Newtonian Telescope Collimation

Written by on Thursday 2nd May 2013 under How-To Guides

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newtonian telescope collimation - Newtonian Telescope Collimation

This guide shows you how to perform an optical alignment your Newtonian telescopes mirrors and get the most from your reflector telescope. It will look at using a homemade Cheshire eyepiece as well as using a laser collimator for accurate results.

 

 

Newtonian Collimation with Laser Tool
Newtonian Collimation with Laser Tool
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 as catadioptric telescopes are sealed units and have a corrector plate, while refractors have a different optical system using combinations of glass lenses. These types of telescopes have there own alignment methods.

Poor alignment in a Newtonian telescope will make objects difficult to resolve, give 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) since the optics are more sensitive to temperature changes and vibrations / knocking.

I have read many articles in magazines, forums and even the (poorly translated) 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 myself by experimentation. The result is this article which, I hope, 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 or omissions 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 telescope mirrors, is one of the most powerful designs. Newtonian telescopes 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.

Newtonian Telescope Cutaway
The yellow lines represent the path light takes as it enters the telescope, reflects off the primary mirror onto the secondary mirror and finally towards the eyepiece or camera.

Why is Collimation Important?

Because telescope mirrors 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 affect 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 and is the process of ensuring that the light path from the object you are viewing is as close to perfect through the telescope body and focuser, into the eyepiece and onto the camera or eye. 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.

Newtonian Primary Mirror
With the primary mirror removed from the tube, the three clips securing the mirror can clearly be seen.

Does my telescope need collimating?

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 2). 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 3).

Testing Newtonian Collimation
Figure 2: Testing Newtonian Collimation
Testing Collimation
Figure 3: Testing Collimation

Newtonian Telescope 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).

Caution: It is important that you do not over tighten any of the screws as they can put excessive stress on the delicate optical surfaces and may damage the surface.

What You Will be Looking At

View through the focuser with no eyepiece
Figure 4: 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.

DIY Collimating Cap
Figure 5: DIY Collimating Cap
Primary Mirror Surface with central marker
Figure 6: 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 8 ) 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 7 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 6 shows the location of the clips and how they hold the mirror in place (as well as the dust on mine).

All three clips should be visible when correct.
Figure 7: All three clips should be visible when correct.
Primary Clips when Poorly
Figure 8: Primary Clips when Poorly aligned.

At the top of the spider, there are three screws that control the alignment of the mirror (Figure 9). 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.

Alignment screw locations.
Figure 9: Alignment screw locations.

Aligning the 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 6 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).

Rear of Primary mirror with protective plate removed.
Figure 10: 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 10 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 7 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.

Very Bad Alignment
Very Bad Alignment
Poor Alignment
Poor Alignment
Good Alignmnet
Good Alignmnet

Using A Laser Collimator

WARNING: DO NOT LOOK DIRECTLY AT THE LASER BEAM. DO NOT STARE INTO BEAM OR VIEW DIRECTLY WITH OPTICAL INSTRUMENTS. DO NOT ALLOW THE LASER BEAM TO BE REFLECTED INTO YOUR EYE BY ANY OF THE MIRRORS! DO NOT ALLOW THE LASER BEAM TO ESCAPE THE TELESCOPE TUBE.

A laser collimator is an accurate way of aligning the optics as long as it is aligned properly itself. The collimator 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 very badly out of alignment, but 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 laser 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.

Antares Laser Collimator
Antares Laser Collimator
Laser dot on Primary Mirror
Laser dot on Primary Mirror

For best results check the alignment by rotating the laser 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 laser collimator or focuser are most likely to be out of alignment.

Laser Collimation
Laser Collimation

References and Links:

 

 


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21 thoughts on “Newtonian Telescope Collimation”
  1. Pingback: Laser and visual cap collimation: I can achieve one or the other, but not both. - Page 2
  2. Pingback: Telescopen | Cynosura
  3. Thank you very much for this easy to understand and fear-taking explanation of colliation! It helped me a lot!

  4. This has helped me very much with my school science project. No other website explained this in such a clear, easy to understand way. Thankyou !  :-) 
    Collin, UK

  5. I just recieved my first newtonian telescope today. An old Celestron Nexstar 130GT...I was excited until I got it outside and learned quite quickly what I had on my hands...a dusty, spider web filled, and horribly out of focus telescope...Junk? =(

    After some hopeful searching, I ran across your site, crossed my fingers, and took some quick notes. A careful cleaning and a few timid adjustments and already the image was getting better.

    I still cannot see the quad stars clearly in Orions Nebula...but it is only a matter of time now!

    I only hope my refractor doesn't get jealous. =)

    Thanks!
    Grass Valley, CA.

  6. I don't know what to say, thank you so much. I have two OTM's and only mount (goto) and a lot of changing takes place as my tastes change, so keeping my mirrors aligned is of paramount importance. I feel more confident about taking on this task and so much happier spending time looking up rather then in. not to mention the joy of bring the wonder of the universe to my children in sharp clear focus.

    thank you Perfect Astronomy.

    Craig
    Pittsburgh, Pa

  7. Greetings Perfect Astronomy!

    Thought I'd take the time to add to the chorus of approval for a well written, and easy to understand guide. As an entry level astronomer who has just been handed an Infinity 114EQ-D that has been sitting in a garage for 15 years, you can't believe how much I was panicking when I disassembled the mirrors, thinking "Woah boy, this is going to be painful to realign". Now I am now more excited than ever to get the mirrors back from resilvering, get this fine apparatus reassembled and aligned, and do some serious astronomy.

