How to collimate a telescope

First you need to be sure that your telescope needs collimation by testing it. Then you have to adjust the mirrors in the correct order, working from the eyepiece end downwards.

A star test
A star test

Collimation means accurately aligning the optical components of the telescope to give the best image possible. This is usually only needed after purchase for reflecting telescopes, such as Newtonians and catadioptrics. Refractors do need to be collimated, but they have simpler designs and the collimation usually stays fixed once they are made. Because the mirrors of reflectors need to be recoated from time to time, it’s usually necessary to provide them with adjustments so they can be brought back into line after recoating.

There is no reason why a well-made reflector should not stay properly aligned for a long time, but unfortunately many commercial instruments are out of collimation even when new, and may need to be collimated to give good images.

People are often scared that they might make things worse, and are reluctant to meddle, but as even Celestron’s manual points out, the rewards of correcting bad collimation are well worth the effort. And judging by some of the questions I have been sent, collimation is easier to do than to spell! It’s colli-mation, not coll-mination.

Is it necessary? Testing your telescope

The easiest way to test your telescope for collimation and other defects is to observe a bright star. Do this outside, not through a window, and allow the telescope to cool down to night-time temperature or thermal effects will undoubtedly affect the image quality.

With an eyepiece in place, and the star central within the field of view, slowly defocus the star image. As it goes out of focus it will get larger until you see a disc of light, probably containing brighter zones.

If you are using a reflector, you will see a circular shadow within the bright disc. This is the shadow of the secondary mirror. In a well-collimated telescope, this should be dead central within the disc (but only when the star itself is centred in the field of view, remember). In extreme cases, the defocused disc is some kind of weird crescent shape, in which case the telescope is badly misaligned.

If the circular shadow is not dead central, you will need to collimate the telescope. This is possible while looking at the star, but doing this makes a difficult job much more tricky because as soon as you start to adjust the mirrors, the star moves away from the centre of the field of view. So it is best to carry out the task during daylight, or in a brightly lit area.

While you are looking at the defocused star, however, you can check other things. Is the outline of the bright disc completely even and circular? A common fault is to find that there are permanent blobs or streaks at the edge, often arranged at 120º to each other. This is a sign that the mirror is too tight in its cell. It should be held just tightly enough that it won’t move, without putting any strain on it.

You may also see other shadows. The legs of the spider assembly will also be visible, and there is not much you can do about these as they are part of the structure of the telescope. There may be a rectangular shadow jutting into the edge of the field of view. This is probably the focusing tube of the eyepiece, and is a sign that the telescope has not been well designed. For every departure from a plain, perfect disc of light, you are losing performance.

Check the shape of the disc on either side of focus. If it shows a radically different structure on either side, the mirror is probably badly made. If, as you go through the focus position the disc is slightly elongated in one direction, then the other, there is a fault known as astigmatism somewhere in the system. Neither of these faults can be corrected. Many telescopes I have seen show slight astigmatism, and as long as you can get a good star image it is not too serious. You may find that it is worse with low-power eyepieces than at higher powers.


In a nutshell, to collimate your telescope you start from the eyepiece end and work your way down to the main mirror. You first adjust the secondary mirror so that you can see the main mirror centrally within it, then adjust the main mirror so that you can see the top of the tube and the secondary centrally within that. What follows is this procedure in detail, for the faint-hearted.

Step 1 Before you start to collimate your telescope, you need one essential accessory – a dummy eyepiece of sorts, with a central hole. This could be nothing more than a piece of card with a hole punched in it, stuck over the eyepiece draw tube with the hole dead centre, or you could make something a bit more elaborate if you want. 35mm film cannisters are excellent raw materials for this. The size of the central hole is not critical. Its purpose is just to make sure you look right down the centre of the drawtube, so about 5 to 7 mm is fine.

Step 2 In conditions bright enough that you can see inside the telescope tube, look through this hole and work out what you can see. When collimating, the key thing is to adjust everything from the outside inwards. The outside edge of your view is the end of the draw tube, and beyond that is the secondary mirror. To start with, ignore anything that the secondary reflects. Its outline should be centrally placed within the outline of the eyepiece tube. If it is not, serious adjustment of the secondary position is called for.

