Collimating A Newtonian Reflector Telescope – A Guide to Set-Up and Alignment

Collimation is the process of aligning your telescope's mirrors so they work with each other to provide properly focussed light to your eyepiece. It is a fairly important process for your viewing experience, as if your telescope isn't properly collimated, you will observe images that are out of focus.

Although collimation in Newtonian telescopes is NOT an absolute necessity, correct alignment will greatly improve the quality of the image you see, as well as your ability to resolve images.

Aperture size largely determines the frequency of collimation that is necessary. For example, a 10 inch Newtonian may require collimation every time it is transported, whereas an 8 inch scope will require collimation only after 3-4 times of viewing. A 2 inch scope might not need any collimation at all!

Although it's often seen as a bit of a tedious process, this guide aims to help you guys get your telescopes sorted as quickly and easily as possible.

Is my telescope properly collimated?

To test if your telescope is properly collimated, place a star in the center of the field of view, and move the focuser so the image appears slightly out of focus. You should see a central circle of light (known as the Airy disc) surrounded by several “diffraction rings”.

Reflector telescope diagram
Airy discs

In the image above, you'll see two Airy discs. If the rings are symmetrical around the Airy disc, you're good to go and the telescope is properly collimated. If it doesn't, and you need to collimate your telescope, keep reading.

You will now need a collimating tool. If you don't have one, you can easily make one out of a plastic 35mm film canister, and you should be able to find them at photo development studios, or your local grocery store. Drill or punch a small pinhole - about 2-5mm in diameter – in the exact center of the lid. Then, cut off the bottom of the canister. Refer to the image below for an example.

collimation peephole
Film canister collimation tool

This device will keep your eye centered directly on the focuser tube. If the canister is the correct diameter, it will fit nicely into the focus tube, acting as a peephole. Insert the canister in place of a regular eyepiece.

Alternatively, you can check your collimation by centering a bright star through an eyepiece with high magnification (e.g. 50x). In a misaligned scope, the star is out of focus and will appear as a doughnut shape with a dark center. If your telescope is misaligned, this dark patch will be perfectly centralised within the bright ring. This is actually the shadow cast by the secondary mirror, and you may also see the spider veins (as in the figure below). When the optics are in need of alignment the dark patch will be off center.

collimation donut
This telescope needs collimation

Collimation – it's a painless process

Film canister collimation tool
Newtonian Reflector Diagram

Remove the lens cap that covers the front of the telescope, and look down the optical tube.

At the bottom you will see the primary mirror (see figure above) held in place by 3 clips about 120° apart.

At the top, you will the small oval secondary mirror, that's held in a support and tilted 45° toward the focuser, outside the tube wall.

To collimate, you will need to go through 3 steps in progressive order. Firstly, align the secondary mirror. Secondly, align the primary/main mirror only after the secondary mirror is aligned. Finally, fine tune these adjustments to ensure perfect collimation.

Aligning the Secondary Mirror

At this point, you're trying to ensure that the secondary mirror is perfectly centered underneath the focus tube. Ignore the reflected image on the secondary mirror for know, and concentrate singularly on ensuring that the secondary mirror is physically centered within the focus tube.

To do this:

1.Point the telescope at a lit wall, and insert the collimation cap (film canister) into the focuser in place of an eyepiece. Look through the collimating cap, and twist the focus knob until the reflected image of the focuser is out of view.

2.If you can't find the three clips that hold the primary mirror in place, adjust the 3 bolts on top of the secondary mirror holder. You might need to use an Allen key/wrench or Phillip's screwdriver. Alternate by loosening one clip and tightening the other two progressively, and stop when you see all 3 mnirror clips. Make sure that all 3 small alignment screws are tightened to secure the secondary mirror in its position.

Collimation alignment
All three clips should be visible when correct.

collimation allignment
Primary clips when poorly aligned

Although the mirrors are held in place to the telescope by both clips and screws, bumps and knocks will affect alignment of the telescope, and you will have to re-collimate from time to time. For example, if you bring your reflector or Dobsonian telescope out on a drive, chances are you will have to re-collimate one you set it up again. Shifts from warm to cool temperatures will also lead to a misalignment, due to the expansion and contraction of glass and metal.

Although the majority of telescopes leave manufacturers reasonably collimated, delivery to you may cause misalignment.

Aligning the primary/main mirror

The 3 locking screws at the back of your telescope
The 3 locking screws at the back of your telescope

1.Now, you will have to find the 3 locking screws at the back of your telescope (directly behind the secondary mirror) and loosen them by a few turns.

2.Run your hand run the front of the telescope, while you look down the peephole and in to the focuser. You will see the reflected image of your hand.

3.Tilt the secondary mirror until you are able to see the entire primary/main mirror in the secondary mirror (you can use the image of your hand to guide you). Now, go back and make sure the secondary mirror is still centered beneath the focus tube.

4.If it isn't, adjust the spider cane screws and then check to see the primary/main mirror is centered on the secondary mirror. This is a bit of a tedious process, but keep going until everyone looks perfect.

5.Once that's done, adjust the main mirror. This is the most important step, with the greatest bearing on image quality. The aim here is to point the center of the primary mirror at the center of the secondary mirror.

6.To do this, you will have to mark the exact center of the primary mirror, usually done with a sticky-ring enforcer for paper, although many telescopes come with a central marker from the factory. Alternatively, you can use a marker to dot the center of the mirror – don't worry, this won't affect your viewing experience at all as the center of the mirror isn't used for viewing.

7.Once the primary mirror is aligned to the center of the secondary mirror, take a flashlight and shine it over the peephole. You will now be able to tell if the reflected light is on the center of the primary mirror. The likelihood will be that it's not, and will require some adjustment.

8.Your goal should be to adjust the set screws until the spot of light is perfectly centered within the peephole. IT helps to get a friend to help you – have them adjust the adjusting screws according to your directions when you look through the focuser. Congratulations, you've now collimated your Newtonian reflector!

Fine Tuning

At this point your telescope should be collimated. The ultimate test is to point the telescope at a distant star – Polaris, the North Star, is a good one. Ensure that the star is perfectly centered in the eyepiece and then rack the focuser in and out of focus. When the star is out of focus you should be able to see the shadow of the secondary assembly and the spider vanes. The shadow of the secondary assembly should be centered within the light circle created by the star. If it's not, you will need to adjust the main mirror set screws in the direction of the dissymmetry. The adjustments needed at this point will be very minute so be careful not to over do them.