How to collimate a telescope — and how to tell whether yours needs it

What collimation actually is

A Newtonian reflector does three things to light in sequence. A curved primary mirror at the bottom of the tube collects it and folds it into a cone. A small flat secondary mirror, angled at 45 degrees near the top, catches that cone before it converges and throws it sideways out of a hole in the tube. The focuser holds the eyepiece over that hole, where the cone comes to a point.

Collimation is the business of making those three components agree about where the axis is. The primary must point its cone squarely at the secondary; the secondary must be positioned and tilted so it sends the whole cone straight up the focuser drawtube; the eyepiece then sits on that same axis. When they agree, a star focuses to a tight point surrounded by a faint symmetric ring. When they do not, the cone arrives off-centre and the star grows a flare on one side — the same defect at every magnification, always pointing the same way.

The cost is resolution, and it is not small. An eight-inch mirror is capable of splitting a double star 0.57″ apart, which is the theoretical figure its aperture buys. A miscollimated eight-inch does not reach that number, because the light that should have been concentrated into the central diffraction disc has been smeared into one wing of it. You paid for aperture and are getting the performance of something smaller.

Nothing about this is delicate work. The adjustments are ordinary screws, the tolerances are visible to the naked eye through a hole, and mirrors do not go out of collimation by being looked at. The reason it acquires a reputation is that almost every explanation starts with the procedure instead of the geometry, so the reader is turning screws without knowing what the picture is supposed to look like when it is right.

Which telescopes need it, and which do not

Newtonians and Dobsonians need it. A Dobsonian is a Newtonian on a simple altazimuth base, so the optics and the collimation question are identical. Both mirrors sit in adjustable cells with springs and screws behind them, the tube is open at one end, and the whole assembly gets carried, tilted and set down. Movement is designed in, because the mirror has to be adjustable to be made at all.

Refractors essentially never need it. The objective lens is a cemented or spaced group fixed in a cell at the factory, and on most beginner refractors the cell offers nothing to adjust. If a refractor genuinely is out — usually after being dropped — that is a workshop job, not a driveway job. Every 90 mm refractor in our set falls in this category: there is nothing on them for you to collimate, and nothing you should try to.

Schmidt-Cassegrains and Maksutovs sit in between, closer to the refractor. The tube is sealed, the primary is usually not user-adjustable, and only the small secondary has adjustment screws behind the front corrector. They hold alignment for years of ordinary use and need attention mainly after rough shipping. The important practical difference is that the star test is the only way to check one — a collimation cap tells you nothing useful about an SCT.

Fast mirrors drift out of tolerance sooner

Two Newtonians of the same aperture do not need the same care. The steeper the light cone — the lower the focal ratio — the smaller the region of good focus, and the sooner a given amount of mirror tilt becomes visible in the eyepiece. This is why an f/4 imaging newt is collimated obsessively and a long f/8 is famously forgiving. The reflectors in our set, ordered by their own published optics:

The focal ratios are computed from each listing’s own published aperture and focal length rather than read off the marketing copy. The tolerance column is an ordering, not a measurement: it says which of these four will show miscollimation first, not how far out any particular example is.

The star test: how to tell if yours is out

You do not need to check collimation on a schedule. You need to be able to tell in thirty seconds whether it is worth checking, and the star test does that. It also has the advantage of testing the entire optical train in the state you actually use it — tube, mirrors, focuser, eyepiece, all at once.

1. 1Put the telescope outside and leave it. A mirror warmer than the air throws off heat currents that produce exactly the soft, unstable image people blame on collimation. Thirty minutes is a reasonable minimum; a large mirror on a cold night wants longer.
2. 2Fit your highest-power eyepiece and centre a moderately bright star. Not the brightest one you can find — glare hides the pattern. Centre it properly: an off-axis star in a fast Newtonian shows coma, which looks like miscollimation and is not.
3. 3Focus, then defocus slightly, a small fraction of a turn either side. The star opens into a small disc crossed by rings, with a dark hole in the middle where the secondary mirror's shadow falls.
4. 4Look at where that dark hole sits. Centred, with the rings evenly spaced all the way round, means collimation is fine and you should stop. Pushed to one side, with the rings crowded on one edge and spread on the other, means the mirrors disagree — and the direction of the offset is the direction of the error.
5. 5If the pattern boils and will not hold still long enough to judge, that is atmospheric seeing, not your telescope. Try again on a steadier night rather than turning screws.

“Highest-power eyepiece” is where a lot of beginners come unstuck, because the two eyepieces in the box are usually a low-power and a medium-power one. The test wants enough magnification to spread the diffraction pattern out where you can see it. On the eight-inch Dobsonian in our set — a 1200 mm focal length — a 6 mm eyepiece gives 200×, and the same eyepiece behind a 2× Barlow gives 400×, which is still inside the 406× conservative ceiling that aperture supports. That is the range the test lives in.

