How to Collimate a Telescope

In order for your telescope to work to its maximum accuracy and efficiency, you will have to give some attention to its optics. One of the most important steps (if not the most important step) you will have to take is collimating your mirrors. It seems like a complicated, confusing and difficult process from the distance, but once you understand what the purpose of it is, you will have no trouble performing this procedure. Here is a step by step guide to help you through it.

Why Is Collimation Important?

To understand why collimating your telescope is not only important but necessary too, you need to know how its optics work. Reflector telescopes use two mirrors – a primary one that collects light before reflecting and directing it to a focal point; and a secondary mirror that feeds the gathered light to the eyepiece, producing the images you observe.

The problem starts when the components of your optics shift out of alignment, and that is something that is inevitable in reflectors. When this occurs, be it from mishandling your telescope, neglect, overuse or even something as simple as transporting or moving your telescope, the optics will not line up and your observations will become blurry, inaccurate or impossible to fine tune for clarity.

Collimation is the process of resetting your optics (both mirrors and your focuser) so that they are clear, precise and neatly aligned.

Do Refractors Need Collimation?

Refractor telescopes do not use mirrors, they work with two convex lenses. These lenses still have to be properly aligned, for the same reason that reflectors do, but the difference is that lenses are typically set in place by the manufacturers permanently and rarely shift. This does not mean that a refractor will never fall victim to misaligned optics, so you may have to collimate at exceptional times. It is more likely that your refractor will need collimating because of severe mishandling rather than being a normal and expected part of maintenance, as it is with reflectors.

How Often Should You Collimate Your Telescope?

This depends on you and your telescope. There are varying opinions on how regularly reflectors need collimation.

Most astronomers agree that in the case of standard reflectors and larger telescopes you should check or test your collimation every time you use your telescope. This is just routine though and does not suggest that collimation is necessary with every use. Other astronomers go by feeling or observation. Regular maintenance is recommended regardless of collimation, so there are some who choose to collimate routinely, for example, every three or so months.

There are two rules of thumb when it comes to collimation, though.

The first is that if you ever move or transport your telescope – especially if you have traveled far or along particularly bumpy roads – you should collimate. Motion is no good for your telescope’s optics and it is highly likely that they will shift position after a journey. By the same token, if you ever bump, or worse – drop, your telescope, you will have to collimate to make sure that your device is not damaged.

The other rule is to go by what you see. If you can tell that there is something off with your telescope’s picture – say it is stopped focusing as it used to, it’s picture is blurry, or there are other distortions in the image – it is a clear cue to collimate.

How to Tell if Your Telescope Needs Collimation

Sometimes distortions in viewing are not a problem with the telescope’s hardware. Atmospheric fluctuations, like temperature, can cause stars to twinkle, and your telescope may produce blurry or unfocused images because of this. There is a simple way to test whether or not maintenance is necessary if you notice that your telescope is not focusing but you are unsure if its due to conditions or collimation.

It is called the star test, and it is quick and easy to do no matter where you are. One opinion is that it is more efficient than using collimation tools, because it pinpoints the problem based on what you use your telescope for, and is therefore more realistic.

  • Choose a bright star (preferably the brightest one in your field of view). The higher it is, the less it will be affected by atmospheric conditions.
  • Aim your telescope at the star and make sure that it is centered in your view.
  • Tune your telescope so that the star is in-focus and note the pattern of the rings that form.
  • Now, tune your telescope so that it is out of focus. The rings should be identical in both settings. If not, it implies aberration.
  • When your star is in focus, the rings should be perfectly concentric. If they group to either side, or appear lopsided, your telescope needs collimation.

How to Collimate a Reflector Telescope

Now that you have determined that there is a problem with the alignment of your optics, you will have to correct it. It seems like an intimidating process, but so long as you understand the desired result, you can choose the method that is most convenient for you.

