This guide focuses on the optical alignment, or Collimation, of Newtonian telescopes. The guide is based on a SkyWatcher Explorer 200, however the principals are the same for any Newtonian.
One of the main disadvantages of Newtonian and Dobsonian Reflector Telescopes is there constant need for optical alignment. Dobsonian telescopes differ only by the mount and can therefore be considered to be a Newtonian Reflector. The requirement to align optics are limited to Newtonian Telescopes. Catadioptric Telescopes have a corrector plate and are sealed units and Refractors have a different optical system using lenses. These type of telescopes have there own alignment methods. Poor alignment in a Newtonian will make objects difficult to resolve, poor detail in planets and nebulae, create images flaring, decrease contrast and greatly reduce the enjoyment from ones telescope. Collimation becomes more important with smaller focal ratios (f/6 and below).
I have read many articles in magazines, forums and even the instruction manuals, and they all seem to confuse the issue and I never really understood what was going on. Eventually I gave in and tried to work it out for my self by experimentation. Hopefully this article will be of help to those of you who look at the manual and see a complex set of diagrams and can’t relate them to your own telescope. Please remember that this is a combination of my understanding of several documents and my own experimentation and as such if you notice any errors please inform me. I have kept the diagrams to a minimum and where possible photographed what you should be seeing through the eyepiece.
Basic Anatomy of a Newtonian Telescope
This type of telescope was invented by Sir Isaac Newton (1643-1727) and although being one of the simplest, consisting of a hollow tube and two mirrors, is one of the most powerful designs. Newtonians have a large Primary Mirror at one end and a smaller, Secondary Mirror at the other. The secondary mirror is positioned near the opening of the telescope and is positioned at 45? to the primary, focusing light through a small opening in the side.
Figure 1: Newtonian Diagram
Why Collimate?
Because the mirror needs to held in place without causing stress to the optical surface, it cannot be permanently attached to the telescope. It is held in place by clips and set screws. Every bump and knock will effect alignment of the primary mirror because it is not firmly attached, as will moving from warm to cool air and back again, due to the expansion and contraction of the glass and metal. Most telescopes leave the factory with a reasonable alignment, however the delivery may not be as smooth and alignment will suffer.
The process of optical alignment is called Collimation. It must be noted that it is not absolutely necessary to collimate a Newtonian, it won’t stop working, however a correct alignment will greatly improve the quality of the image and the ability to resolve images.
You can check your collimation by centring a bright star in an eyepiece that will give you a high magnification (e.g. 50x). When the star is out of focus it will appear as a doughnut shape with a dark centre. On an aligned telescope this dark patch will be perfectly centralised within the bright ring. This is actually the shadow cast by the secondary, and you may also see the spider veins (Figure 3). When the optics are in need of alignment the dark patch will be off centre.
A more accurate test uses even higher magnification (e.g. 100x), or as high as seeing will permit. When just out of focus you should see an Airy disk form. This looks like a series of concentric rings around the star. They should be all centralised and circular (Figure 4).
Figure 3: Testing Collimation
Figure 4: Testing Collimation
The Process of Collimation
The photographs in this guide show a very bad alignment to illustrate the methods involved. It was not always possible to capture the view on camera, but where possible this is explained in the text or with simulated images. Most of this procedure can be done during daylight hours, however you will need to be outside to let the telescope cool down to normal temperature. Fine tuning should be done immediately before a viewing session to ensure that it is as accurate as possible. I suggest checking the alignment during dusk hours and fine tuning as soon as you can resolve bright stars (after the telescope optics have cooled down).
What You Will be Looking At
Figure 5: View through the focuser with no eyepiece
This image shows the view through the focuser with no eyepiece installed. It is sometimes confusing as you see reflections of reflections within the field of vision. You will be able to see your eye in the reflection of the secondary mirror reflected in the secondary. In this example you can see the camera lens off centre.
Aligning the Secondary Mirror
The first stage is to align the secondary mirror with the optical axis of the telescope and focuser. This stage however does not have to be carried out each time - its pretty much set and forget.
It is important that all the available light is focused into the eyepiece, and this can only be done by aligning the secondary mirror to reflect the whole circumference of the primary. This can be seen visually by observing the three clips on the primary circumference. The mirror should be at an angle of precisely 45? from the primary and it is fairly easy to set this using a collimating cap. This sometimes comes with the telescope, or can be purchased separately. Alternatively you can use an old 35mm film canister - simply cut the end off and drill a small hole in the cap. This simple canister is the perfect size for a 1.25″ focuser.
