How Do I Know If My Telescope Needs Collimation?

You’re so excited to use your new telescope.

You take it to your favorite stargazing site and point it to a patch of sky where you intend to search for the Andromeda galaxy. But then you can’t see anything more than blurriness, no matter what you try to focus on with your scope!

Does blurriness mean that you need to collimate your telescope?

If what you can see through your telescope is very blurry, this is indeed a sign that you need to collimate – or, in other words, align – your telescope.

Collimation can be defined as a process in which you align all your telescope’s components.

This is important to enable the telescope to collect more light so that you can see objects clearly. With that in mind, let’s explore how to collimate your telescope so you can get back out there and star-search successfully.


How To Tell If Your Telescope Needs To Be Collimated

While blurry images are sometimes a hint that something is “off” with your telescope, there is a test you can do to find out if your telescope needs to be collimated. To do it, you’ll need a dark sky, a bright star in it, and your telescope!

  • With your telescope, focus on a star that’s very bright, such as Sirius.
  • Try to center the star in the field of view. Focus on it and zoom in as much as you can.
  • Then, carefully start defocusing the star. You want to see a diffraction pattern of concentric circles appear around it. 
  • Basically, this refers to circles around the star that might look a little wiggly. If the circles you see are not concentric, then your telescope needs to be collimated.

Easy, huh?

Extra Notes When Doing This Test

  • Make sure you do this on a clear night when there’s no haziness in the air, as that could interfere with your image.
  • Use an eyepiece (or one that’s used with a Barlow lens), so you can increase the magnification of the star to around 200x or more.
  • If you notice what can be described as bleeding edges around the star, that’s a sign your telescope needs to cool down, so you’ll have to wait for it to climatize outside before proceeding. Remember that temperature changes can affect how your telescope functions!
  • When you look at the star, you should see circular rings around it when it’s out of focus, but you should also see a bright center when it’s in focus.  

The above test is known as the “star test,” and it’s completely tool-free, which is a bonus. Although it’s accurate, it can be a little tricky for beginners to master right away, but you’ll get the hang of it! 

How To Collimate Your Reflector Telescope

If you have a reflector telescope, which makes use of two mirrors in its design instead of lenses, then you can try this quick way to collimate it.

  • First, you’ll need to locate the secondary mirror that’s mounted on the front of the telescope tube. You’ll see that it’s held in place by an object such as a glass pane. You’ll also notice adjustment screws on it.
  • Find them, and then gently – and without touching the mirror – try to tighten or loosen the secondary mirror where the screws are placed. This will enable the circles in the diffraction pattern to move a bit so that the circles become more concentric. It’s like you’re pulling and pushing them – but really gently. 
  • Now, obviously, while doing this, you will need to ensure that you have a clear view of the pattern. You could use a device that connects to your telescope so you can see the image clearly, or ask a friend to help you out.
  • When you start tugging at the screws, you should concentrate on one instead of tugging them all as that can be confusing. You need to use very gentle and subtle movements because tiny changes will have an impact on the telescope’s mirrors.
  • Pay attention to what screw you moved and how much you moved it, as well as in what direction, so you prevent further confusion.  
  • When you can see that the diffraction pattern becomes more out of focus, you will have to move the screw back to the location it was in originally and try to move it in the opposite direction to see if that fixes things. Or, try a different screw. On the other hand, if the circles look better, keep moving that same screw to continue improving it.

Equipment You Need To Collimate Your Telescope

Equipment You Need To Collimate Your Telescope

There are different ways in which you can collimate your telescope, so let’s check out some popular methods.

Collimation Cap Method

A collimation cap is otherwise known as a sight tube, and it’s basically like a plug that you put in the focuser of your reflector telescope.

It can be used to check that your telescope’s secondary mirror is aligned correctly with your focuser, but you can also use it to ensure that both mirrors are aligned with each other. Bonus.

To collimate your telescope with a collimation cap, you will first need to put it in the focuser and view the primary and secondary mirrors.

If the telescope is collimated, everything will be lined up well. If they seem out of sync, then you’re going to have to proceed with the next steps to fix the collimation issue.  

  • Take a look into the cap and you’ll notice its underside is reflected in the telescope’s primary mirror. You’ll also see that there’s a reflection of the hole in the cap that you’re currently looking through – it looks like a black dot on the mirror’s surface, right?
  • You want to adjust your primary mirror. You will do this by turning the collimation knobs (they’re located at the scope’s back end) until you can see that the dot is in the middle of the mirror.
  • It’s pretty simple to do, but take your time. When you move a knob and see that it centers the dot a bit more, continue. If you see that it brings it more out of sync, stop and turn the knob in the opposite direction or try a different knob.
  • Once you’re done with that, you need to align the secondary mirror to ensure that it will bring all the primary mirror’s light to the telescope’s eyepiece in an effective way. Look in the collimation cap at the outer circumference of the primary mirror as well as the outer edge of the secondary one. It’s common to find that the secondary mirror has many adjustment screws, which can feel stressful, but you don’t have to move them a lot.
  • A tape measure should be used to measure the distance between the top of the tube and the focuser’s center. This should be the same as the distance from the top of the tube to the middle of the secondary mirror (via Gary Seronik). Adjust the screws back and forth if the measurements are out of whack – you can do this by loosening the big nut or screw.
  • Then, adjust the tilt so that the primary mirror’s outer edge and the outer edge of the secondary mirror are concentric. You might need a hex wrench or screwdriver for this.
  • You should work with the focuser racked all the way in. When you see the primary mirror’s outer edge almost touching the outer edge of the secondary mirror, then you know you’ve struck gold.