Use a Cheshire Collimator

Cheshire eyepieces (or Cheshire collimators) are thought to be the best way to collimate a telescope. The tool itself is a peephole that you insert into your focuser, rather than an eyepiece. Cheshire collimators do not contain lenses or mirrors but have crosshairs in place. When light falls on the black build of the Cheshire tool, your telescope’s mirrors reflect and therefore magnify the crosshairs through the peephole or eyepiece. To use one to collimate a telescope, this is what you have to do:

  • Adjust your secondary mirror so that the central spot of your primary mirror is centered in your view. You might notice reflections, but you can ignore those for now.
  • Now, adjust your secondary mirror so that the center mark hits the point where the Cheshire collimator’s crosshairs intersect.
  • Once the center mark is perfectly centered and aligned with the crosshairs, adjust, or fine-tune, as necessary, to make sure that the reflections you noticed in the first step line up neatly and do not cause interferences.

Use a Collimation Cap

If you are lucky, your telescope will have come with a collimation cap made to fit it. If not, they are affordable and relatively easy to find and make a big difference in collimating your gear.

  • To best use a collimation cap, you will need markings on your primary mirror. Most telescopes come with this pre-set, but if not, you are going to have to mark the center of your primary mirror yourself which can be tricky and inaccurate.
  • The first step, of course, is to install your collimator cap. In most cases, this is done by inserting it into the rear of your telescope.
  • Looking through your telescope’s eyepiece, you will want to adjust your primary mirror so that the mark and the ring of your collimator cap are centered perfectly in your view. Try to get the marker inside the ring.
  • Now, align your secondary mirror so that the primary is centered. This might require some fiddling because you can adjust your secondary mirror by rotating, tilting it, or moving it back and forth. Play around to determine what the problem is, and adjust it accordingly.
  • Recheck your primary mirror, and repeat the process if it has fallen out of the center. There may be a little back and forth needed in adjusting your mirrors, but in no time they will align and your telescope will be collimated.

Use a Laser Collimator

Laser collimators are handheld tools that you can attach to your telescope to perfectly center your device’s optics. They are highly effective but can be somewhat tricky to put into practice, as even the slightest wobble can cancel out your collimation. Still, they are handy and well worth the effort. You may just need a little bit of practice.

  • You may need a hand when it comes to laser collimating. One person will operate the laser and inspect the eyepieces and optics, while the other adjusts the mirrors. One person can achieve this, but it will take extra time.
  • Your laser and your telescopes tube have to be perfectly still, and perfectly straight. Wobbling and shaking will interfere with the accuracy of this process. Prop both up in such a way that looking through your telescope will not cause unnecessary movement.
  • Shine your laser into the tube of your telescope. It will shine directly into the center.
  • As with a collimation cap, you will have to play around with both your primary and second mirror to line them up and bring them into focus without distortions or interferences.
  • Adjust your mirrors so that the dot of the laser is centered to the primary mirror’s marker. You will know that your telescope is aligned when the marker perfectly fits the laser inside it, in the center of your field of view.

How to Collimate Without Tools

Though using tools makes collimation all the more easy, it can be done without them. Note that there are a few cons to this, such as a higher risk of inaccuracy in your guesswork, as well as the need to look at stars which might be obscured or distorted by the same atmospheric factors mentioned earlier. That said, this is a great way to touch up your collimation, or for emergency collimation if you are out in the field and do not have access to your tools.

  • Select a star that is around magnitude 2 (Polaris is a good choice) and center it in your telescope.
  • Make sure that your eyepiece is powerful enough and will give you 25x magnification per inch of aperture.
  • Using the star test, tune your telescope so that it is out of focus and the rings around the star are visible. You will notice that there is a dark gap right in the middle of these rings. If your telescope needs collimating this dot will be off center.
  • Next, you are going to have to adjust your telescope’s aim so that the black dot is centered and the rings are concentric. Do not worry if the star itself is not centered in your field of view in this step. What is important is that the rings and dot are concentric.
  • Now it is time to collimate your mirrors. Without adjusting the focus, shift your mirrors so that the now-concentric star is once again in the center of your view. As with the other methods, you are going to have to methodically switch between your primary and secondary mirrors to achieve this.
  • Finally, lower the magnification of your eyepiece and slowly bring the star back into focus. The black dot should not shift, and neither should the concentric rings.

