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How To Collimate a Telescope? A Beginner’s Guide

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Written By: Zane Landers
Updated:
Category: Learn

Collimation scared me so much that I opted not to buy a Newtonian reflector for my first telescope and ended up buying a not-so-great NexStar 4SE, a Maksutov-Cassegrain type of telescope. Newtonians and Schmidt-Cassegrains are the telescopes types that are most likely to lose good collimation, as they have the most moving parts.

However, later, I realized that collimation is nothing to be frightened of and is quite an easy process even if its a Newtonian telescope. If I can assemble and set up my telescope properly, I’m also capable of collimating it correctly. It’s that simple.

Good collimation is required with any telescope or optical system to achieve good, sharp views across the entire field. Below is how I collimate my telescopes that belong to either of the 4 major telescope types: refractors, Newtonian reflectors, Schmidt-Cassegrains (SCT), or Maksutov-Cassegrains (Mak).

Believe it or not, refractors can be out of collimation occasionally. Funny enough, I’ve had it happen more often with expensive refractors than cheap ones.

An out-of-collimation refractor will usually show elongated stars like coma in a Newtonian, like in the image below.

out-of-collimation refractor
This shows what a star looks like in a refractor, starting from a perfect collimation to a heavily mis-collimated stage, both in focus (top section) and out of focus (bottom section).

Adjusting the collimation of the objective lens is usually done with either 1 or 2 sets of 3 screws at the front of the lens cell, which can be accessed by pulling off or unscrewing the dew shield. Just adjust until the star looks perfectly symmetrical, both in and out of focus. If your refractor doesn’t have any screws in its lens cell, you’re out of luck.

Newtonians are the most likely to need collimation. I recommend checking collimation every time you take your Newtonian out for observation, as there is usually a 50% chance it needs to be tweaked at the very least.

No matter what method of collimation you’re opting for, you need to know how to adjust the mirrors, both primary and secondary (if needed).

Your primary mirror will always have 3 collimation bolts/knobs at the back of its supporting cell. Most mass-manufactured telescopes also have 3 locking bolts or screws, which are, in theory, meant to keep the collimation more stable during usage.

Three collimation screws and three locking screws in the back of a dobsonian reflector's optical tube
Collimation screws (marked orange) and locking screws (marked red) in a typical newtonian reflector (this image is from one of my dobsonian reflectors)

In practice, these 3 locking screws do nothing and serve as a mere hindrance to collimation. In some cases, these could hit the back of your primary mirror if the scope is dropped and damage it as a result. I’d advise you to just remove the locking screws.

Adjusting the collimation bolts adjusts the tip and tilt of the primary mirror for alignment.

The primary rests on a series of supports on a ring or cell, which sit on springs that loosen/tighten when the collimation bolts are adjusted.

Barring a huge bump or similar, the secondary mirror in a Newtonian rarely goes very far out of alignment.

screws in secondary mirror
Screws in a typical reflector’s secondary mirror housing

You can usually adjust your secondary’s tip/tilt with 3 or 4 small Allen or Philips head screws recessed in the spider/secondary mirror holder. You can rotate the secondary mirror by loosening a center screw or nut and retightening it. Be careful not to loosen it all the way or your secondary mirror will fall onto the primary and damage both.

The Methods for Collimating Newtonian Reflectors

Newtonians are unique in that you can collimate them with several different methods.

Newtonian with a Cheshire or collimation cap

Collimation caps usually come with most reflectors nowadays, and if you lack one, you can get a film canister (rare these days), cut the bottom off, and poke a small hole in the lid just wider than your pupil to force you to center your eyeball on it.

Buying a Cheshire collimation eyepiece is probably preferable if you can afford it, though.

Collimating your Newtonian with a Cheshire or collimation cap is simple. First, make sure the secondary mirror is aligned as in the above image. Your secondary is aligned if you can see the entire primary mirror with an even ring of the tube walls around it.  Next, you must align the primary.

To align the primary, adjust the three collimation bolts until you have a view looking like this:

final collimation view

The secondary should be centered in the primary, and if you’ve done a good job aligning the primary it should appear centered in your view as well.

Lasers can dramatically speed up collimation and are very simple to use. The only catch is that on many cheap lasers, the beam itself may need collimation! You’ll need to throw together some sort of simple V-block or jig (or use your scope’s accessory tray) and adjust the laser collimator’s small Allen screws until the beam/dot doesn’t move when the laser is rotated.

To align your secondary, turn the screws (if you need to) until the laser beam is centered on the primary mirror – adding a center dot of some kind to your primary can help with this, but I don’t find it to be a necessity. Here’s what a scope with an aligned secondary and misaligned primary looks like.

telescope with aligned secondary and misaligned primary mirrors

Now here’s what a perfectly collimated scope will look like with a laser:

perfectly collimated scope

Notice how the laser dot perfectly hits the small hole it originates from.