    More power to those with the knowledge, and more importantly, the ability to impart it!

    - ATSystems, Sydney, AU.

  8. I follow and completed the collimation procedures with a laser collimator; However I had a slight problem: the secondary mirror is not round when I looked down the focusing tube it's like a egg shape; I tried to adjust it with the center screw and the 3 support screws with not much a success. Is this ok to be slightly not round or what is the best way to adjust it.

  9. Wonderful!

    I got so wrapped around the axle last night I went to bed to rediscover collimation. Your column brought it all back!

    Background: Ironically, I actually make living aligning complex optical systems. Hubble, JWST, KPNO - spending every working day in labs loaded with sophisticated alignment test sets of my own design. That puts bread on the table and feelin' my oats.

    So holiday weekend and I install the recoated 29" mirror and fold (98.5%!) in my giant Dob, super clear night and anticipating 2nd light after a 4-year hiatus... And I honestly forgot how to align a Newtonian! Everything badly out and I keep fiddling with the folding flat and get no further. I had forgotten that it is the PM that brings its own dot in. So I’m headed back up to the observatory now and sure it will come in nicely.

    Nuances: Your column is excellent, clear and to the heart of good alignment. A few additional subtleties...interesting but possibly more confusing than important: For a large, fast system (e.g. my 29” f/4.5 Dob) the elliptical flat should have its own axial dot that is displaced from center toward the PM. This is because the side of the ellipse closer to your eye collects a larger field angle than the farther side. So to get all the circles absolutely concentric, one needs to correct for that. This will result in a corresponding asymmetry in the final signature when the system is fully-aligned. Second point – nothing magic about 90 deg fold. It can really be any angle and still perform perfectly. Some commercial scopes actually are designed for 80, 70, 60 to help displace the eyepiece farther down the tube and it “aims up” somewhat. This can help when it takes a warehouse ladder just to get to the focuser! Third point – Readers may have noticed the 29” is an old Coulter mirror. Jim Jacobsen had a hard time making them and finally gave up. Astigmatism was the biggest problem. My mirror has unacceptable astig but I fixed it with a few springs! There are 12 around the back with thumb screws. I bend the astig out by pulling (slightly) on the short radius to make it match the long (orthogonal) one. Result is a perfect image from an intentionally-stressed imperfect mirror!

    But back to earth here – your column is the clearest tutorial on fundamental Newtonian collimation that I have ever encountered. Thanks! Tom Dey Springwater, NY

  10. At first I thought collimation would be hard, but since I found these instructions it's been real easy. I don't have to use my collimation tool anymore.

  11. Exelent instructions I was realy stuck before reading this as my lazer didn't have any instructions at all
    Thanks Harry

  12. Yep. Tore down a used Bushnell Deep Sky reflector today just to see what makes it tick. Purchased used for a very good price and figured it would be a learning experience anyway. The primary is pretty much imovable, but after removing the secondary, inspecting it, cleaning it, and reading this article it was a breeze to collimate. Also made a collimating cap out of a 3X Barlow case that slipped right in to the 0.965 eyepiece holder. Very cool. Thanks very much for the simple, understandable information.

    Dan
    North Carolina USA

  13. The Orion instructions for their laser collimator is especially difficult to figure out.

    Right here we have the best instructions, actually the only instructions I at last understand.

    Well done!

  14. Thank you so much for this great help! I'm a total noob to telescopes, after reading this i took a look and my only one and first telescope (6 inch newtonian) and i realized that was totally out of line!... i did the alignment and the result was like day and night when looking at Saturn!!!

    Thanks again !

    Marcelo figuera
    Utah, USA

  15. Thank you so very much for this great page , it was a GREAT help !!!!!!!!!

    Its always fantastic when you find something you were looking for to help you understand and do your own bit of DIY , this page is just fab , THANK YOU !!!

    Martin
    Cork Ireland

  16. This guide was very helpful and very clear. In my 63 years have owned many refractors but this is my first reflector and thus the first collination process. The manuel with the telescope was not very clear on the process and your instructions and pictures made the process very understandable. Thanks for the help.

  17. You made this totally understandable! My secondary was way off. It was so easy to do with the 35mm cap. Can't wait to try the Fraunhofer diffraction pattern on my 12inch dob. (if the skies ever clear)
    Thank you so very much for such clear instructions!
    Jan
    Michigan,USA

  18. Hello, read with interest, i just did a laser Collimation on my SkyWatcher 200 mm fully flocked crayford focuser and after 30 mins learning tinkering its according to the laser perfect when i look through an eyepeice just befor contact i can see my eye perfectly centered then i focus however it remains as a dull blob in the centre of the eyepeice, what to do its driving me insane

    Thanks Damian.

  19. Very well explained Old Sport.
    Perhaps you could explain secondary mirror offset in the collimation process. This aspect was confusing to me at first in my early attempts. I still don't understand it fully.

  20. This is one of the most usefull guide i ever saw on the internet!
    It took me 3 hours, but i got it, i collimicated my scope!
    it was reaaly badly colimicated, and this is the only guide that speak in understandable languge and includes picturs.

    Thank you very much!

    Or Cohen  :)  Israel.

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