How to Collimate a Reflector Telescope — and What Tools to Buy

Collimation is the alignment of a reflector telescope’s mirrors so that light from a star is focused to a single point on the optical axis. A well-collimated Newtonian reflector produces sharp stars. A poorly collimated one produces stars with a comet-like tail on one side of the field — coma — regardless of how good the mirror quality is.

Most beginners with a reflector telescope have experienced bad collimation without knowing it. The good news: checking and correcting collimation takes 3–5 minutes once you know how.

Tool	Price	Best for
Cheshire / sight-tube combined	~€28	Recommended first purchase, most accurate
Basic laser collimator	~€35	Fast, less precise without a baffle
Baader Laser + Cheshire combined	~€65	150mm+ scopes in regular use
Howie Glatter TuBlug	~€100	Advanced use, maximum precision

Why Collimation Goes Wrong

A Newtonian reflector has two mirrors: the primary (large, curved, at the bottom of the tube) and the secondary (small, flat, mounted at 45° near the front). Light enters the tube, reflects off the primary, reflects off the secondary, and exits through the focuser to the eyepiece.

For this to produce a sharp focus, both mirrors must be precisely aligned:

The secondary must be centred under the focuser
The secondary must be tilted to direct light exactly to the primary’s centre
The primary must be tilted to reflect light exactly back up the optical axis to the focuser

Any knock, vibration during transport, or thermal shift can move these alignments. Dobsonians, which are moved regularly, need collimation checked before most sessions. Reflectors in fixed observatories can go weeks between checks.

How to Know if You Need to Collimate

The star test: At high magnification on a bright star, slightly defocus the image. You should see concentric, symmetric rings around the star’s disc. If the rings are asymmetric — thicker on one side — collimation is off. The thicker side points toward the direction the secondary needs to move.

The cap test (daytime): Remove the eyepiece and look down the focuser tube with the lens cap on the front of the telescope. You should see the secondary mirror centred in the focuser tube, the primary mirror reflection centred in the secondary, and your own eye’s reflection centred in the primary’s reflection. If these concentric circles are off-centre, collimation is needed.

The Collimation Tools

Cheshire Eyepiece Collimator (~€20–€35)

A Cheshire is a hollow cylinder with a peephole at one end and a reflective 45° surface inside with a small central hole. You insert it into the focuser and look through the peephole. The 45° reflective surface illuminates the primary mirror, and you centre the reflections manually.

The Cheshire is slower than a laser but more accurate, and does not require a precisely machined focuser. It is the tool recommended by most experienced observers for final collimation. A combined Cheshire/sight tube unit (~€28–€35, from Baader or GSO) combines the two functions.

Best for: Final collimation of primary mirror alignment.

Price: ~€18–€35, astronomy dealers

Laser Collimator (~€25–€45)

A laser collimator inserts into the focuser and projects a laser dot onto the primary mirror, which reflects it back. The goal is to return the dot to the centre of the laser collimator exit hole. Fast, dramatic, and requires checking that the laser collimator itself is accurate (they can be knocked out of calibration).

The laser is excellent for checking secondary mirror alignment quickly. It is less reliable than a Cheshire for primary alignment because small errors in laser manufacture become significant. Using both — laser for the secondary, Cheshire for the primary — is the approach most experienced observers recommend.

Best for: Quick secondary check, initial rough alignment.

Price: ~€25–€45, Amazon EU

Baader Laser Collimator with Cheshire (~€65)

Baader’s combined laser/Cheshire unit addresses the reliability issue of laser-only collimators by incorporating a Cheshire function in the same tool. The laser confirms secondary position; the Cheshire confirms primary alignment. Baader’s manufacturing tolerances are tighter than generic laser collimators, reducing the false-reading problem.

For a Dobsonian used regularly, a quality combined tool is worth the investment over a cheap laser. The Heritage 130P, Skyliner 200P, or any instrument used at least weekly justifies the price.

Price: ~€60–€70, Baader dealers

Howie Glatter TuBlug (~€95)

The Howie Glatter is the reference standard precision laser collimator. A bore-sighted laser diode with verified < 0.1° angular error means false readings are essentially eliminated. Used with a Glatter holographic attachment, it generates a circular projection pattern visible from the mirror end for single-person operation.

Justified for telescopes with apertures above 250mm where collimation errors become more critical. For 130–200mm instruments, the Baader combination collimator achieves functionally identical results.

Price: ~€90–€110, specialist dealers

Step-by-Step Collimation (Newtonian / Dobsonian)

What you need: Your collimation tool, a small Phillips screwdriver (for secondary), and 5 minutes.

Step 1 — Secondary mirror centre: Look down the focuser with the Cheshire or cap. The secondary mirror should appear centred in the focuser tube. If not, loosen the secondary spider vane screws and slide the secondary cell up or down the spider until centred. Re-tighten.

Step 2 — Secondary tilt: The reflection of the primary mirror in the secondary should appear centred within the secondary. Adjust the three tilt screws on the secondary holder (usually a central push-pull screw system) until the primary reflection is centred.

Step 3 — Primary mirror tilt: The reflection of the secondary (and your eye or the Cheshire cross-hair) should appear centred within the primary reflection. Adjust the three primary mirror tilt screws (usually at the back of the telescope tube — push-pull pairs) until all reflections are concentric.

Step 4 — Star test: At 150–200× on a bright star, confirm concentric defocused rings. If good: done. If asymmetric: return to Step 3 and make a small adjustment in the direction of the thicker ring.

The first time takes 20–30 minutes. By the fifth session, 3–5 minutes.

Which Telescopes Need Collimation

Newtonian reflectors: Yes, every session or whenever transported.

Dobsonians: Yes, same as Newtonians.

Refractors (lens telescopes): No. Refractor lens elements are factory-set and do not require user collimation.

Schmidt-Cassegrain (SCT) and Maksutov-Cassegrain: Rarely. These sealed catadioptric designs hold collimation well. Some experienced owners never collimate; some check annually.

The Practical Summary

For a Heritage 130P, Skyliner 200P, or any Newtonian/Dobsonian reflector:

Buy a combined Cheshire/sight tube (~€28) when you buy the telescope. Doing this check before every session takes 3 minutes and is the single highest-value maintenance action.
If your telescope is 150mm+ and used frequently, upgrade to the Baader Laser/Cheshire combination (~€65).
Check collimation after every transport. Check it again after the telescope reaches thermal equilibrium at the observing site (mirrors change shape slightly as they cool).

For the telescopes this process applies to, see Best Dobsonian Telescopes 2026. For the complete upgrade picture, see Best Telescope Upgrades and Accessories 2026.

Some links on this page are affiliate links. If you purchase through them, Orion News earns a small commission at no extra cost to you.