Can You Make Your Own Collimation Cap?

You certainly can! Here’s how.

What you’ll need

  • An eyepiece cap or film canister – just cut off the end of the canister
  • Small drill bit
  • Paper
  • Pencil


  • Take your eyepiece cap or film canister and drill a hole through it. This hole has to be done exactly in the middle of the item.
  • To ensure that the hole will be exactly in the center before you go ahead and drill it, take a piece of paper and trace the eyepiece cap or canister on the paper. Cut it out. Fold the paper in four symmetrical squares, then unfold it. The spot where the lines intersect is where the middle is.
  • Then, with some tape, fasten the paper onto the cap (or canister cap). Make sure that it’s all even and lined up, then you can go ahead and drill a hole through the middle of it.

How To Collimate A Refractor Telescope

Is your refractor telescope feeling out of whack? Here’s what you need to do to collimate it. 

  • First things first, find the three mounting screws that are on the objective lens on the front of the telescope tube.
  • Remove the lens shade from the front of the tube and you’ll see that next to those screws is a collimation screw that looks like an Allen screw.
  • You have to loosen the mounting screws while this Allen screw is moved in or out. You should only do one or two full turns with the mounting screws.
  • Then, point the telescope at a bright star in the sky.
  • Put the star in your field of view and use a high-power eyepiece that’s between 4mm and 6mm in order to focus on it.
  • You should ensure that the star is centered properly in the field of view.
  • Now, look at the diffraction pattern. As with our previous instructions, if it’s out of focus, you need to collimate it.
  • Loosen the mounting screws about one turn and move the Allen screw to see if the changes do anything. If not, try another set of screws. If it makes a difference, continue until you see that you’ve perfected the view.
  • When you’re done, make sure you find the star back in the center of the field of view to check that it’s perfectly centered and symmetrical this time. 

Other Tools You Could Use To Collimate Your Telescope

Other Tools You Could Use To Collimate Your Telescope

Cheshire Eyepiece

A Cheshire is an eyepiece that you can use to collimate your telescope. It’s basically a sight tube that has a hole in the top (through which you look) and a surface that’s tilted to 45 degrees that’s aimed at a hole in the side of the telescope.

You can find some Cheshire eyepieces that have cross-hairs at the bottom of the tube to help you align the secondary mirror of a reflector telescope. It’s very easy to use and accurate.

Here’s how to use it:

  • Put the Cheshire in the focuser while looking at the reflection of its face in the primary mirror.
  • Turn the primary mirror’s adjustment screws so you can move the reflection all the way until it is centered in the middle of the primary mirror.
  • Since most mirrors will have three adjustment screws or pairs of pull-push adjustments, try to use only two of them. Move slowly and only use the third screw if the other two didn’t help you collimate the telescope.
  • Take your time and move slowly.

Note: the Cheshire eyepiece won’t always be exactly centered in the shadow of the secondary mirror, but this isn’t a problem because the secondary mirror is slightly out of sync.

Laser Collimator

This telescope collimation method is really quick to do, but you’ll need a laser collimator. This is put into the telescope’s focuser and it produces a beam. If the beam doesn’t hit the secondary mirror, that’s a sign the telescope is out of sync and needs to be collimated. You’ll have to point the beam at a wall so that you can make adjustments to it.

Here’s how to do it:

  • First things first, you should put the laser collimator in the eyepiece holder of the telescope. Make sure that it is pointed towards the back of the telescope.
  • Aim it at a wall and turn on the laser collimator. You should not see a red dot on the wall otherwise, that shows you that the collimation is seriously off. If you don’t see a dot on the wall from the laser, this also means you can safely look into the optical tube in order to collimate the telescope, so don’t skip this step!
  • Now you can go ahead and adjust the telescope’s secondary mirror. You’ll need an Allen wrench for this one. Look into the optical tube with the laser on and you’ll see where the laser hits the mirror. You’ll also see another dot in the center which is from the primary mirror.
  • Carefully loosen or tighten the tiny adjustment screws. When you loosen or tighten them, this will push the secondary mirror of the telescope and bounce the laser into a different direction. You need to adjust these screws so that the red dot of the laser will be inside the small circle on the primary mirror.
  • Don’t tighten the screws too much – this can cause the secondary mirror holder to break!
  • Once you’re satisfied, you want to adjust the primary mirror on the back of the telescope.
  • Loosen its thin and long lock screws so that you can turn the three adjustment knobs.
  • These work in the same way as the screws for the secondary mirror do. Turn the knobs until you can see that the laser has slowly moved to the center of the bullseye on the target. One of the benefits of using a laser collimator is that you can look at its target view when making adjustments to the screws and knobs, without having to run around to the other side of the telescope.

Related Questions

Do you need to collimate refractor telescopes?

These types of telescopes are usually permanently collimated when they’re produced at the factory so you will probably never have to collimate them. However, if you’ve bumped or dropped them hard, it’s worth checking them out to be sure. 

How often should you collimate a reflector telescope?

It’s worth bearing in mind that some telescopes, such as reflectors, will need to be collimated whenever you want to set them up, like if you’ve moved them from their stargazing location to a different one. So, you will have to check them regularly.


Telescope collimation doesn’t have to be something overwhelming that puts a black cloud over your excitement about using your telescope.

As we’ve seen in this article, it’s actually really easy to do, and there are different ways in which you can do it. Even if you own a reflector telescope that needs to be collimated regularly, it can be done simply and effectively without a hassle – and without many tools.

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