How to collimate a Newtonian reflector telescope

What collimation is and why a Newtonian needs it

Collimation is, as Celestron defines it, “the proper alignment of your telescope’s optical elements — lenses and mirrors.” In a Newtonian reflector this specifically means the primary mirror (the large one, at the bottom of the tube) must send the light beam exactly to the center of the secondary mirror (the small, angled one near the focuser), which in turn must redirect it with the same precision toward the eyepiece.

As the Asociación Astronómica de España explains, Newtonians “require ongoing maintenance” because they depend on the perpendicular alignment of two independent optical axes — something a refractor, with its fixed, sealed front lens and no internal mirrors to adjust, doesn’t need at all. Transporting the tube to the observing site and temperature changes are the causes of misalignment that the sources consulted mention most consistently; AstroGEDA is particularly emphatic that reflectors, and Dobsonians in particular, are “more prone to falling out of collimation” precisely because they have to be moved from place to place.

Signs that your telescope is out of collimation

Before touching a single screw, it’s worth confirming the problem is really collimation and not focus, dirty optics, or atmospheric turbulence. The signs the various guides consistently point to are:

• Elongated or “V”-shaped stars instead of round points, even with good focus.
• Loss of contrast and detail: Jupiter’s belts blur, small lunar craters become hard to distinguish.
• Off-center secondary mirror shadow when slightly defocusing a bright star — instead of appearing in the middle of the diffraction disk, it shows shifted to one side.
• Non-concentric diffraction rings (Airy disks) in that same defocused-star test.

Tools: homemade cap, Cheshire, or laser (and why the sources disagree)

There are three common types of tools, and it’s worth being upfront here: the sources consulted don’t agree on which is most reliable.

• Homemade collimation cap: a cap with a small hole in the center that’s inserted into the focuser drawtube. Celestron explains how to make one from a 35mm film canister (about 1.25” in diameter), drilling a hole 1/16” to 1/8” in the center of the bottom — an “inexpensive” solution that works for initial alignment, though it’s less precise than a commercial tool.
• Cheshire collimator: a tube with crosshair reticle. The Asociación Astronómica de España calls it “the gold standard for visual collimation,” and Astro Baby explicitly recommends it over the laser for beginners; guides such as Gary Seronik’s likewise advise against the laser for beginners, though his recommended alternative is the homemade collimation cap, not the Cheshire.
• Laser collimator: it’s inserted like an eyepiece and projects a red dot that must be centered on a mark on the primary mirror. It offers speed, and sources such as ElGranObservatorio recommend it as the first choice — but several independent sources warn of a practical, not theoretical, problem: laser collimators that are poorly calibrated from the factory. A user on the Astronomo.org forum reported that of several units tested, “up to the third or fourth one… were quite badly out of collimation.” Astro Baby outright discourages them for use without prior experience, and the Agrupación Astronómica de Málaga “Sirio” considers them “insufficient to guarantee proper alignment” in its comparison against Catseye precision tools.

Using the Cheshire for the initial precise alignment and, if you own a laser, verifying with it afterward for quick field adjustments is the synthesis proposed by the Astronomo.org forum thread (and one that seems reasonable given no other source contradicts it) — without treating them as interchangeable or trusting a laser without first checking it’s properly calibrated.

The process: secondary mirror first, then primary

All the guides consulted — from Celestron to Astro Baby — agree on the general order of the process, although each breaks it down into a different number of steps (two, three, or four depending on the source). The summary that keeps repeating is:

1. Align the secondary mirror first: with the collimation tool inserted into the focuser, adjust the secondary’s screws (typically 3 Allen screws around a central screw) until the primary mirror appears fully round and centered in the reflection.
2. Align the primary mirror next: adjust its 3 collimation screws, spaced 120° apart, until the reflection of the secondary itself (or the primary’s center mark, depending on the tool) is centered in the reticle.
3. Tighten the locking screws paired with each adjustment screw, so the mirror doesn’t shift from vibration or transport.

Important — this is what no source consulted spells out: exactly which screw to turn, and in which direction, to correct an observed offset depends on the specific telescope model and the collimator you’re using, so no generic guide can give you that instruction precisely. The real process is iterative, driven by observation: you adjust a screw a little, check the result in the collimator (or, at night, on the test star), and correct in the direction that moves the image toward center. High Point Scientific suggests a practical trick for identifying which screw needs adjusting at a given moment: placing your hand’s silhouette in front of the tube’s opening to visually pinpoint the problem area before touching any screws.

A warning repeated by several technical sources: don’t touch the secondary mirror’s central screw (the one holding it to its holder, distinct from the tilt screws) — it’s factory-set, and touching it can needlessly alter the telescope’s field illumination.

Fast reflectors (f/5 or faster): more demanding, but not zero tolerance

Here too the sources consulted diverge somewhat, and it’s worth laying that out as-is rather than flattening it into a single sentence. The Asociación Astronómica de España states that in “fast” systems (f/4-f/5) “tolerance drops drastically” compared with slower instruments. Astro Baby, on the other hand, notes that even fast telescopes “have some margin” and that small errors don’t end up being noticeable in practical observation, though it warns that these same telescopes naturally show collimation patterns that are “slightly off-center” due to the secondary’s eccentricity, which can be mistaken for a real error. Taken together, a fast Newtonian demands more care and precision when collimating, but that doesn’t mean any tiny deviation ruins the image — the “perfect” pattern of an f/5 itself isn’t identical to that of an f/8.

A real-world example of this case is the Celestron AstroMaster 130EQ, an f/5 Newtonian reflector: because of its short focal ratio, it’s worth checking its collimation somewhat more often than with a slower Newtonian, and paying attention to whether the asymmetry seen in the collimator is simply the natural pattern of a fast tube rather than an actual error.

How often you should check collimation

The sources don’t offer a single figure, and it’s not worth inventing one here: Sky at Night Magazine notes that the frequency “depends on the focal ratio — shorter instruments require more frequent adjustments”; AstroGEDA recommends checking it “before observing sessions”; and Visitando el Universo simply advises checking it “periodically,” without pinning down an interval. The one thing several sources indirectly agree on is that transport is the most common trigger for a noticeable loss of collimation, so checking collimation — even with just a quick look through a homemade collimator — after moving the telescope is a reasonable habit even if it doesn’t require readjusting anything every time.

Final check: the star test

The final, and according to several sources the most reliable, step is the star test: point at a moderately bright star, defocus it slightly at high magnification, and observe the resulting diffraction pattern. With good collimation, the concentric rings and the secondary mirror’s shadow appear centered; with poor collimation, the pattern looks off-center, asymmetric, or even heart-shaped (a sign of a mirror being pinched by its cell). This step confirms collimation regardless of which tool — Cheshire, laser, or homemade cap — was used beforehand, and it’s the criterion that decides whether it’s worth continuing to adjust screws or the telescope is already ready for observing.

Who this guide is for

If you own a Newtonian reflector — open-tube, like most Dobsonians and Newtonians on an equatorial mount — collimation isn’t optional: it’s the maintenance step that makes the difference between a sharp image and a blurry one with the very same telescope. The good news is that, once you get the hang of it, adjusting with the right tool usually takes just a few minutes. Start with an inexpensive tool (a homemade cap or a Cheshire) before moving up to a laser collimator, and always verify the result with a real star test before calling the session good.

Further reading

• Celestron — How do I collimate my Newtonian (aligning the mirrors in daylight)
• Celestron — How can I make my own collimating tool for my Newtonian reflector
• Asociación Astronómica de España — Colimación de un telescopio Newton
• Astro Baby’s Collimation Guide