tilt or the primary.

Step two: how do I tilt the secondary mirror?

With centring and rotation correct, look at where the primary mirror's outline appears reflected in the secondary. Using the three small tilt screws around the secondary's mount, adjust until the primary mirror's full circular outline, including its edge clip and the reflection of the focuser drawtube, appears centred and complete inside the secondary's reflection. Through a Cheshire, this means the reflected image of the primary mirror should appear as a full, evenly framed circle with no crescent of missing edge on any side.

Step three: how do I adjust the primary mirror?

Only now move to the primary mirror, at the back of the tube, which has its own three tilt screws, usually paired with three locking screws. Through the Cheshire, look for the small centre spot marked on the primary mirror, a sticker or dot every collimatable Newtonian ships with specifically for this purpose. Adjust the primary's tilt screws until that centre spot appears exactly in the middle of the Cheshire's crosshair, with the reflected image of the crosshair and the Cheshire's illuminated ring symmetric around it.

This is the step a laser collimator handles quickly and repeatably: with the laser seated in the focuser, adjust the primary's tilt screws until the returning beam lands precisely back on the target ring around the laser's own aperture. Because primary tilt is a straightforward beam-in-beam-out geometry problem, a laser is genuinely excellent here, which is why it remains a useful tool despite being unsuitable for the secondary step.

Why can't a laser collimator set the secondary mirror reliably?

A laser collimator only tells you where a fixed beam lands after bouncing off both mirrors. It cannot tell you whether the secondary mirror is centred under the focuser or correctly rotated to begin with, because those are questions about the secondary's physical position relative to the focuser opening, not about where a single beam happens to reflect. If the secondary is off-centre, a laser can still be adjusted to bounce back cleanly by compensating with primary tilt, producing a result that looks correct on the laser's target but leaves the actual optical axis skewed and the illuminated field of the eyepiece uneven. This is the most common reason owners collimate with a laser alone and still see soft, one-sided stars: the tool confirmed the wrong thing perfectly. Use a Cheshire or collimation cap for the secondary, and save the laser for the primary tilt step where its speed is a genuine advantage.

How do I confirm collimation with a star test?

The star test is the final step and the one that checks the whole system under actual starlight rather than a tool's approximation of it. Point at a moderately bright, isolated star, ideally one reasonably high in the sky to minimise atmospheric distortion, and centre it using your shortest, highest-power eyepiece. Slowly defocus the star slightly in both directions, inside and outside focus, and look at the pattern of diffraction rings that appears around the small central disc.

On correctly collimated optics, those rings appear concentric, evenly centred around the star on both sides of focus, like a small bullseye. On misaligned optics, the rings look noticeably lopsided or crescent-shaped on one side, with more rings visible on one edge than the other. If the rings are off-centre, revisit the primary tilt step; persistent asymmetry after a careful primary adjustment usually points back to the secondary centring or rotation from step one. Atmospheric turbulence, seeing, can make the rings shimmer and boil even on perfectly collimated optics, so judge the test on a night with reasonably steady air and do not over-correct chasing a symptom the sky is causing rather than the mirrors.

Putting the sequence together

Secondary centring and rotation, then secondary tilt, then primary tilt, then a star test to confirm, in that order, every time. Skipping ahead to the primary mirror because it feels like the "real" adjustment is the most common way this goes wrong, since every later step is judged relative to whatever came before it. Once you have done the sequence a couple of times it takes under five minutes, worth doing before any serious session and always worth doing after transporting the telescope. For the rest of what a telescope needs between sessions, see telescope maintenance and storage, and if you are choosing between a Newtonian and a design that skips this maintenance entirely, the trade-offs are laid out at refractor versus reflector. First-night setup steps that come before any of this, including cool-down time and dark adaptation, are covered at using a telescope for the first time.