Collimating a Refractor Telescope – A Guide to Set-Up and Alignment

(with Adjustable Objective-Lens Cell)


Collimation – in refractor telescopes – is the process of aligning the lenses of your telescope. This ensures that the light that the lenses collect are able to focus on the right spot on the back of your telescope. This ensures that your eyepieces are able to function correctly.

Collimation is a simple process, despite its reputation as being tedious. Here's how to get your refractor's collimation sorted:

Remove the dew cap on the front of your telescope – then look through the scope. You'll see your lenses held in a threaded cell. The cell will be held in place by 3 pairs (6 individual) screws spaced 120 degrees apart (Fig. a).

There will be two kinds of screws. The first are larger Phillip head screws (cross-head), and they hold both ends of the cell to each other.

The smaller, buried Allen screws push against a ledge within the tube, allowing it to tilt slightly against the tension provided by the Phillip's screws.

So how do I know if I need to collimate my telescope?

The easiest way to collimate your refractor telescope uses Polaris (Fig. b) – the North Star, and an eyepiece.

The North Star is the outermost star in the handle of the Little Dipper, located in the constellation Ursa Minor.

In order to collimate your telescope, you will have to alternately loosen and tighten each set of screws until the image you see through the telescope is properly centred.

With this method, ensure that your telescope isn't polar aligned – in fact, if possible, try to intentionally point your telescope mount's head due east or west.

Now then, here's the process.

Firstly, use the lowest powered eyepiece you own – it will be the one with the largest number (or longest focal length) in mm. Use this eyepiece to acquire – or spot – Polaris, and place Polarins in the center of your eyepiece view.

Secondly, switch to an eyepiece of a higher power, and keep the image centered.

If your telescope is in focus, the star image you will see – known as an Airy disc - will have a bright point in its center, and fainter rings that move outwards in sequence. Refer to the image (Fig. c.) for examples of images through a collimated and off-collimated image. As you will see, an off-collimated image will have its brightest point off center.

So, I'm going to need to collimate my refractor. How do I do this?

If you find that your telescope requires collimation, start by taking out your star diagonal and look at your off-focus image. This will allow you to gauge the deflection of the lenses.

You will now have to loosen the pair of screws – slightly – on the side of the cell you spotted the deflection through the star diagonal.

To do this, you will have to:

Firstly, slacken the Allen head screws., then tighten the Phillip's head screws against them once more.

Secondly, re-check the image through your star diagonal, and move the image to the centre of any eyepiece you own.

Lastly, If you find that the image you see is even more off-center (i.e. it seems worse), repeat step one, but in the opposite direction. Alternatively, slacken the other two Allen screws slightly.

You will now have to keep repeating steps 1 – 3, until you have a perfectly collimated telescope!

Once you have a round, perfectly-centered star image (Airy Disc), you're all set!

It helps if you get a friend to help you with collimating your refractor telescope. Have your friend adjust the screws according to your directions while you look through an eyepiece.