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View from a Scope | July 25, 2021

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How to collimate a telescope

How to collimate a telescope

By collimating your reflector you are fine tuning your viewing experience. The process of collimation involves lining up all the mirrors of your telescope so that the reflected light is directed directly up the focus tube to your eye


When I first bought my own telescope a Skywatcher 150p, I had read a lot about the need for these to collimated and that this procedure is difficult and time consuming. I then came across these items on ebay, a laser collimator (£20), and I thought to myself brilliant this will put all my worries to rest, I mean whats more accurate than a laser.

My Mistake

I had used this collimator since day one, until I posted a question on stargazers lounge as to why my laser pointer would move as i rotated the collimator in the focus tube. At this point the guys there informed me that my laser collimator was probably not collimated.

The realisation dawned on me that my telescope was never collimated and that I was probably making things worse by chasing this laser dot around the mirror.

So take note that cheap collimators from ebay are probably not the best tool to use for collimating. There are more expensive laser collimators that will work perfectly but for now i decided to do it the manual way. Don’t be scared doing this, but also if it is your first time don’t start this before a viewing session. Give yourself a couple of hours, or in my case a weekend. Once you do it once it will all make sense.

What do you need?

To do a complete manual collimation you will need the following.

While researching how to do a manual collimation I came across a number of sites recommending the following guide. Astro Babys Guide to Collimation

This is indeed an excellent guide and the one that I found made most sense to me. So I wont be making any claims of doing anything drastically different, I will just highlight some of the things I found helped me.

Step 1. Focus Tube and Secondary Mirror Alignment

The first step is to get the secondary mirror lined up properly. You do this by placing the collimating cap into the focuser and looking through the small hole. We are trying to get the secondary mirror to appear as a circle in the centre of the view through the collimating cap.

A secondary mirror that is well aligned will look like this. (Excuse the flash from the camera in the reflection).


The first time I looked through the focuser at directly at the secondary mirror I was lost with all the reflections going on.

Astrobaby suggests putting card behind the mirror and in front of the primary mirror to rule out these reflections, this helps a lot. I still had to from time to time wave my hand behind the secondary mirror to get my orientation right, be careful not to touch the glass with your fingers, It is a pain to wipe clean.

Here is the image above labeled.


A secondary mirror that is out of alignment will look oval or will not be centered in the focuser tube view. To fix that we need to adjust the tilt and the height of the mirror.

This is done with the screws and on top of the secondary mirror holder.

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An important note here is that when you are working on the secondary mirror ensure that your telescope is in the horizontal position, You don’t want tools or screws or worse the secondary mirror itself falling onto your primary.

When adjusting the secondary mirror, you use the middle screw to help centre the mirror in the focuser circle, Tightening and loosening the screw allows you to move the mirror up or down.

Once the mirror is centered in your view, you use the allen keys to adjust the tilt to get that circle shape.

One thing I found that helped was to loosen the allen screws and the centre screw so that the mirror is very loose (always keep a good hold of it so that it does not fall). Then while looking down the tube move the mirror to a position that looks right.

Then very carefully tighten the centre screw and allen keys, You will never be able to hold the mirror in the exact right place while tightening the screws, but it will allow you to get close enough so you can fine tune it.

Be sure not to over tighten any of the screws, finger tight is good enough. Once you are happy you have the secondary in the middle of the focuser tube it is time to move onto the next step.

Remember we are using the collimation cap for these steps, the cheshire will be used in step 3.

Step 2. Secondary and Primary Mirror Alignment

Something to to note here is that while doing step 2 you may upset some of your settings in step 1. Don’t worry about this you just need to go alternate between step 1 and step 2 until you get both right.

OK the goal of step 2 is to line the secondary and primary mirror up. You do this by looking through the collimation cap, your are trying to get the 3 clips of the primary mirror into view on the secondary.

So your view through the collimation cap should look something like this.



Notice you can see the 3 clips that hold the primary mirror around the edge.

Your goal is to get a similar view through the collimation cap.

You can adjust the view (the tilt of the secondary) by using the 3 allen screws mentioned above.


As I mentioned before doing this may knock out what you have done in step 1. So when you can see the clips, go back to step 1 and ensure that the secondary mirror still appears as a circle and is centered in the focuser tube.

Patience is the key skill here, This part almost drove me insane, I went back to 7 or 8 times over two days but out of no-where it came together and the good news is that once done you shouldn’t really have to do it again, unless the telescope gets a big bang (pun intended) or you remove the mirror for cleaning.

One tip I will give you though is that small movements is all that is necessary on the allen screws. So take it slow and keep checking your progress and if you can see the clips creeping into position try not to be tempted to go for broke and make one big adjustment. Keep it small steps and you will get it right.

Step 3. Align the Primary Mirror

As a reward for getting through the pain of aligning the secondary mirror, you get to align the primary mirror, which thankfully is an awful lot easier.

To align the primary mirror you need to use your Cheshire, so insert this into the eyepiece holder.

Now angle the Cheshire so the silver reflective bit (which is at a slant) is directed towards a light source.

When you look through the Cheshire you will see a number of elements,



You will also see a small circle on the primary mirror.

The goal here is to use the adjustment screws behind the primary mirror to align the crosshairs and primary mirror circle.

You may notice as in the image above that the secondary mirror is offset a bit, this is normal in fast scopes. My own one here is an F5 scope.

Once you have everything lined up you can do a final check with the with the collimation cap, replace the cheshire with the cap and look through it again.

You should see the primary mirror circle and in the centre of that a black dot, the black dot is the hole you are looking through on the cap. this should be in the centre of the circle.

I had to alternate between the cheshire and collimation cap a few times until i was happy that both were lined up.

Star Test

Obviously the ultimate test is a live test, This will confirm how good or bad your collimation is.

To star test you will need a bright star and very good seeing conditions. you will need to have your telescope cooled down as well.

1. Put a high magnification eyepiece into your focus tube. I use a 6mm eyepiece for this

2. Centre the star in the eyepiece and focus. It should be a small point of light when focused correctly.

3. Once focused, defocus very slightly. You should see an airey disk. If this airey disk is a perfect circle you have good collimation.

Below is a very helpful table of common conditions you may see when doing your star test. this is taken directly from Astrobaby’s collimation guide. Which I can recommend highly enough.



I hope this guide help when you are manually collimating. The first time it did i thought it was the worst thing in the world, but once i understood how the mirrors tilted and moved it became a lot easier.

I am now relatively happy with my collimation, but would like to get a proper laser collimator someday to confirm my settings.

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