How To Collimate a Dobsonian Telescope (diagrams included)

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If you’ve got a Dobsonian, at some stage you’ll need to collimate it. How do you do that? It may seem overwhelming. To help you with that, the following takes you through the steps on how to collimate a Dobsonian telescope. Once you get the hang of it, it’s a painless process…

how to collimate a Dobsonian telescope

A Dobsonian is a reflector telescope, it uses mirrors, a primary and a secondary, and these need to work together. Hence, the reason for collimation.

I cover the features that make a Dobsonian special in my article I wrote about the advantages and disadvantages of a Dobsonian telescope.

Here, I aim to help you with this seemingly tricky maintenance routine of collimation, so as to keep your Dob giving you the satisfaction you desired when you purchased it.

Instructions that come with the telescopes generally summarize how to collimate a Dobsonian, but these are often not well written or hard to understand.

As well, some people prefer written step-by-step instructions that they can read on their mobile device or print out so they can easily refer to it, rather than having to access a video each time.

How to collimate a Dobsonian telescope – in a nutshell

I give full details below, but as a heads up, here are the collimation steps in summation:

  1. Test the telescope using the out-of-focus star method. If it is off-center, it needs collimation. Note the side closest to the center circle of light – you will need to adjust the mirrors to bring it back to the center.
  2. Get your collimation cap (DIY or bought) or laser.
  3. Take off the large lens cap on the front of the telescope.
  4. Using the collimation cap in place of an eyepiece look through the focuser for the 3 clips of the larger primary mirror at the back.
  5. Align the secondary mirror, alternating adjustment of the screws on top of the mirror holder until you see the 3 clips.
  6. Check out the larger primary mirror at the back. Align this using its 3 adjusting screws at the base.
  7. Test again using the out-of-focus star method to see if the collimation is now correct.
  8. If not, tweak it by repeating the process.

How do I know if I need to collimate my telescope?

To know whether or not your telescope needs collimating, there is a simple test using out-of-focus star images, which is also referred to as the star test for telescopes.

Star testing a Dobsonian for collimation

There are 3 steps to the method of using out-of-focus star images…

  1. On a dark night, position your telescope so that a bright star is in the center of the field of view.
  2. Now, adjust the focuser so what you see is slightly out of focus. You should see a central circle of light (the Airy disc) with many diffraction (Fresnel) rings around it.
  3. Check to see if the rings are symmetrical around this central circle of light. If not, e.g. they lean to one side, your telescope needs collimation.
how to collimate a Dobsonian, start with the out-of-focus star test

Magnification for telescope star testing

6-inch Dob150x magnification (max 300x)
8-inch Dob200x magnification (max 400x)
10-inch Dob250x magnification (max 500x)

You might find that the best magnification for star testing, at least, to begin with, is 25x for each inch of aperture. As examples, for a 6-inch, the magnification would be 6 x 25 (150x) and for an 8-inch Dobsonian, 8 x 25 (200x).

You could then fine-tune up to 50x per inch (300x for 6-inch and 400x for 8-inch — about the maximum limit), where viewing conditions permit.

What is the primary mirror of a telescope vs the secondary?

Reflectors, like the Dobsonian, use two mirrors. The secondary mirror is the smaller oval-shaped one at the top of the tube that is angled toward the focuser. The primary mirror is the larger mirror at the back of the tube. It captures the light and sends it back to the angled secondary mirror at the front that then reflects it through the focuser.

This all makes sense, but let’s face it, not all telescopes are the same and it can be confusing when you’re new to using astronomy telescopes.

It is good to know which is which and to get familiar with your telescope design early in the piece to avoid confusion and possible errors when trying to collimate.

Adjusting the mirrors for alignment

The collimation described here uses a collimation cap and starts with the secondary mirror.

Secondary mirror at front

Have a look at the secondary mirror. It is held in a support with a central bolt behind it. This larger screw moves the mirror up and down.

Around the central bolt are three screws. These screws adjust the mirror’s angle – normally 45º towards the focuser. You use these alternately to adjust and align the secondary mirror with the primary mirror.

Screws to adjust – secondary mirror

How to align the secondary mirror

First, point the telescope at the wall that’s well lit.

Using the collimation cap in place of an eyepiece, look through the focuser for the 3 clips of the larger primary mirror, inside, at the back of the tube.

Primary mirror clips that are inside the tube at the base

Turn the focus knob until you see the base of the tube, i.e. the focuser is not in the reflected image.

While disregarding the reflected image of the collimation cap (or your eye), look for the 3 clips.

If you can’t see the three clips, alternate the adjustment of the three screws on top of the secondary mirror holder (see image above) until you do. When you loosen one, tighten the others to compensate for the slack.

Once successful, tighten all three screws to ensure the secondary mirror is secure.

Primary mirror at back

Now check out the primary mirror. It is held in place by screws that are spaced apart (about 120º apart for a 6-inch Dob) at the back of the tube. It is these that you’ll adjust for the primary mirror.

Screws at the back to adjust the primary mirror

How to align the primary mirror

You may need someone to help with the primary mirror, depending on the size of your telescope.

First, loosen the three adjustment screws just a little (a few turns). If you’re not sure which ones are the adjustment screws, see the image above.

Now while looking through the collimation cap in the focuser, run your hand around the front of the telescope. The aim here to see which way the primary mirror is out of alignment using your hand’s reflected image.

Stop your hand at the point where the image reflected in the secondary mirror is closest to the edge of the tube (also the edge of the primary mirror).

Look at the back of the telescope and see if there is an adjusting screw corresponding to that point. If so, loosen it. If not, tighten the adjusting screw on the opposite side.

Whichever screw is applicable, adjust it gradually until the primary mirror is aligned, i.e. the image of your hand reflected in the secondary mirror is in the center of the view.

This is where a friend could help. Your friend could do the adjustment while you observe how well it’s aligning in the focuser.

Retest the collimation

With good observation conditions, take the telescope outside and with an eyepiece, perform the star test (see instructions above). If necessary tweak the alignment of the mirrors using the star as the center.

Why do you need to collimate a telescope?

I have covered collimation in regard to SCT telescopes, which have a mirror and a lens. Dobsonians have only mirrors.

Collimating a Dobsonian involves aligning the mirrors so they work together to focus light to the eyepiece properly.

You’re going to get better views and hence better satisfaction with using your Dob.

Even the Hubble Space Telescope gets collimated.

Info sources

Collimating the Hubble Space Telescope

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