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Collimation of an SC telescope

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Collimation of a SCT.

The collimation or alignment of a telescope is very important if you are going to get the best from your equipment, at first it may well look very difficult and many can be worried about touching anything which is normally no bad thing.

Basically all SC telescopes have three screw adjusters located on the outer surface of the secondary mirror which sits on the other side of a housing in the middle of the collector plate. Some people fit a system known as Bob’s Knobs which is just an extension replacement screw system that makes the job easier. Most adjustment is done with the use of an Allen key and some with Philips type screw heads, if this is the case great care must be taken when performing the task. The good thing about this type of scope is that they hold their collimation very well indeed so this will only need to be done occasionally. It is a good idea to perform this with the use of a torch until such time you are comfortable with the job, night vision is not important to carry this out and it is always better to be safe than sorry.

Choice of star.

I always select a star that is not too high up in the sky, select something of at least 2nd magnitude that is about 45 degrees above the horizon, this will make it easier to view the star and to easily adjust the screws. On larger scopes you will always find your arms are just a few inches too short making it essential to make journeys to the front of the scope.

First steps.

You have selected a star and placed it in the center of the field of view, on driven scopes there is no problem of the motors holding this position. I do not know of an un-driven SC telescope but if there is one out there then select Polaris as all stars appear to revolve around it. I use an eyepiece giving a power of between X80-X100, a bit little more or less will not matter. I also collimate without the diagonal fitted but some people do it with it in place, the choice is yours

Once the star in the centre then de-focus it so that you see a large starlight circle with a blacker circle somewhere within, this is the shadow of the secondary mirror assembly, if it appears in the centre then the scope is fine. I always go for a fairly large star circle as I find it easier.

Let’s say the scope needs adjustment.

Whilst looking at the defocused star take note of where the shadow is offset within the light circle, now place your finger on one of the screws at the front if you can and see where the shadow falls, be careful not to touch the glass collector plate, you will see you fingers shadow on the light circle. You need to locate the screw that needs to be adjusted first, move the finger around until you see the shadow at the thinnest part of the off centered circle, once you have located this you know the first screw to adjust. Using the drive motors at a very slow slewing speed move the de-focused star to the edge of the field, this needs to be in the same direction that the darker shadow of the mirror is off-set within the brighter star circle.

Careful place your Allen key or screwdriver in the selected screw head and turn it a little, then check the image, if it has disappeared from view you have moved it the screw the wrong way, so turn it back the opposite direction . Correct action at this point will bring the star back to the centre and will have moved the shadow of the secondary to a point nearer the middle. Never turn the screw more than one full turn, it will never be required to turn this far for normal alignment. If you can do this whilst looking through the scope it is so much easier, however on larger scope it is a process of elimination to find the correct screw at first. In most cases the screws will only require a quarter of a turn or less. It must be noted that on some occasions small movements of two of the screws will be required to achieve this.

Once you are getting the shadow closer to the centre of the starlight disc you may find you need to adjust other screws, the same applies very small movements until you have the star as close to the middle as you can see. If it is near or in the middle you will start to see a number of small thin concentric rings of light around the shadow at the centre.

Increase of power.

I normally increase the power using an eyepiece to give X150-X200, then repeat the process using very small adjustments of 1/10 of a turn or less, until the shadow looks exactly in the centre of the light from the de-focused star. Check the image after every action and if a mistake has been made undo that action. Look for the concentric light rings again.

I often repeat the process using an even higher power of over X300 but this is not always necessary, if you are happy with what you have done focus the star and have a look at the diffraction rings, the scope should now be in good collimation and ready for use. A good final check is select a star of around 4th magnitude and check for a clean point of light, this is known as the Ariy disc and offen has concentric diffrations rings around it on brigher stars. Any adjustments made at this stage will need only the very tiniest of screw movements, be careful not to undo the work you have done.

This seems very worrying and difficult at first but once you have done this a few times you really will wonder what all the fuss was about. The scope should now be in good collimation for a night of astronomy.

Clear skies to all,


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Nice 1 Alan, that's great mate - i think once you have owned an SCT for a while it becomes only natural to think if your getting the best out of the scope.  When you take a step back and just think what your about to do, you realise that there is only one mirror that needs adjustment - not like the 2 mirrors of a Newt/Dob and as we all know that the seeing and the atmosphere will have a far greater effect on the scope than a slight mis - alignment will create - as you say the SCT's seem to hold collimation well - so long as you have the screw/knobs fairly tight as to hold the secondary still - if there's a loose screw/knob on the secondary this has the tendency to allow the secondary a certain amount of movement, so its worth ( if the scope is out of collimation) just tightening the three screws/knobs, giving even tension on the secondary - as you say - they may only need increments of a 1/16 of a full turn to achieve perfect collimation.

I've also found that if your doing any imaging on the Planets just to check the orbiting Moons of said planet - wether Jupiter or Saturn, and have a quick look at the images of the Moons - as this will give a good indication of perfect collimation - or any flaring/off centre alignment.

A great Post Alan and thanks for posting.


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I have thought about using APT with a DLSR afocal with a high power EP, so as i adjust, the image in APT on the PC screen will change should work ok just need a clear sky and make it the first task in the evening......but thats a nice write up and will help people that are not to sure........

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  • 2 weeks later...


Yes one of the other Moderators told me about this as with larger scopes it makes it easier, with mine my arms need to be a few inches longer but I have done it so many times now I could almost do it in my sleep. Know what you mean about clear skies though.


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