If you don't own a high-power eyepiece

Celestron Celestron AstroMaster Telescope Accessory Kit – 1.25" Eyepieces & Filters

Celestron's listing states this kit contains a 6mm Plössl, a 15mm Kellner, a 2× lens and three filters. The 6mm is the part that matters here: it is the cheapest route to star-test magnification on a mid-focal-length reflector. If you already own something in the 5–7mm range, buy nothing.

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$79.95 · 2,673 reviews

One honest caveat on any 6 mm Plössl, this one included: eyepieces of that design and focal length put your eye very close to the glass, which some people find uncomfortable and spectacle wearers generally cannot use at all. The listing does not publish an eye-relief figure, so we are not going to quote one.

The daylight check

There is a cruder version that needs no sky at all. Point the tube at something bright and featureless — a lit wall, or blue sky — take the eyepiece out, and look down the empty focuser tube. You will see the secondary mirror, the primary reflected in it, the little clips around the primary’s edge, and your own eye reflected back at the centre of it all. Everything in that view should be concentric. If the primary looks like it is sliding out of the frame on one side, you have found your answer without waiting for dark.

The three tools, and when each is enough

The daylight check above is imprecise for one reason: your eye is not held on the axis. Every collimation tool ever made is a solution to that single problem. They differ in how precisely they hold your line of sight, and in how much they cost.

1. A collimation cap — costs almost nothing

A cap that fits the focuser like an eyepiece, with a small hole drilled dead centre. Looking through the hole forces your eye onto the axis, which is the whole trick. The classic home-made version is a 35 mm film canister lid with a pinhole in the middle, and it works because a hole in the right place is the entire mechanism.

Check the box before buying anything: several Dobsonians ship with one. The listing for the Celestron StarSense Explorer 12″ Dobsonian, for instance, states that a collimation cap is included alongside the eyepiece and finder.

A cap is genuinely enough for a great many people. On a moderate focal ratio, used carefully in good light, it will get you close enough that a star test shows concentric rings. What it cannot do is show you the centre spot on the primary mirror clearly, which makes the last increment of precision a matter of squinting.

2. A Cheshire eyepiece — the honest sweet spot

A Cheshire is a sight tube with a polished 45-degree face and a side window that lets daylight in. That illuminated face reflects off the mirrors and shows up as a bright annulus against which the primary’s centre spot is unmistakable. Most versions combine a long sight tube with crosshairs, which is the part that sets the secondary — a job a cap does poorly and a plain laser does not do at all.

It has no batteries, nothing to align, and nothing that can be wrong with it in a way that lies to you. If you buy exactly one collimation tool and intend to own a reflector for years, this is the defensible choice.

3. A laser collimator — fast, and carrying a trap

A laser collimator drops into the focuser and fires a beam down the tube. Adjust the primary until the returning dot lands back on its own aperture, and you are done. It works in the dark, on your own, at the eyepiece end where the screws are — which for a large Dobsonian in a field is a real advantage over walking back and forth.

Here is the part the listings do not mention: a laser collimator has to be collimated itself. Its beam is only useful if it leaves perfectly parallel to the barrel that holds it. Cheap units frequently arrive with the beam off-axis, and a laser whose beam is tilted does not fail obviously. It gives you a confident, repeatable, wrong answer, and you will faithfully turn the primary’s screws until the mirror matches the tool’s error.

Testing one takes a minute. Lay the collimator in a V-shaped support — two books, a drawer runner, anything that holds it level — aim it at a wall several metres away, and slowly roll it about its own long axis. A good laser leaves the dot sitting still. A bad one sweeps the dot round in a circle, and the size of that circle is the size of the error you would otherwise have transferred to your mirror. Better units have adjustment grub screws so you can correct this; the cheapest often have none.

Two smaller caveats belong with it. A laser is only as centred as the focuser holding it, so a drawtube with any slop moves the beam every time you nip the thumbscrew. And a plain laser is a poor way to position the secondary mirror — it checks the return path, not whether the secondary is round and centred under the drawtube. The barlowed-laser method, where a Barlow between laser and focuser turns the return into a shadow of the centre spot, sidesteps beam-alignment error neatly, but it is a technique for someone who already owns both, not a reason to buy in.

Our honest ordering: buy a cap or make one, add a combination Cheshire when you want the last increment of precision, and add a laser only if you collimate in the dark, collimate often, or own a fast mirror that will not stay put. Buying a laser first is the common mistake, because it is the one that photographs well.

Doing it: secondary first, then primary

Order matters, and it is always the same. The secondary decides where the light cone is aimed; the primary is then adjusted to agree with it. Set the primary first and you will simply undo that work when you move the secondary. Work indoors in good light the first time, with the tube horizontal so nothing you drop can fall onto the mirror, and never touch a mirror surface — not to clean it, not to steady it, and never the small centre spot, which is a deliberate marking and not dirt.

The secondary

Look through the cap or sight tube at the secondary. Two separate things are being fixed here, and they use different screws. The central bolt in the middle of the secondary holder moves the mirror along the tube and lets it rotate; the three small screws around it tilt it. First, loosen the three tilt screws slightly, and use the central bolt to slide and turn the secondary until it appears as a round outline sitting centred under the focuser. Then tighten the three screws in small alternating increments to tilt the mirror until the whole primary, with its retaining clips visible all the way round, appears centred in the secondary’s reflection. Nudges here are small — an eighth of a turn is a large adjustment.

