How to Collimate a Telescope – Your Complete Guide to Better Focus

Learning that you need to collimate the mirrors of a reflector telescope can come as a shock if you’ve never owned on before.

However, if your night sky views weren’t what they once were, or you can’t get your telescope to focus, then it’s probably time for you to collimate it.

Don’t worry, collimation is a perfectly normal activity for all owners of reflecting telescopes and, with our guide, is simple to achieve. Keep reading to find out more.

What is Collimation?

Collimation is an odd word that simply means to align the mirrors in your telescope in such a way that the light they collect is perfectly focussed.

All reflector telescopes, whether Newtonian and Dobsonian, work by collecting light with a primary mirror at one end of the telescope tube. This is shown as the large blue area on the right of the diagram below.

How reflector telescopes work
Visualisation of how reflector telescopes work (source)

The primary mirror reflects and focuses light towards the secondary mirror, which is mounted near the opening of the telescope and shown as the small blue rectangle on left of picture. This secondary mirror diverts light rays by 90° sending them to an eyepiece mounted on the side of the telescope, shown as a red line and cross, above.

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To achieve peak performance with your reflector, the primary and secondary mirrors must perfectly align with each other, and the secondary mirror must be perfectly aligned with the eyepiece.

Over time, with movement and use, your telescope’s mirrors lose their alignment. Collimation is the process we follow to put the mirrors back into their ‘sweet spot’ for crisp images of celestial objects.

Collimating a Reflector Telescope in Four Simple Steps

If you’ve never done this before, don’t worry!! We’ve created an easy-to-follow video guide and broken it down into a few simple steps which you can follow along with below.

The only tools you’ll need are a laser collimator, a hex key (allen key in the UK) to adjust your secondary mirror, and a phillips screwdriver. You’ll learn more about laser collimators below, but click here to grab one on Amazon if you don’t yet own one.

Laser collimator on a table
Laser collimator

Step 1: Check for Rough Alignment of Mirrors

The first step is to see that your mirror is not so far out of alignment that you have more drastic action to take.

Take a look down the focuser with no eyepiece in place. You should be able to see the 3 retaining clips holding the primary mirror.

Primary mirror seen through draw tube with retaining clips at 12, 4, and 8 o'clock positions
Primary mirror with retaining clips at 12, 4, and 8 o’clock positions

If you do, move on to the second step. If not, turn your secondary mirror until you do, you may need to loosen the locking screw with a phillips screwdriver to achieve this. The slope of the secondary mirror should face the focusing tube.

Step 2: Aligning Your Secondary and Primary Mirrors

Our first ‘proper’ step (as you should never normally need to make adjustments at step one) is to fine-tune the secondary mirror so that it aligns with the primary.

To do this, we’ll first install our laser collimator. This goes into the draw tube where you’d normally fit an eyepiece. Make sure the target on the collimator is facing the back of the telescope, i.e. the end with the primary mirror.

Lock the collimator firmly in place – we want it rigid and square and not moving about whilst collimation is in progress.

Adjusting screws of a secondary mirror in reflecting telescope
Secondary mirror – three adjusting hex screws and locking phillips screw

Switch the laser on. Looking down the open aperture into the telescope you’ll see a laser dot on the surface of the primary mirror. What we need to do at this stage is move that dot so it’s in the exact center of the mirror.

Many mirrors (like the one in the video) already have there center marked with a ring. If not, you will need to do this yourself with this guide.

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Use your hex key to adjust any two of the three screws on the secondary mirror and move the laser. You may need to undo the cross-head screw a little to allow you to do this. Ideally, carry this step out with the telescope laying on a flat surface so you don’t drop the hex key down the tube and onto the primary.

As you turn the adjusting screws, you’ll notice the laser dot move quite violently. The best way to make progress is to make small adjustment, pause to check where you’re at, before making the next turn.

When you’re happy that the laser is centered, tighten the locking screw again with the philips screwdriver.

Laser dot from collimator centered on primary mirror in telescope
Laser dot centered on primary mirror

Step 3: Collimating the Primary Mirror

Our penultimate step is to adjust the primary mirror. We do this using the adjusting screws on the bottom of the telescope.

Adjusting screws for a primary mirror of a reflector telescope
Adjusting screws for primary mirror (largest) and smaller locking screws

This is where it becomes important that the target in the laser collimator is facing the primary mirror. At this stage we’re trying to move the laser dot to the center of the target and we can only do that standing at the end of the telescope with the adjusting screws.

If you can already see the laser on the target, then you don’t have much work to do. If there is no red dot visible then it will take more trial and error to get aligned – but stick with it!

Before making adjustments, loosen the locking screws holding the primary mirror in place. Now, slowly, move one of the adjusting screws to see where it moves the lazer in your target. If it’s going towards the centre of your target, stick with it. If not, try another adjusting screw.

You’ve successfully collimated your primary mirror when you’ve moved the laser dot in the target’s center. At this point, tighten the locking screws again and make your final checks.

Laser dot almost centered in laser collimator
Laser dot almost centered in laser collimator

Step 4: Final Checks

As you tighten the locking screws, you may find that the laser pops back out of the target’s centre. If that happens, all you need do is some very small tweaks to the adjusting screws to finish the collimation off.

Make one last check inside the scope to see that the laser dot is in the center of your primary mirror still. If it is, you have a perfectly collimated scope and can look forward to stunning detail in tonight’s sky!

Telescope Collimation Video

We recorded our own guide to laser collimation to make it easier to follow along with the steps above:

Summary

There are just four simple steps to collimating your reflector telescope:

  • Check you have good approximate alignment of mirrors
  • Collimate the secondary by moving the laser dot to the center of the primary
  • Collimate the primary by centering the laser dot in the collimator target
  • Make final checks and tweaks

It is easier to collimate your scope regularly because it only needs small tweaks. If you’ve left it a long time, or never done it before, it will probably take a bit longer to put right.

Whatever your situation, you need not be worried about collimating any longer.