In the case of catadioptrics, it is not usually possible to adjust the position of the secondary within the tube, as this is fixed in place. In my experience, even in a Newtonian the secondary is not often grossly misaligned, except perhaps in home-built telescopes, so I will not dwell on the possibility now. Get the hang of the other adjustments first.

Step 3 Now look at the reflection in the secondary. You should see the outline of the main mirror somewhere within the outline of the secondary. Ideally it should be central and symmetrical, but if not you will have to realign the secondary. This is possible in both Newtonians and catadioptrics.

There should be a central screw or rod that moves it in and out, and there will probably be three screws for adjusting its alignment. On a cat these screws may be hidden by a blanking plate. You may need to slacken off the central screw slightly, which also holds the mirror in position, before you can adjust the outer three. Your aim is to be able to see the outer main mirror dead centre within the outline of the secondary. Make tiny adjustments so you can see which way each screw moves the mirror, and keep adjusting the other two screws so as to keep the mirror fairly well held. If you loosen one, tighten the other two, and vice versa – you don’t want it so loose that it can slop around and ruin your adjustment.

Step 4 Once the secondary is properly adjusted you can now look even more centrally, still through your dummy eyepiece. What can you see reflected in the main mirror? Ideally, it should be the top end of the tube, with the secondary central within that. If you can see more of one end of one side of the tube than the other, with the secondary also off-centre, you now need to turn your attention to the three adjusters at the back of the main mirror.

On catadioptrics there are no such adjusters available to you, and adjusting the secondary is as far as you can go. On most Newtonians you will find three pairs of screws. One of each pair is a locking screw, while the other is the adjuster.

Slightly loosen the locking screws, and by trial and error (made much easier by a helper doing the adjustment while you look through the dummy eyepiece) adjust the other screws to get the secondary central. Again, take up the slack with the other two screws as you adjust one. Eventually you should get everything in line and you will be able to see your own eye looking up at you from the dead centre of the main mirror. Tighten the locking screws, but not to battleship standards. You don’t want to break the casting of the mirror cell.

Now everything should be lined up and tight. Check by swinging the telescope around a bit and make sure the alignment is good in all tube positions. If not, one of the mirrors could be loose in its cell which is another matter. In most cases, however, your telescope should now be properly collimated.

These instructions are rather long, to allow for most possibilities, but in practice the job is quite simple and should take only a matter of minutes with a bit of practice. The shorter the focal ratio (eg f/4), the harder it is, but the more essential.


You may find it hard to judge the centre of the main mirror when looking through the eyepiece. If you can get to the mirror there is a simple trick that might help. Get a small blob of Blu-tack, and by measurement with a ruler stick it dead centre. This sounds drastic, and you might worry about it affecting telescope performance, but the central part of the mirror should always be in the shadow of the secondary anyway and never contributes to the image.

An alternative is to stretch two pieces of thread from one side of the tube to the other, close to the main mirror, so as to define the centre of the tube where they cross.

Paul Sutherland

Paul Sutherland

I have been a professional journalist for nearly 40 years. I write regularly for science magazines including BBC Sky at Night magazine, BBC Focus, Astronomy Now and Popular Astronomy. I have also authored three books on astronomy and contributed to others.
Paul Sutherland

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Paul Sutherland

I have been a professional journalist for nearly 40 years. I write regularly for science magazines including BBC Sky at Night magazine, BBC Focus, Astronomy Now and Popular Astronomy. I have also authored three books on astronomy and contributed to others.

One thought on “How to collimate a telescope

  • 06/23/2009 at 11:39 am

    Thanks for your collimating instructions, but I'm a neophyte and I just don't "get" them.

    When looking through the fake eyepiece I made following your instructions I see the cross-hairs in my viewfinder and they seem correct. Then again, I can see them fine without the fake eyepiece too.

    As for the circle thingy it too looks good save for this dark chunk on the side which you say means the equipment is defective.

    It is a reflecting telescope on an equitorial base and has an aperture of 5.1 and a tube length of 900, if that means anything.

    I bought it from an astronomy equipment store where they sell every imaginable size and shape…from 200$ to 12,000$.

    Mine cost around $500 {not cheap for me) and comes with 2 lenses and a Barlow, to which i've added a 6mm lense and a four-filter set.

    The instruction pamphlet is useless and I can't even do the meridian flip properly. I've joined an astronomy club which might help. If not, the thing goes back from whence it came and I take up golf.

    Thanks again,

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