This is the step you may never have to repeat. A secondary that has not been dismantled tends to stay where it was put, which is why the routine check most people describe is really only a primary adjustment.

The primary

Behind the primary cell you will find either three large adjustment screws or, more commonly, three adjusters paired with three locking screws. Loosen the locks first if yours has them — forcing an adjuster against a tightened lock is how cells get damaged. Then, looking through the cap, turn one adjuster at a time and watch the reflection move. The target is the centre spot on the primary sitting under the crosshair, or under the hole in your cap, with everything in the view concentric. Change one screw, look, change the next: chasing two at once is how people end up an hour in. Snug the locks gently when the picture is right, then look again, because locking can move it slightly.

Then go outside and star test. The daylight adjustment gets you into the right neighbourhood; the star is the only judge of whether the light cone actually lands where the eyepiece expects it. If the defocused rings are concentric, you are finished, and the screws should not be touched again just because you are enjoying yourself.

How often this really needs doing

Much less often than the internet implies. Collimation is a popular subject to write about, which is not the same as a frequent problem to have.

A solid-tube Dobsonian that lives indoors and gets carried carefully to the same patch of garden can hold usable collimation for weeks or months. The same telescope loaded into a car boot, driven over a rough road and lifted out at a dark site should be checked when it comes out — not because the drive ruins it, but because thirty seconds with a cap costs nothing and a wasted night costs a night. Frequency is set by handling, not by time.

Collapsible tubes complicate the picture slightly, and manufacturers make a specific claim about it. Sky-Watcher’s listing for the Sky-Watcher Heritage 150 states that the collapsible tube design “retains collimation” through collapse and extension, and the same claim appears on their larger Flextube models, which use preset indentations in the struts to return the optics to the same position. That is their claim, published by them, and we have no way to confirm it. Treat it the way you would treat any manufacturer claim about mechanical repeatability: plausible, worth a check the first few times, and cheap to verify with a cap.

Schmidt-Cassegrains such as the eight-inch NexStar in our set can go years untouched. Refractors, as above, are not part of this conversation at all. And before you blame collimation for a soft image, run through the cheaper explanations first: a mirror still cooling, a night of poor seeing, dew on the secondary, or a magnification well past what the aperture will support. Those account for far more disappointing evenings than misaligned mirrors do.

We do not list a collimation tool we would stand behind

This is the part where an affiliate page normally sells you a laser. We are not going to, because we cannot do it honestly: the accessory kits we carry are eyepiece, Barlow and filter sets, and not one of them contains a collimation cap, a Cheshire or a laser. We checked each listing’s published contents rather than assuming, and inventing a recommendation to fill the gap would be worse than admitting it.

So the recommendation for the tool itself points away from us. Make the cap — a film canister lid and a pin genuinely does the job — or buy a combination Cheshire from an astronomy specialist rather than a general marketplace, where you can see the construction and ask whether a laser ships collimated and adjustable. Cloudy Nights will tell you which current models hold their alignment, which is exactly the kind of question our method cannot answer. They pay us nothing.

What we can point you at with a clear conscience is the eyepiece, because the star test is useless without magnification and the eyepieces in most boxes do not reach it. That is a real gap in a beginner’s kit, it is one we can verify from the published contents of the listing, and it improves every night of observing rather than only the five minutes you spend behind the tube with a screwdriver.

Where this leaves you

The one accessory this page will actually recommend

• Contains a 6mm Plössl — the star test needs high power, and most bundled eyepieces stop well short of it
• Also holds a 2× lens and Moon and colour filters, which are useful on the nights when nothing needs collimating
• On a 1200mm reflector the 6mm works out at 200×, comfortably inside the 406× ceiling that aperture supports
• Not the right answer if you already own an eyepiece in the 5–7mm range — in that case you need nothing from this page

Contents as published by Celestron on the listing. We have not observed with this kit, and the eye relief of any 6mm Plössl is short enough that spectacle wearers should think twice.

Celestron

Celestron AstroMaster Telescope Accessory Kit – 1.25" Eyepieces & Filters

$79.95

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What we can tell you, and what we cannot

There is no observatory behind this page. Nobody here has collimated the telescopes named on it, and there is no reviewer with an invented biography attached. What that leaves is work you can check: the optical figures come from each listing’s own specification table, never from a product title, and the magnification and resolution numbers are computed from those specifications with the formulae this site publishes.

What that method cannot judge is how a particular laser collimator behaves after a year in a cold bag, whether one brand’s secondary holder creeps, or which supplier is currently shipping units with a beam that will not adjust. Those are questions for people with the hardware in their hands. The geometry, though, does not change with the season, and the geometry is the part most explanations leave out.

If you are choosing the telescope itself rather than maintaining one, our aperture-first list of the best telescopes covers which designs put your money into the mirror. Reflectors take collimation as part of the bargain, and it is a small part.