Collimation and Troubleshooting: Keeping Your Views Crisp

1) What collimation is (and who needs it)

Collimation is aligning a telescope’s optical elements so light travels down the intended optical axis and forms the sharpest possible image at the focal plane. When collimation is off, you can still “reach focus,” but fine detail (planetary bands, tight double stars, crisp lunar edges) softens and stars may look asymmetric across the field.

Why reflectors need it

In a Newtonian reflector, the primary mirror and secondary mirror must be aligned precisely. Small mechanical shifts (transport, temperature changes, bumps, mirror cell settling) can move the mirrors enough to degrade performance—especially at faster focal ratios (lower f-number), where tolerances are tighter.

Why some catadioptrics need it

Many Schmidt-Cassegrains (SCTs) and some other catadioptrics allow secondary mirror adjustment. They typically hold collimation better than Newtonians, but when they do drift, the symptoms (asymmetric star test, mushy planetary detail) are similar. Maksutovs often hold collimation well and may not be user-adjustable; if they are not adjustable, troubleshooting focuses on other causes.

Who usually does not need routine collimation

Most refractors are factory-aligned and not meant for frequent user collimation. If a refractor shows severe misalignment, it’s often a mechanical issue (focuser tilt, lens cell shift) best handled carefully or by service rather than constant tweaking.

2) Collimation tools: what they do and how to use them responsibly

Collimation tools help you reference the optical axis. The key idea: use more than one check (or at least one tool plus a star test) so you don’t “perfectly align to a wrong reference.”

Continue in our app.

• Listen to the audio with the screen off.
• Earn a certificate upon completion.
• Over 5000 courses for you to explore!

Or continue reading below...

Download the app

Collimation cap (simple, reliable baseline)

A collimation cap is a dust cap with a small peephole (often with a reflective underside). It centers your eye and makes mirror reflections easier to judge.

• Best for: Beginners, quick checks, primary mirror alignment, and sanity-checking other tools.
• How to use: Insert in the focuser (or in an adapter that fits snugly). Look through the peephole and assess whether the primary mirror center mark is centered in the reflected reference.
• Responsibility tip: Make sure the cap sits squarely (no wobble). If your focuser has compression rings, tighten consistently.

Cheshire (high-confidence primary alignment)

A Cheshire eyepiece uses a bright reflective surface and a peephole to show the primary’s center mark relative to a reference ring. It is excellent for setting the primary mirror tilt.

• Best for: Accurate primary adjustment in Newtonians; repeatable results.
• How to use: Insert the Cheshire, illuminate it indirectly (room light or a dim red light angled at the side window), and adjust the primary until the center mark is centered in the Cheshire’s bright ring.
• Responsibility tip: A Cheshire doesn’t “fix” secondary placement by itself; it mainly confirms where the primary is pointing.

Laser collimator (fast, but verify it first)

A laser projects a beam down the focuser axis. It can speed up alignment, especially in the dark, but it can also mislead you if the laser itself is miscollimated or if it sits tilted in the focuser.

• Best for: Quick secondary tilt adjustment, rough alignment, and field touch-ups (with verification).
• How to use (basic): Insert the laser snugly, turn it on, adjust the secondary so the beam hits the primary center mark, then adjust the primary using the return beam (if your system supports it) or with a better primary reference (Cheshire or barlowed laser).
• Responsibility tip: Never assume a laser is accurate out of the box.

Verification steps to avoid mis-collimating with a misaligned laser

Do these checks before trusting laser results:

• Rotation test (laser self-collimation check): Place the laser in the focuser (or a V-block on a table aimed at a wall). Turn it on and slowly rotate it. If the dot draws a circle, the laser is miscollimated. A good laser keeps the dot in the same spot (or nearly so).
• Seating test (focuser fit check): Insert the laser, lightly tighten, note the dot position on the primary, then loosen and re-seat the laser in a different orientation. If the dot shifts noticeably, the issue may be focuser slop, adapter tilt, or the laser barrel tolerance.
• Cross-check with a cap/Cheshire: After laser alignment, confirm the primary center mark position with a Cheshire or cap. If they disagree, trust the Cheshire/cap for primary tilt and investigate the laser.
• Prefer barlowed laser for primary tilt (if available): A barlowed laser method uses the shadow of the center mark and is far less sensitive to small laser misalignment. If you don’t have a barlowed setup, a Cheshire is the safer primary reference.

