How to Collimate your Newtonian Reflector

What is collimation anyway? Simply put collimation is the process of aligning the focuser, secondary, and primary mirrors (lenses, mirrors, prisms, eyepieces) in their proper positions. For a Newtonian reflector telescope to achieve crisp and well focused views of planets and other heavenly bodies all of the mentioned components have to be accurately aligned with each other.

For a beginner like myself this can be no small task. I spent many hours attempting to collimate my Meade DS114 reflector with very little success. There are many websites on the Internet that explain step by step how to collimate your telescope. One that I found to be very helpful was FAQ about Collimating a Newtonian telescope by Nils Olof Carlin. This site goes into detail concerning the different sorts of errors that occur when any one of the above mentioned components is misaligned. Most of the material in the site went over my head, but I was able to understand the steps needed for collimating my own telescope.

This is a picture of what you will see when looking down the focus tube of your telescope. Now, one very important tool which you will absolutely need is a peephole. Relax, you can make one out of a 35mm canister. You will need to cut out the bottom of the canister and drill a hole about 2-5mm in diameter in the exact center of the canisters cap. To the right is a picture of the peephole I use. The canister is just the correct diameter and will fit nicely into the focus tube. The purpose of the peephole is to ensure that you are looking directly down the center of the focus tube (this is very important).

Looking down the focus tube with the peephole in place you will be able to see the following; the focus tube, the secondary mirror along with the secondary mirror holder and clip, the reflection of the main mirror, the spider vanes, and finally the reflection of the secondary mirror. At this point what you are looking for is that the secondary mirror is perfectly centered underneath the focus tube. Don't worry about what or what you can't see on the secondary mirror at this point, just concentrate on ensuring that the actual secondary mirror is physically centered in respect to the focus tube.

This step is what caused me the most difficulty with my telescope. If the secondary mirror is off to the left or right of center you will need to adjust the spider vane screws on the outside of the telescope tube to bring the mirror assembly back to center. Be sure to unscrew one vane screw while screwing in the other. If the secondary mirror is too far in or out of the tube you will need to loosen the three set screws on the secondary mirror assembly and then either screw in or out the center screw which holds the secondary mirror in place. This process should bring the secondary mirror directly underneath the focus tube. I like to rack the focuser in to the point where the secondary mirror just covers the focus tube opening. In this way you will be able to make fine adjustments until everything looks symmetric.  The focus tube itself could be badly misaligned and might need to be centered, although this was not the case with my telescope. Just check the focus tube to be sure it isn't grossly out of place. 

 

Here is my telescopes spider assembly which holds the secondary mirror. I am holding one of three plastic tubes which were over the wire rods that hold the secondary assembly in place. I'm not sure why Meade added these small tubes to the design, but they are the reason why I could not collimate my telescope. With them in place I could not adjust the spider vane screws to allow for the left and right movements of the secondary mirror just explained above. I had to take the spider assembly apart and remove the three tubes. In short order after that I was able to collimate the telescope.

The next step is to center the main mirror's reflection on the secondary mirror. To accomplish this you will turn your attention to the three set screws on the center of the spider assembly directly behind the secondary mirror. While looking down the peephole, loosen these three screws and tilt the secondary mirror until you are able to see the entire main mirror on the secondary mirror. Lightly tighten the set screws and proceed to make fine adjustments to them until you are confident that the main mirror's reflection is perfectly centered on the secondary mirror.  After this step you might need to go back and ensure that the secondary mirror is still centered underneath the focus tube. If not, adjust the spider vane screws again and check that the main mirror is still centered on the secondary mirror. Do these two steps until everything look perfect.

The next step is to adjust the main mirror. Here is where a spot on the center of the main mirror comes in very handy. I made the center spot on

my telescope's main mirror by first taking the whole main mirror assembly off. I then took a few measurements of the main mirror, and with a compass I drew a circle on a piece of paper of the same diameter as the main mirror. I cut the circle out with scissors and folded it in half and then in half again. I then cut out a small piece from the tip of the folded paper. Once I unfolded the paper I placed it on top of the main mirror, and with a red marker I made a very small mark in the center of the paper were I had cut out a hole earlier. Now I had a spot in the exact center of the main mirror. I then took a circular paper reinforcement ring and placed it around the center spot. Don't worry about putting something in the center of the main mirror as this area of the mirror is not utilized for viewing due to the fact that it is always in the shadow of the secondary mirror assembly.

Once the main mirror with the center spot is put back in place on the telescope you can use the peephole as a rudimentary Cheshire. Basically, what you want to do is take a flashlight and shine it over the peephole which in my case is somewhat transparent. If you look down the peephole you should see the center spot on the main mirror, and a ring of light around it due to the flashlight shining through the peephole. Most likely the center spot will not be in the center of the peephole, so adjustment to the main mirror will be necessary.

The main mirror assembly normally has three set screws and three lock screws. Unscrew the lock screws a bit, and then screw in or out one set screw at a time until you get an idea of what each screw does with respect to the center spot. The goal is to adjust the set screws until the center spot is perfectly centered within the peephole. In my case the paper reinforcement ring is just smaller than the peephole, so when the flash light shines down the peephole I can adjust the main mirror until I end up with a perfectly spaced light ring round the center spot. 

At this point your telescope should be collimated. The ultimate test is to point the telescope at a distant star using an eyepiece one to two times the power of your aperture value (9mm eyepiece in my case). Be sure that the star is perfectly centered in the eyepiece and then rack the focuser in and out of focus. When the star is out of focus you should be able to see the shadow of the secondary assembly and the spider vanes. The shadow of the secondary assembly should be centered within the light circle created by the star. If it's not, you will need to adjust the main mirror set screws in the direction of the dissymmetry. The adjustments needed at this point will be very minute so be careful not to over do them.

It is possible to use a laser collimator to assist in the collimation process. The easiest way to go is just to purchase a laser collimator, but it is possible to build one yourself. I made one from a rifle bore sighting laser and a 35mm canister cap. I drilled a hole in the center of the cap large enough to allow the laser to snugly fit in place. This particular setup is not very precise so I just use the laser collimator as a rough guide during collimation. A good laser collimator is a great tool, and I recommend buying one.

Collimating your telescope doesn't have to be a nightmare. In the process of collimating my telescope I learned how it was designed, how to take it apart, and how to put it back together. Frankly, I feel much more comfortable working with my telescope now. At first I did not feel comfortable at all with taking the main mirror out and putting a center spot on it, or taking the spider assembly apart. Nevertheless, I had to do these things to correctly collimate the telescope, and I am glad now that I mustered up enough courage to take the thing apart and get the job done. The crisp, well focused images which I am now getting from my telescope makes all of this hard work worthwhile.