Collimating a Newtonian Telescope

A collimator is a tool used to "collimate" or align the optical system in a telescope. Using a 35-mm canister is an inexpensive and fairly accurate way to align the optics of your Newtonian telescope. A 35-mm film canister will usually fit into the 1.25 inch eyepiece holders on the focusers of most telescopes. Fuji Film canisters fit better than those from Kodak. 

You will need to do two things to collimate the optical system.

1. Put a dot of some sort exactly in the center of your primary mirror. A dot of paint or a small stick-on dot are commonly used. You do not need to fear that it will cut down on the light gathering capability of your mirror or otherwise damage the mirror, since it will be obstructed by the secondary mirror, the one in the spider, anyway.

2. Put a small 1/8 inch hole exactly in the center of bottom of the 35-mm film canister.

The procedure is as follows.

1. Insert the canister with the hole into the eyepiece holder, instead of an eyepiece.

2. Look at the mirror in the bottom of the telescope through the hole in the canister. You should see a reflection of the hole in the bottom of the canister. Be sure to do this in a well lighted room or outdoors, or you may not see the hole reflected in the mirror.

3. If the hole in the canister is exactly aligned with the dot that you put in the center of the mirror, you do not need to do anything else, as the optical system is collimated.

4. If the hole in the canister is not aligned with the dot in the center of the mirror, then adjust the position of the primary mirror using the three set screws in the base of the mirror mount until it is. You may need to have two people to help do this, one to look, and one to adjust. Also, this may be difficult, as some mounts may not have screws, especially Dobsonians, or they may not have wing nuts. You will have to work this out with your individual telescope and its configuration. A modification may be in order.

5. When you have adjusted the mirror by adjusting the screws until the dot on the mirror exactly lines up with the reflection of the hole in the bottom of the film canister, Voila! Your optical system is collimated.

If you want a truly accurate collimation, you will ultimately need to get a laser collimator. The cost may vary from less than $100 to more than $250, depending on the vendor.

It is important that you have a pretty good collimation on your optical system, or the image quality can suffer significantly. Note that this procedure applies mostly to Newtonian reflector telescopes. It is not advisable for the user to collimate a Schmitt-Cassegrain telescope, such as an LX200 or a C-8. Also, refractor telescopes should not need to be collimated at all.

Ed Flaspoehler

Another Look at Collimating

Look through the 1/8th hole, and look to see that everything is centered or concentric. If not, then make your adjustments as follows.

Extend the focus mount close to its maximum height and, with no eyepiece in place, look down at the circle of the bottom of the eyepiece tube. You should see a reflection of the main mirror in the diagonal . You may also see part of the inside of the tube, an off center image of the diagonal and your own eye in the center of that, as shown in "A" The diagonal is adjusted to bring the image of the main mirror's outline (ignore the reflections in that mirror for now.) until the outline of the main mirror is concentric with the outline of the eyepiece tube- as shown in 'B'.

It can help to keep your eye located in the center of the eyepiece tube - if you find a plastic 'cap' or insert which fits the tube, and drill a small hole ( about 1/8" or less) in it so as to locate your eye centrally.

Once you have it set-up so that it looks like the drawing 'B' you move to the rear of the telescope. At this point it helps to have someone to assist. Then the 3 screws at the back against which the main mirror is resting are adjusted, one at a time to bring the reflected image of the diagonal and its 'spider' or support, into the center of the field of view.

Finally you should arrive at an adjustment where the image will appear as in 'C'. You will also see the reflection of your eye, in the center of the diagonal. When you arrive at this point - your optics will be mechanically aligned as accurately as you can make them. Any further refinement of these adjustments can only be done by using special 'alignment-eyepieces' or by adjusting on the slightly out of focus image of a bright star. For normal viewing - the adjustments given above should provide good images when viewing through the telescope.

Paul Greenhalgh