How to Collimate a Telescope

So, you’ve finally gotten your first telescope. The one you’ve been saving and saving for. Of course, you can’t wait to use it, but there’s one problem you hadn’t counted on when you try to look out of it. The star you focused on, the one you’ve been making all of your wishes on and want to see as up close as possible, now looks like a donut-shaped blob with a dark hole in the center.

It’s ok. Don’t rush to pack up your telescope and return it to the store. What you have is a collimation problem. A what? You need to know how to collimate a telescope. What that means is to line up the mirrors in your telescope so that they are positioned in such a way that the light they gather will be perfectly focused. The fact of the matter is that all reflector telescopes work in the same manner, by gathering light with the main mirror at one end of the telescope tube.

Here are some instructions as to how to collimate a telescope to help you out.

Collimating Your Telescope

If you want to get peak optical performance from your telescope, it’s essential that the optical components are correctly lined up to each other — which is known as optical collimation. All new telescopes need to have this done. It’s the only way to get good, clear views of the whole field out there.

Now collimating your telescope shouldn’t be any big problem, even for a beginner.

You need to be careful, however, while carrying out this series of steps, and refer to the instructions given by the manufacturer to avoid damaging your new telescope.

In this article, we are going to show you how to collimate a telescope for the two most common types of telescope — Newtonian and Schmidt-Cassegrain.

Collimating a Newtonian Reflector

You may not know this, but Newtonian telescopes are the most apt to require collimation. In fact, some telescope hobbyists actually advise checking the collimation each and every single time you use your Newtonian for observing because there is around a 50/50 chance that it will require at least a little adjusting.

What makes Newtonian telescopes different from all the other reflector telescopes out there is that you can collimate them using several different ways. Placed in the order of accuracy and complication they are as follows:

1. Employing a Cheshire collimator or collimation caps

2. Employing a laser collimator

3. Star testing

Adjusting the Mirrors

The main mirror will have 3 collimation bolts/knobs located in back of the supporting cell. Usually, a telescope will have 3 locking bolts or screws. The purpose of these is to make the collimation more stable. In reality, though this doesn’t work and they wind up being a hindrance in the collimation process. These should be removed.

Tuning the collimation bolts regulates the tilt and tip of the main mirror for alignment. This mirror sits on a series of supports which are located on a ring, which sits on springs that loosen or tighten when the collimation bolts are tuned.

Unless it receives a hard hit, the secondary mirror found in a Newtonian seldom gets out of alignment. When it does it can be fixed using 3 or 4 small Allen or Philips head screws fond recessed in the secondary mirror holder. The secondary mirror can be rotated by loosening a center screw and retightening it — but watch out not to loosen it all the way because doing so will cause your the mirror to fall onto the primary mirror and both will suffer serious damage.

Using a Cheshire or Collimation Cap

These days, the majority of reflectors have collimation caps, and if you don’t have one you can get a Cheshire is better though, but costly.

It’s a simple matter to collimate your Newtonian with a collimation cap. All you have to do is make sure the secondary mirror is aligned. Then you align the main mirror.

Next, you align the main mirror by turning the three collimation bolts until the secondary mirror is centered within it.

Laser Collimating

Lasers are wonderful for greatly increasing the rate of collimation and are easy to use as well. The collimator’s small screws must be adjusted until the beam of the laser doesn’t move.

When aligning your secondary mirror, if necessary, turn the screws until the laser beam is focused on the main mirror. Ultimately, the laser beam should perfectly hit the small hole it comes from.

Collimating on a Star

This is a basically easy, tool-free way of reinforcing what you’ve done and making sure that it turned out successfully. Aim your telescope at the North Star, or the brightest star you can find, switch over to an eyepiece with high-power and then slightly unfocus it.

If the shadow of the secondary mirror is focused on the diffraction rings of the star, that’s perfect and your collimation is right where you want it. If that’s not the case, all you have to do is turn the collimation bolts until it becomes centered — this may cause your target star to move outside your field of vision, but you’ll find that it will always move in the direction the mirror has been tilted.

Collimating a Schmidt-Cassegrain

These telescopes seldom require collimation, but you should still check every time you use them. The process with Schmidt-Cassegrains is a little more difficult than with a Newtonian, but it’s still within the ability of a beginner.

Since you can’t use a Cheshire or a laser to collimate, the only choice you have is collimating on a star.

Nearly all Schmidt-Cassegrain telescopes have a total of 3 collimation screws on the holder of their secondary mirror. Most SCTs will use Allen head/hex key screws.

Collimating an SCT on a star is very akin to doing so on a Newtonian except that different tools are needed for the adjustments. All you need to do is aim the telescope at a bright star and unfocus it. Then work the screws until it is exactly centered. Remember that even something as small as a 1/4 turn of a screw may do the job of collimating your telescope.


The above sums just about all you need to know regarding how to collimate a telescope. You can see that it’s nothing earthshattering to have to undertake. Just a little time and patience are all you need for success just like anything in astronomy.