When they are no longer exactly in line, the image goes soft: stars never become sharp points, planets stay without detail. Putting the mirrors back in line is called collimation. It sounds daunting, it takes five minutes, and the tool costs the price of a meal. This page says how to tell whether it needs doing, in what order, and what not to touch.
The short answer
- In daylight, take out the eyepiece, rack the focuser all the way in and put in the collimation cap: a cap with a small hole in the middle.
- Look through the hole. You should see circles nested inside one another, well centred: the focuser tube, the small mirror, the large mirror reflected in it, and in the middle the ring marked at the centre of the large mirror.
- If the centre ring does not surround the reflection of the hole, turn the screws at the back of the tube, the large mirror's, until it does.
- In the evening, check on a bright star slightly out of focus: the shadow of the small mirror should sit in the middle of the disc.
Why a Newtonian drifts
The large mirror sits at the bottom of the tube on three springs, set by three screws. A car journey, a tube put down a little hard, a change of temperature, and it tilts by a fraction of a millimetre. It is not a manufacturing fault, it is the design: an adjustable mirror is a mirror that can go out of adjustment. A refractor or a Maksutov, whose optics are fixed at the factory, has neither the problem nor the adjustment.
The 'faster' the telescope, meaning the smaller its focal length over its aperture, the tighter the tolerance. A 130/650 or a 150/750 (f/5) forgives less than a 114/900 (f/8). And it is precisely tabletop and ordinary Dobsonians that sit at f/5 or f/6.
How to tell whether it needs doing
Two checks, one by day, one by night. By day, the collimation cap shows in thirty seconds whether the circles are centred. By night, the star test is the only judge: it is the image you look at, not the tool.
- Wait until the telescope has cooled, about an hour: a mirror that is still warm blurs the test and looks like misalignment.
- Aim at a bright star high in the sky and centre it with the eyepiece that magnifies most.
- Turn the focuser until the star becomes a small disc of rings with a dark hole in the middle: that is the shadow of the small mirror.
- If the hole is in the middle of the disc, the collimation is good. If it sits off to one side, the large mirror needs adjusting.
Step by step, with a collimation cap
Do it in daylight or in a lit room, with the tube horizontal or tilted slightly down, so that a dropped screw does not land on the mirror. A white wall or a sheet of paper behind the small mirror helps you see the outlines.
- 1. The small (secondary) mirror, only if needed. Look through the cap: the large mirror should appear whole and centred in the small one. If an edge is cut off, adjust the three small screws on the secondary holder, a little at a time, each one a little.
- Do not touch the centre screw of the secondary holder: it holds the mirror in place, and loosening it lets the mirror turn or drop into the tube.
- 2. The large (primary) mirror. At the back of the tube there are three adjusting screws and, often, three locking screws. Loosen the locking screws slightly, then turn one adjusting screw at a time, a quarter turn, looking through the cap.
- The aim: the ring marked at the centre of the large mirror should surround the reflection of the cap's hole exactly. When it does, tighten the locking screws, without forcing, and check again.
- 3. In the evening, confirm on a star. If the shadow of the small mirror is still off-centre, a touch on the large mirror, looking in the eyepiece, finishes the job.
What about a laser collimator?
A laser collimator slides into the focuser and puts a dot on the large mirror, then its return on a target. It is quicker, especially on a large Dobsonian where you cannot look into the focuser and reach the screws at the same time. But a laser is only right if it is itself aligned, and cheap ones are not always. A laser that is out of true sets the telescope out of true, with great confidence.
- Before using it: put it in the focuser without tightening, switch it on, and rotate it. The dot on the large mirror should stay still.
- If it traces a circle, it is the laser that is off, not the telescope. It adjusts by its own screws, or goes back.
- Tightening the focuser can also tilt it: check the dot again once it is clamped.
ExperienceOpinion, not physics — shared by nearly everyone who has helped a beginner.
A cheap laser used as it comes, unchecked
If it is not aligned itself, it misaligns a telescope that was fine.
- Instead
- A collimation cap, then the star test; a laser later, checked by rotating it in the focuser.
- What changes
- The cap costs 8 to 20 €, and is often already in the box.
When to leave it alone
- When the star test is good. Perfectly concentric circles in the cap are not the goal; a sharp star is.
- When the telescope has just come out of a warm house or the seeing is poor: the blur then has nothing to do with the mirrors.
- On a refractor, a Maksutov or a smart telescope: their optics are set at the factory, and taking them apart does more harm than good. A Schmidt-Cassegrain can be collimated, but by a different method from this one.
- On a 'Bird-Jones' Newtonian, whose focuser holds a corrector lens: the adjustment means taking it out and putting it back, which is not first-night work.
- The ring marked at the centre of the large mirror: do not remove it. It sits in the shadow of the small mirror and takes nothing from the image.
What a beginner can ignore
Advanced collimation guides talk about secondary offset, focuser squaring, Cheshire eyepieces and autocollimators. All of it is true, and none of it matters for a first year of visual observing. The secondary is set at the factory and rarely moves; what drifts is the primary, and three screws put it back.
Questions people ask
How do I know if my telescope needs collimating?Look at a bright star slightly out of focus once the telescope has cooled: if the dark shadow in the middle of the disc is not centred, it needs collimating.
In daylight, a collimation cap shows the same thing in thirty seconds.
How often should a Dobsonian be collimated?Check every time you go out; it takes thirty seconds.
Adjust when it is off-centre: often after a car journey, less often if the telescope stays at home.
Do I need a laser collimator?Not to start.
The collimation cap is enough for visual observing and never goes out of adjustment. A laser is quicker, but only if it is aligned itself: rotate it in the focuser, and the dot should not move.
In what order do I adjust the mirrors?The small (secondary) mirror first, and only if the large one does not appear whole and centred in it.
Then the large (primary) mirror, by its screws at the back of the tube. In most cases, only the primary needs a touch.
Does a refractor need collimating?In practice, no: its lenses are fixed at the factory.
Nor does a Maksutov or a smart telescope. Collimating at home is for Newtonians and Dobsonians.
Can I remove the ring at the centre of the mirror?No, and it does no harm: it sits in the shadow of the small mirror, so it takes no light from the image.
It is the mark you collimate against.
Method and changes
What this page rests on and when it changed, computed from the catalogue — no date below is typed by hand. There is no price history yet, so this log says what was read and when, never whether it went up or down.
- First published
One price per model: the cheapest observed offer, with its date. Models are ordered by that price, never by what a link pays. How we choose.
Written and maintained by the person behind About.