How to Collimate a Schmidt-Cassegrain Telescope

The above methods are for reflector telescopes because they are not built for the easiest collimation. In the case of Schmidt-Cassegrain models, all the collimation tools you will need will come built-in to the telescope. Still, there is a trick to it.

  • As with Newtonians, you must do the star test first to be sure that your telescope needs collimating in the first place. The only difference is that rather than looking for concentric rings, you will notice a donut shape. The dark circle is the shadow of your secondary mirror. If it is off center, you will have to collimate.
  • Generally speaking, your Schmidt-Cassegrain will have specific collimation screws attached to its secondary mirror. Collimation is as easy as adjusting these screws until the dark circle is centered to your star.
  • You can simplify this, by reaching around your telescope and bringing your own hand (or finger) into view. Move your hand to either side of the misaligned donut, and then check which screw is closest to, or directly opposite, it.
  • Adjust the screw so that the mirror’s shadow becomes centered. You may notice that it brings the star out of focus (or sharpens it), so you might have to play around to find the perfect setting.
  • The result should be a concentric donut shape, with your secondary mirror’s shadow right in the middle.

How to Collimate a Refractor

Finally, although refractors should just about never need collimation you might find yourself having to correct the alignment of the lenses in rare circumstances. Honestly, it might be simpler to take it to a professional, as unlike the methods for Schmidt-Cassegrains and Newtonians, collimating a refractor is quite a complicated process. For this reason, if you ever find that your refractor must be collimated, I strongly recommend taking it to a professional rather than attempting it yourself, but here are some general instructions for the sake of knowing.

  • Firstly, you will have to learn which screws in your telescope adjust which components. Remove the cap from your telescope and look inside it. You will notice that your lenses in a threaded cell. You should see three pairs of screws. The larger screws hold this cell in place, while the two smaller ones adjust the position of the cell, allowing it to tilt. You will have to adjust these screws, so make sure you have the appropriate tools to do so.
  • Now, you are going to have to do a star test. However, your telescope should not be polar aligned. It helps to deliberately aim your telescope’s head due west or east.
  • Using your eyepiece with the highest focal length, center Polaris in your view. Then, replace your eyepiece with one that has a lower focal length and make sure that Polaris is still centered. If it is not, it is likely that you will have to collimate.
  • If it is centered, pay attention to the image that you see. The star will have a bright point with sequential rings extended outward from it. As with the star test for other telescope makes, if the bright spot is off-center, your lenses need aligning.
  • To fix the problem, remove your diagonal and look at the out of focus image. This will help you determine which direction you need to collimate your lenses to.
  • Now you have to adjust the screws. Be gentle and turn them slowly and methodically. First, tighten (or loosen) the smaller screws, and then adjust the larger ones accordingly.
  • Check the image through your diagonal in-between to see if the adjustments have brought it closer to being centered. You may notice that your adjustments have thrown your lenses off even more. If so, you will have to adjust the screws in the opposite direction.
  • Repeat this process as many times as necessary, until you have a perfectly centered, concentric image through your diagonal. One that is in order, you can adjust your focus for clarity.


Telescope collimation is a necessary (if not slightly tedious) procedure that will improve your stargazing significantly. The important thing to remember is that no two telescopes are the same so you will have to collimate yours according to its type, optics, and even model.

Some telescopes come with collimating caps to make your job easier, while some, like the Schmidt-Cassegrain variety do not require tools at all. Refractors are typically the most difficult to accurately collimate, but to compensate for this they rarely need collimation.

In any case, if you do not trust your own ability, or do not feel like doing it yourself, you can take your telescope in for tuning, repairs, and alignment. Just keep in mind that Newtonians/ Dobsonians require frequent collimation, so it is more efficient to learn how to do it yourself.

There are tools available to help you out, like Cheshire collimators, laser collimators, and the aforementioned collimation caps. If you are willing to put some extra time into it, you could always collimate without spending extra on these tools.

Ultimately, it does not matter how you choose to collimate your telescope, so long as you do. It is essential maintenance that will remove interferences, distortions, and inaccuracies from your observations.