More expensive lasers offer smaller dot sizes, reticles, holographic displays, etc. with theoretically more accuracy, but on almost all instruments I’ve found that the sub-$50 collimators work just fine once the alignment of the laser itself is tweaked.

That’s really all there is to it. Collimating with a laser is super easy.

star collimation

Collimating on a star is a relatively simple, tool-free method, but with a long scope, it’s annoying to fiddle with the screws and then comes back to the eyepiece. However, I still recommend at least checking collimation on a star to be sure you’ve done a good job. Point your scope at Polaris if you’re in the northern hemisphere, or a bright star near the pole in the southern hemisphere, switch to a high-power eyepiece and slightly defocus it.

You should see one of the images on the left.

If the “donut” (the shadow of the secondary mirror) is centered on the diffraction rings of the star, your collimation is good. If it isn’t, adjust the collimation bolts until it is centered – this may result in your target star moving outside the field of view, but it’ll always move towards the direction the mirror has been tipped – if you’ve tightened a bolt, it’ll move toward that direction, and if you’ve loosened it’ll move away from the direction of that bolt.

I recommend using #1 or #2 and at least quickly checking with method #3 once you’re observing.

Before doing anything with your collimation tool, make sure it is square with the focuser and not tilted by the thumbscrew. I do this by pressing the collimator firmly into the focuser, then quickly tightening the thumbscrew. If you have a compression ring in your focuser instead of the thumbscrews, you won’t have to worry much about this issue.

SCTs don’t need collimation very often, but it’s still worth checking every session – particularly if you plan on doing astrophotography or high-magnification planetary/double star viewing.

The collimation process with Schmidt-Cassegrains is a little more tedious compared to a Newtonian, but it’s nothing to be afraid of.

You can’t really use a laser or Cheshire to collimate a Schmidt-Cassegrain, so collimating on a star is the only real option you have.

Pretty much all Schmidt-Cassegrain telescopes have 3 collimation screws on their secondary mirror holder.

3 collimation screws on their secondary mirror holder

The scope shown here oddly has Philips head screws, but most SCTs use Allen head/hex key screws. You can buy thumbscrews (sold as “Bob’s Knobs” among others) to replace these, but I’ve found that the thumbscrews seem to work their way out of collimation more often and lead to people at star parties and outreach events ruining your collimation by fiddling with them.

star collimation

Collimating an SCT on a star is a similar experience to a Newtonian apart from the adjustments requiring tools. Just defocus a bright star and adjust the screws until the “donut” is perfectly centered:

Keep in mind that even ¼ of a turn of a screw may be all you need due to the optical nature of SCTs.

There are some odd methods involving Cheshires and laser collimators to collimate SCTs, but it’s not as simple as dropping them into the scope like with a Newtonian so I personally wouldn’t bother.

Maksutov-Cassegrains typically rarely need collimation. But if yours is out of collimation, you’re in luck.

Most Maks have small screws behind the primary mirror, which should allow you to tilt it. Apart from the collimation being done via the primary instead of the secondary mirror, the overall experience is basically like collimating a Schmidt-Cassegrain.

Zane Landers

An amateur astronomer and telescope maker from Connecticut who has been featured on TIME magazineNational GeographicLa Vanguardia, and Clarin, The Guardian, The Arizona Daily Star, and Astronomy Technology Today and had won the Stellafane 1st and 3rd place Junior Awards in the 2018 Convention. Zane has owned over 425 telescopes, of which around 400 he has actually gotten to take out under the stars. These range from the stuff we review on TelescopicWatch to homemade or antique telescopes; the oldest he has owned or worked on so far was an Emil Busch refractor made shortly before the outbreak of World War I. Many of these are telescopes that he repaired or built.

3 thoughts on “How To Collimate a Telescope? A Beginner’s Guide”

  1. Thanks for all this info. I’m a beginner with new Astro Fi 130mm scope and I have been able to collimate with its red dot finder but then they ask you to align further by clicking on 3 different stars and not only can I not work this but also I don’t understand why this must be done. The guy at the shop can’t explain this so I need help badly and hope you might explain PLEASE !!?

    Reply
    • I am sure you’ve figured this out by now, but using the red dot and aligning to 3 stars is the process for aligning the telescope to the stars for viewing. Collimation is the process of aligning the telescope’s internals with each other. Hopefully you got it sorted out! – Adam

      Reply
    • I realise this is old but it may help others. The 3 star alignment is not the mirror alignment. Collimating is alignment of the mirrors.

      3 stars alignment is so the scope computer knows where it is looking at the night sky. When used with the date, time and location the 3 star alignment resolves the scope direction to objects and allows for tilt, in case it isn’t level.

      Manual telescopes are recommended for starting your journey. That is a poor job done by the shop.

      Reply

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