3) Step-by-step Newtonian collimation flow (secondary placement → primary → star test)

This flow separates tasks that beginners often mix together. You are aligning (A) the secondary’s position, (B) the secondary’s tilt, and (C) the primary’s tilt. Do them in that order.

Before you start: set up for success

• Work in good light the first few times.
• Remove the eyepiece; insert your cap/Cheshire (or laser if verified).
• If your primary has a center mark (donut/ring), that’s your reference. If it does not, accurate collimation is much harder; consider adding a proper center mark using a template made for your mirror size.
• Use small adjustments. If a screw feels stuck, stop and check you’re turning the correct one.

A) Secondary placement concepts (what “placed correctly” looks like)

Secondary placement is about geometry, not sharpness yet. You want the secondary mirror to appear as a centered, evenly illuminated ellipse under the focuser, so the focuser “sees” the whole primary mirror evenly.

• Goal 1 (centering under the focuser): Looking through a cap/Cheshire, the outline of the secondary should look centered in the focuser drawtube.
• Goal 2 (rotation): The reflection of the primary mirror should look centered within the secondary’s outline (not skewed to one side).
• Goal 3 (offset awareness): In many Newtonians, the secondary is intentionally offset slightly away from the focuser and toward the primary for best illumination. This can make “perfectly centered” reflections look subtly non-intuitive. Don’t chase perfection by eye if your scope is designed with offset; prioritize seeing the primary reflection centered in the secondary and achieving good axial alignment.

How to adjust placement:

• Up/down the tube (secondary fore-aft): Adjust the center bolt (the one that holds the secondary stalk) to move the secondary closer to or farther from the primary. Do this gently while supporting the secondary so it cannot fall.
• Rotation: Slightly loosen the secondary center bolt just enough to rotate the secondary holder. Rotate until the primary reflection looks evenly framed.
• Side-to-side centering: Some spiders allow minor lateral adjustment; often you achieve apparent centering via a combination of fore-aft and rotation. Don’t force spider vanes; keep tension even.

B) Secondary tilt (aim the focuser axis at the primary center)

Now you aim the focuser axis so it hits the primary center mark.

• With a laser: Adjust the secondary tilt screws until the laser dot lands exactly on the primary center mark.
• With a cap/Cheshire: Adjust secondary tilt until the primary center mark appears centered under the peephole reference (method depends on tool style; the key is that the sightline from the focuser points to the primary center).

Tip: Secondary tilt adjustments are usually small. If you need huge tilt changes, your secondary placement (step A) is likely off—go back and re-check rotation and centering.

C) Primary mirror adjustment (aim the primary back to the focuser)

This is the step that most directly affects on-axis sharpness.

• Using a Cheshire: Adjust the primary collimation knobs until the primary center mark is centered in the Cheshire’s bright ring.
• Using a cap: Adjust the primary until the center mark is centered relative to the peephole reflection.
• Using a laser (only if verified, preferably barlowed): Adjust the primary based on the appropriate return/shadow reference. If using a simple return beam, be aware it is sensitive to laser alignment and focuser tilt.

Lock screws: If your cell has locking screws, tighten them gently and evenly, re-checking collimation as you go. Over-tightening can shift the mirror or pinch it.

D) Final refinement: star test (the reality check)

Tools align the mechanics; the star test confirms the optics under the sky.

• Choose a moderately bright star high in the sky (to reduce atmospheric distortion).
• Let the telescope reach thermal equilibrium as much as practical; heat plumes can mimic collimation errors.
• Use high magnification (often 20–30× per inch of aperture if seeing allows).
• Defocus slightly inside and outside of focus. The diffraction pattern should look concentric. If the bright rings are offset to one side, fine-tune the primary collimation (very small turns).

Practical approach: Make one tiny primary adjustment, re-center the star in the field (critical!), and re-check. Repeat until the pattern is symmetric.