Figure 6: DIY Collimating Cap
Figure 7: Primary Mirror Surface with central marker
Looking through the hole in the cap, on a correctly aligned secondary mirror you will see all three clips that hold the primary in position. On an incorrectly aligned mirror you will only see two clips holding the primary mirror in place (Figure 9) or if it is very bad just the one clip. In either case you will see the reflected image of the secondary and struts (spider). Figure 8 shows all three clips as they should look.
Spider is the common name for the struts that hold the secondary mirror in place. Ignore all reflected images - we are only interested in the primary mirror clips. Figure 7 shows the location of the clips and how they hold the mirror in place (as well as the dust on mine).
Figure 8: All three clips should be visible when correct.
Figure 9: Primary Clips when Poorly aligned.
They are coloured yellow the third is not visible.
At the top of the spider, there are three screws that control the alignment of the mirror (Figure 10). They are either Phillips or Allen heads and you need to alternately loosen one and then tighten the other two to compensate for the slack. It is best to do this when the Optical Tube Assembly (OTA) is in a Horizontal position or slightly tilted downwards, this way there is no risk of a dropped tool, or any thing else falling down and damaging the primary mirror. When you see all three clips equally as in Figure it is aligned correctly. Don’t worry about the position of the spider or the reflected view. Make sure that all three alignment screws are tightened to secure the secondary mirror in place.
Figure 10: Alignment screw locations.
Primary Mirror
This is the most crucial alignment step and will have the greatest affect on image quality. The aim of this step is to centre the sweet spot of the primary mirror in the focal axis of the eyepiece (or put simply, point the centre the primary mirror at the centre of the secondary mirror). This can only be done by marking the exact centre of the primary mirror. This is commonly done by using a sticky ring enforcer, however most telescopes come with a central marker from the factory. In Figure 7 you can see the centre marked with a ring which has been calculated and set in the factory.
By adjusting the three screws at the back of the primary (you need to remove the protective plate first) you can adjust the angle of the mirror and thus the focal sweet spot. There are three lock screws (the big Phillips head screws) and three adjusting screws (Allen head).
Figure 11 - Rear of Primary mirror with protective plate removed.
By adjusting the screws you need to position the edge of the reflected mirror against the edge of the visible in the secondary and also to align the central point of the primary within the secondary. Believe me, it is far easier to see and do than it is to explain. Figure 9 shows the mirror tilted downwards, so the inside of the tube is visible. By unscrewing the top screw and tightening the bottom two, the mirror is tilted upwards again. When done you should have a view similar to that of Figure 8 where central marker is in the centre of the reflected image of the primary mirror, which is in turn centred in the secondary, bounded by the mirror clips.
Fine Tuning
For this you need to be outside on a cold dark night with clear visibility and good seeing. You need to centre a bright star in the eyepiece and just out of focus you will see the Airy disk phenomenon. This is technically called a Fraunhofer diffraction pattern and is caused because of the wave nature of light creating diffraction patterns through an aperture.
When the optics are correctly aligned, the star will have a series of concentric rings around it. As the alignment is more and more offset, the rings become more and more asymmetrical as shown in the following images.
Figure 12:
How alignment effects the Airy Disk
Using A Laser Collimator
A laser collimator is an accurate way of aligning the optics as long as it is aligned properly itself. The collimators usually come with instructions on how to align them. They are also reliant on the accuracy of the focuser alignment and the absence of any focuser play.
On most telescope optics the laser beam will not leave the telescope unless it is badly out of alignment. You should hold paper or card over any openings to check for the laser beam escaping before looking down them with your eyes.
Once the collimator and focuser are correct, it is simply a matter of adjusting the secondary mirror so that the projected laser dot hits the exact centre of the primary mirror. You then need to adjust the primary until the return beam is exactly on the laser. In the pictures below it is clear that some fine tuning is required as in both cases the laser dot does not hit the centre marks.
Figure 13 - Laser Collimator in eye piece
Figure 14 - Laser dot on Primary Mirror
For best results check the alignment by rotating the collimator in the focuser at 90, 180, 270 and 360 degree intervals. If there is no visible change in position it is aligned correctly. If there is a variation then the collimator or focuser are most likely to be out of alignment.
References and Links:
- A Treatise on Newtonian Collimation by Scott McCluney
- A primer on collimation of Newtonian Telescopes by John Crilly
- A FAQ about Collimating a Newtonian telescope by Nils Olof Carlin
- Howie Glatter’s Laser Collimator Page
- Collimating my Dobsonian Reflector Telescope by Joe Schlatter Jr
- Simple Diagram of a Newtonian Telescope by TMoore
- Simulated Airy disk images created using Aberrator by Cor Berrevoets.
