The basic functions required to collimate a newtonian telescope.
You can can collimate other scopes but the process is different. A refractor for example should never need taking apart and I would only every suggest a professional does this as it can ruin a perfectly good telescope.
The newtonian telescope has two mirrors, the primary and secondary. The primary is the large mirror at the bottom of the telescope and the secondary is the smaller one at the front. The secondary is also positioned level with the focuser.
The light comes into the opening at the top and is reflected off the primary mirror and due the curve of the mirror the light is then bounced into the secondary mirror that is set and a 45 degree angle, this then sends the light down into the focuser and into the viewers eye.
In order to get this light properly focused into the eye we need to ensure the mirrors are properly aligned.
Here you can see an the effects of a bad collimation on a out of focused star. The top star is out of collimation and the star will appear distorted but the bottom star is correctly collimated and will be nice and clear when focused correctly.
I feel that before you make any changed to your telescope:
- Star test your telescope first and make sure it need adjustment. Do not assume it needs adjusting
- The secondary rarely needs adjustment so check it again!!.
- Make sure you have all the tools needed are at hand.
- Give yourself enough time as it can be time consuming and take you time.
- Screws and bolts need to be tight and firm. You are not changing a car wheel.
- Always work with the telescope horizontal. You do not want to drop anything down the tube and damage your mirror. You can if wanted wear gloves to prevent any oils from you hands being transferred onto your mirrors.
Rule 1 – Before starting collimation don’t assume that anything is out of kilter and check each element before adjusting anything. About 90% of collimation errors are down solely to the primary mirror so don’t jump in and start meddling with the secondary unless you are sure that there is a problem. Each stage of this guide will show you how to check the various elements – only if they are out of alignment will you need to make adjustments.
Rule 2 – Make sure you have all of the tools you require ready to hand. Some telescopes will require Allen keys and a screwdriver. You don’t want to be looking for tools when you halfway through collimation.
Rule 3 – Make sure you have budgeted for enough time. Collimation can be a very time intensive process for a beginner. It can be carried out over successive sessions if you need to but don’t plan on collimating on a night when you want to be observing. It will lead to a rush job and frustration – TAKE YOUR TIME and you will be rewarded with good collimation and better viewing through your telescope.
Rule 4 – Do not over-tighten nuts and adjusters. They need to be firm and tight – they DON’Tneed to be tightened until the threads pop.
Rule 5 – Whenever you are working on a telescope ALWAYS work with the telescope tube in a horizontal position. If you drop things into the tube you don’t want them to impact on the primary mirror. I also wear cotton gloves when I am working to make sure I don’t damage anything with sweat from my hands.
Star Test.
This is very simple. Get a star in the middle of you focuser and move the focuser inwards and out wards. This will defocus the star and will start to show the dark area of the secondary mirror. We want this central in the start. If you are only observing and this looks okay then leave it.
You will also need to the spacing of the spider veins that hold the secondary mirror in the middle of the scope. Just using a ruler to check that the length of all the veins are the same. If not you will usually have a thumbscrew or similar on the end of each vein and you just need to slacken/loosen the opposite screws the get it all central.
Collimation Cap & Cheshire Collimation
We now need to check and adjust if needed is the secondary mirror. Place your collimation cap into the focuser and check the view down the focuser will show several reflections as shown below.
Collimation cap
To make things easier you should place some paper down the tube and bend it under the secondary mirror. This will assist you when looking down the focuser. Using the paper will block all these reflection and just show you the focus tube and secondary. You can then loosen the middle screw on the secondary mirror which will allow you to rotate the mirror around. The mirror needs to be central to the focus tube. You will see the round focus tube edge and you need to ensure the circle of the secondary is central to this. If the mirror needs moving upwards or downwards this is adjusted by winding the central screw in or out. I recommend you loosen up the three adjustment screw and then once correct take up the slack by tightening up these three screws
Tip: Most secondary mirrors will either have cross thread or hex bolt screws. These can be replaced with thumb screws readily available from ebay to make adjustment easier.
To make sure your secondary is correct you need to use a collimation cap and wind your focuser fully out. Your view should look like the image below.
Now the secondary mirror is central we need to align it with the primary. Using the three screws at the back of the secondary. We are trying to get the reflection of the primary central in the secondary mirror. This is normally made easier by using the mirror clips around the primary as the image below shows.
The primary adjustment
Now that the difficult part has been complete we finally come to the primary mirror adjustment. At the rear of the telescope you will normally find three pairs of bolts placed at 66 °. One will be the locking bolt and the the larger one is usually the adjustment bolt. You may also have a circular doughnut sticker in the centre of the primary mirror.
We now need to remove the collimation cap and replace it with the Cheshire collimator.
As this is a manual tool it is by far the most reliable tool to use. As with the collimation cap you use the aperture at the end of the collimator and turn the opening on the side towards a good light source. The view will be as below and it is now a simple process of moving the centre of the mirror (doughnut sticker) into the middle of the cross hairs of the collimator by adjustment screws in or out. If you have a long focal length telescope you may need an assistant to move these screws as needed.
Lazer Collimator
Whilst using a lazer can make things easier you need to ensure the lazer is also collimated so that the lazer is being correctly projected onto the mirrors. The lazer projects it's beam down to the primary and back up to the collimator where it is projected onto a disc. We then just view the primary mirror from the top of the telescope and adjust the three secondary screws to get the lazer dot into the middle of the doughnut sticker on the primary. Once done we then need to adjust the primary mirror to get the lazer dot that is projected onto the target of the collimation tool in the centre. This can make the job easier if you have no help with longer focal length scope.
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