Aligning Telescope Optics

Good collimation - step 3
[title type=”modern-h2″ align=”center”]Aligning Telescope Optics[/title]

Its crucial to the quality of the view seen through a telescope to have the optics inside accurately aligned. The process of aligning them is called collimation and is a task that with a little practice you will be performing with ease. Unfortunately though, it is something that you have to repeat over and over again, particularly if you move the telescope around to different observing sites as its very easy to jolt them back out of alignment.

Its much easier to collimate a reflecting telescope than a refracting telescope, the latter should really only be done if you are experienced. Best advice for this is to go along to your local astronomical society or astronomy shop to get assistance. In this section, I describe how you can collimate the optics in a standard reflecting telescope.

Inside a typical reflecting telescope are two mirrors, the big circular ‘primary’ mirror that you can see at the bottom when you look down the tube and the much smaller ‘secondary’ mirror which is up at the open end of the tube, near the eyepiece holder. Both of these mirrors are adjusted in the act of collimation.

Rough collimation is easiest done in daylight and you will find another pair of hands will help make the task easier. So to begin, set the telescope up in daylight, you don’t even need to be outside. Now look through the eypeiece holder (without an eyepiece in place), directly in front of you, you will see the secondary mirror. Using the adjustment screws on the mirror holder (accessed from the open end of the main tube) adjust the mirror so it appears central when you look through the eyepiece tube, like the second image below.

Good collimation - step 1

The next task is to look at the reflection in the secondary mirror and you will see in it, a reflection of the main primary mirror. There are usually three screws on the back of the secondary mirror itself which adjust its tilt. Now, looking through the eypiece holder again, adjust these screws until you can see all of the primary mirror, centered in the reflection of the secondary mirror, just as it appears in the second image below.

Now this next bit is where the other pair of hands come in. Your willing helper will need to be down the bottom end of the tube, behind the primary mirror. If you look at its mount (or cell) you will see three (you may find more on larger mirrors but this is quite uncommon) adjustment screws or maybe wingnuts. These adjust the tilt of the primary mirror. Looking through the eypiece tube, look at the reflection in the primary mirror and you can see in it, another reflection of the secondary mirror (often a primary mirror will have a tiny dot painted in its centre to help alignment) Using the adjustment screws, you will see that gently turning them will move the image around the primary mirror. The first few times, its a little trial and error but adjust the screws until you have the reflection of the secondary mirror in the centre of the primary mirror just like it appears in the second image. If you look carefully, you may even see a reflection of your eye looking back at you right in the centre.

Poor collimation - step 3Good collimation - step 3

That completes the rough collimation. To be sure you are getting the best out of your telescope, you can fine tune the collimation by taking it outside at night. Let the telescope cool down for a while (this will remove currents of warm air in the tube trying to get out into the cool of the night), make sure the atmosphere is nice and stable and the stars aren’t twinkling lots and point your telescope at a reasonably bright star. With a medium power eyepiece, perhaps around 100x focus on the star and see if its a nice crisp pinpoint. If its not, then you may need to tweak the collimation a little more.

While pointing at the star, de-focus it a little and you will see a series of concentric rings. The object is to adjust the screws on the back of the primary mirror cell again so that the circles are concentric and not off centred. The first image shows an example of what an out of focus star image might look like if the collimation is off and the second image is what you are aiming for.

Poor collimation - star imageGood collimation - star image

If you see anything other than a circular shape once this is complete then you may well have other problems with your telescope. The mirror may be slightly misshapen or it may be slightly pinched in its mounting causing it to deform slightly. If this is the case, and you don’t get a nice circular image, its worth seeking assistance, the best place to start would be your local astronomical society.

Collimation Tools

There are a couple of tools you can buy to help you collimate. The first is what’s called a Cheshire Eyepiece and it fits neatly into the eyepiece holder of your telescope. It has a set of wires fixed inside and an angled mirror making the alignment process much quicker and easier.

Cheshire eyepiece

A slightly more expensive accessory is the laser collimator which also fit into the eyepiece holder. It sends a laser beam down the tube of the telescope and produces easy to see red dots of light which show up on the mirror surfaces. In a similar process to that outlined above its quick and easy to adjust the mirror orientation until the beam bounces down the telescope optics and back up into the laser device.

Laser Collimator

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