4) Common symptoms and fixes (what you see → what to try)

Triangular stars (often “pinched optics”)

Symptom: Stars look triangular or have threefold symmetry, especially at higher power, and the shape may stay similar across the field.

Likely causes:

• Primary mirror clips too tight (Newtonian): clips pressing on the mirror edge.
• Over-tightened retaining ring (some catadioptrics) or stress in the corrector/secondary assembly.

Fix: For Newtonians, loosen mirror clips so they do not press on the glass; you want a tiny clearance (the mirror should not rattle, but clips should not clamp). Re-check collimation afterward. If the scope is under warranty and you are unsure, pause and consult the manufacturer’s guidance.

Out-of-focus “donuts” look weird or off-center

Symptom: When defocused, the donut (secondary shadow) is not centered, or the bright rings are lopsided.

Likely causes:

• Primary collimation off (most common).
• Star not centered in the field during the test (very common).
• Seeing/thermal currents causing a shifting pattern.

Fix: Center the star precisely, reduce defocus (small defocus is more diagnostic), and adjust the primary in tiny increments. If the pattern swims rapidly, wait for steadier moments or reduce magnification.

Astigmatism: stretched stars that rotate 90° through focus

Symptom: Slightly inside focus, the star looks like a short line/oval in one direction; slightly outside focus, it rotates about 90°.

Common sources:

• Pinched primary (again): stress can create astigmatism-like patterns.
• Eyepiece astigmatism (especially at fast f-ratios) or a poorly matched eyepiece.
• Your eye: some observers have noticeable astigmatism; it’s more apparent with larger exit pupils.
• Focuser/adapter tilt: mechanical tilt can mimic asymmetric shapes.

Fix workflow:

• Rotate the eyepiece in the focuser. If the astigmatism rotates with it, suspect the eyepiece.
• Try a different eyepiece. If it disappears, you’ve found the culprit.
• Reduce exit pupil (use higher magnification). If it improves dramatically, your eye’s astigmatism may be contributing.
• Check mirror clips/retainers for stress and ensure nothing is overtightened.

Dew fogging (soft, low-contrast views that worsen over time)

Symptom: The view gradually loses contrast; bright objects bloom; flashlight inspection may show a hazy film on the front element/corrector or eyepiece.

Likely causes: Moisture condensing on exposed optics (corrector plates, refractor objectives, finder scopes, eyepieces).

Fix:

• Use a dew shield to slow radiative cooling and reduce exposure to the sky.
• Gently warm optics with a dew heater if available; avoid high heat.
• Keep eyepieces capped when not in use; store spares in a pocket or case to keep them warmer.
• Do not wipe dew off in the field unless you must; if you do, use proper lens-safe methods and minimal pressure.

Backlash (slop when reversing direction)

Symptom: When you reverse a knob (focuser or slow-motion controls), nothing happens for a moment, then it “catches up.” This makes fine focusing or tracking frustrating.

Likely causes: Gear play in the mount controls, loose tension, or focuser mechanism slack.

Fix:

• Focusing technique: Finish focus by turning the knob in the same direction each time (approach from one side) to take up slack consistently.
• Mechanical check: If your focuser has tension/mesh adjustments, make small changes to reduce play without making motion stiff.
• Mount controls: Adjust worm gear mesh only if you understand the procedure; too tight can bind. Otherwise, use technique (always approach from the same direction) and keep balance reasonable.

Mount wobble (image shakes, especially at high power)

Symptom: Touching the focuser, adjusting knobs, or a light breeze causes the image to vibrate for seconds.

Likely causes: Loose fasteners, extended tripod legs, insufficient stiffness, or a long optical tube acting like a lever.

Fix (quick checks):

• Tighten tripod/mount bolts and accessory tray/spreader.
• Keep legs shorter and wider if possible; avoid fully extending thin sections.
• Check that the tube rings/dovetail clamps are tight.
• Use lighter touch and let vibrations settle before judging collimation or focus.

5) Quick diagnostic decision tree (start from “image looks bad”)

START: Image looks bad (soft, weird stars, low contrast, hard to focus)