How to collimate your telescope

Follow our beginners’ guide to collimation and make sure your reflecting telescope is tuned for the task.

Written by Pete Lawrence.

Skill level: beginner

A
a
-
Image Credit: 
Paul Wootton

 


For optimum optical performance, it’s very important that a telescope’s optical components are properly aligned to one another – a process called optical collimation. It’s also a task that should be carried out on all new telescopes. In fact, a telescope with a small focal ratio like f/4 requires more accurate collimation than one with a large ratio such as f/10.

Collimation is one of those procedures that shouldn’t cause any problems, even for a beginner. Do take care with the procedure, though, and be sure to refer to the manufacturer’s instructions so that you don’t damage an expensive telescope.

In this section we show you how to collimate the two most common designs of telescope – Newtonian and Schmidt-Cassegrain. The procedure is different for each, so to be of assistance we’ve included two separate three-step tutorials.


What you’ll need to get started

Adhesive paper reinforcing rings

These paper items of stationery, normally used to reinforce loose-leaf page binder holes, are ideal for marking the centre of a Newtonian’s primary mirror. When placing one on the surface of the mirror, be careful not to touch the mirror with your hands or to drop the ring as you move it to its final position.

Allen key/collimation knobs

If you are collimating an SCT, adjustment is made via the secondary mirror mounted on the back of the corrector plate. Many popular SCTs require you to adjust the supplied hex bolts with an Allen key. Collimation knobs are a simple replacement to the hex bolts that allow you to adjust the secondary’s alignment by hand, without the need for the key.

Cheshire eyepiece

These can be home-built or purchased. In practice, this is one of the most useful tools you can get for your reflector and will make the job of collimation much easier.

Laser collimator

Laser precision makes collimation much easier for beginners. It’s an easy way of bringing your telescope up to peak performance as quickly as possible. However, if you’re a beginner we don’t advise using one for short-focus Newtonian telescopes as the process can be fiddly.


Before you can successfully collimate a Newtonian, you need to mark the centre of its primary mirror. To do this, remove the mirror from the main tube – leaving it in its cell – and record which fixing holes line up with each other. This allows you to replace each item in the same way that it came out.

Cut a piece of paper into a circle that’s the same diameter as the mirror and make a small hole in its centre. Carefully lay the circle over the mirror and make a small dot through the hole with a felt pen. Then stick a marker, such as a reinforcing ring or a small (4mm diameter) circle of black adhesive tape, onto the centre point.

Then carefully place the adhesive marker centrally over the pen mark so that it sticks to the mirror’s surface. The mark will not affect the performance of the mirror because it lies in the shadow of the secondary. Now replace the mirror back into the main tube


Axes aligned

When collimating a Newtonian, all the optical axes need to be aligned. To do this, point the centre of the focuser at the centre of the primary, then adjust the primary so that its centre points at the centre of the focuser. The process can be carried out under daylight conditions.

Roughly align the secondary so that the reflection of the primary mirror looks approximately centred through the focuser. Use the adjustment screws on the secondary holder for this purpose. With small one-eighth turns on one screw, slowly adjust the position. If the screws are not spring-tensioned, carry out the opposite adjustment on the other two screws to hold everything together. Avoid over-tightening the screws.

Adjust the secondary so that the centre of the focuser points at the mark in the centre of the primary. A Cheshire eyepiece or laser collimator will certainly help make this step more straightforward – simply make sure that the centre marker is on the crosshairs of the Cheshire eyepiece or is illuminated by the laser collimator’s light spot.

The final step involves pointing the centre of the primary at the centre of the focuser by adjusting the screws at the rear of the primary mirror cell. With a Cheshire eyepiece or laser collimator in place, rotate one screw by one-eighth of a turn, keeping a mental note of which screw it is. Your goal here is to make the reflection of the crosshairs line up with the real crosshairs or, if using a laser collimator, to make the return spot hit the centre of the collimator’s target area.

A second pair of hands is a real help with large scopes as it means one person can adjust while the other monitors. The screws on the back of the primary cell are normally spring-loaded, so you can adjust one screw and leave the others alone. If you run out of adjustment turns, release all the screws by a number of turns and restart the collimation process.


The Schmidt-Cassegrain

The process is slightly different for Schmidt-Cassegrain telescopes and relies on adjusting the secondary mirror fixed to the inside of the corrector plate. An Allen key is normally used to turn the hex bolts that adjust the position of the secondary. Be careful not to over-tighten these bolts, or let them become so loose that the secondary detaches from the corrector plate. A set of collimation screws makes adjustments more straightforward – especially in the dark.

Schmidt-Cassegrain collimation is best done using a moderately bright star after letting your telescope cool down to outside temperatures. In fact, it’s a good idea to choose a night when the stars aren’t twinkling too much.

First centre the star with a 200x eyepiece. Then defocus it slightly so that you can see a small set of rings around a central hole. If the rings appear bunched on one side, move the telescope so that the bunched edge is closest to the edge of the field of view. Then carefully adjust the collimation screws one at a time, so that the star moves back into the centre of the field of view. If the rings continue to appear offset, repeat the process.

Once this procedure is complete, swap the 200x eyepiece for one with a 600x power and then repeat the entire procedure.


Step-by-step guide to collimating a Newtonian telescope

Step 1 – Look through the focus tube and centre the secondary mirror in the view. Use the three adjusting screws to roughly align the secondary mirror so that the centre spot of the primary mirror is more or less in the centre of the view.

 

Now adjust the secondary mirror so that it appears circular when viewed. Don’t pay any attention to the myriad reflections you can see at this point.

 

 

 

Step 2 – Adjust the secondary mirror’s position so that the centre spot on the primary mirror appears central on the crosshairs of a Cheshire eyepiece. If you have a laser collimator, ensure the laser spot is on the primary mirror’s centre marker. Ignore the reflection of the secondary in the primary – this gets corrected in step 3. Steps 1 and 2 only need to be carried out infrequently.

 

 

 

Step 3 – Adjust the position of the primary mirror, using the screws at the rear of the cell, so that the reflections are aligned. The best tool for this job is a Cheshire eyepiece. A laser collimator also delivers good results: adjust it so that the return spot is centred on the collimator’s target. It’s a good idea to carry out this stage at regular intervals to get the most out of your telescope.

 

 


Step-by-step guide to collimating a Schmidt-Cassegrain telescope

 

Step 1 – To collimate an SCT, use a 200x eyepiece and point it at a moderately bright star; work direct, without the use of a diagonal. Defocus slightly so that you can see the star’s diffraction rings surrounding a central hole.

 

If the rings appear bunched at one side, move the telescope slightly so that the bunched area is closest to the edge of the field of view.

 

 

 

 

Step 2 – Adjust the secondary adjustment screws, one at a time, by a small amount (one-eighth of a turn) and attempt to re-centre the star. Once this has been done, look at the ring pattern again. If it’s still bunched to one side, repeat from step 1. If you have collimated properly, you should see a set of concentric rings when the star is centred in the field of view.

 

 

 

 

 

Step 3 – Finally, once the 200x defocused star rings look concentric, increase the magnification to 600x and repeat the process from step 1. With a higher magnification in place, be aware that a small adjustment can move the star image a long way. An SCT should ideally be collimated at the start of each observing session – the process takes no more than a couple of minutes.

 

 

 

 

This article first appeared in the September 2005 issue of Sky at Night Magazine
Like this article? Why not:
How to locate Barnard's Star
previous feature Article
How to find an exoplanet
next feature Article
We use cookies to improve your experience of our website. Cookies perform functions like recognising you each time you visit and delivering advertising messages that are relevant to you. Read more here