Newtonian Telescope Collimation

Before every observing session, it is advisable for the user of a Newtonian telescope to perform a quick check of the telescope's state of collimation, or optical alignment, to ensure peak performance. This is a simple task that should be second nature to any Newtonian telescope user.

For brevity, let's assume that the telescope has been constructed properly with the focuser perpendicular to the mechanical axis of the tube, the primary mirror centered in the tube, and a well constructed and concentric secondary mirror holder and spider assembly centered in the tube or placed at the proper offset for large, short focus optical systems.

(Just a quick note about this offset: It can be, and usually is - ignored! In a 6" f/8 system it amounts to only about 1/16 of an inch. Even in a 12½ inch f/4 telescope it usually amounts to less than 3/16 of an inch! The only reason for worrying about this small amount is when you are concerned about fully illuminating a large field at the focal plane using the very smallest diagonal that you can - with a given focuser height. Experience has shown that most telescopes come equipped with (and most first-time builders tend to choose) diagonal mirrors that are a little larger than their uses for the telescope will really justify. Therefore, in the interest of simplicity, we will ignore it too!)

The illustration below depicts the view into a telescope's focuser with the mirrors in proper collimation. (All of the following illustrations are oriented such that the skyward end of the telescope is pointing toward the top of the page.)

As you look into the focuser, you should see a series of concentric reflections. Beginning at the outside edge, you see the end of the focusing tube, the secondary (or diagonal) mirror in its holder, and the reflection of the primary mirror at the bottom of the tube.

Reflected in the primary mirror, you should see the secondary mirror in its holder with the spider vanes, the end of the focuser tube, and at the very center, a reflection of your eye looking back at you! The object of collimation is to bring all of these reflections concentric with each other.

Rough collimation needs to be done in daylight or twilight, or any other condition where there is sufficient light inside the tube to view the mechanical parts. Fine collimation is done under the stars.

The first step is to acquire a collimation tool.

The simplest and most bang for the buck is just a dummy eyepiece; a piece of material that will fit into the focuser, with a small (1/8 inch or so) central hole that serves to center your eye in the focuser tube. A plastic 35mm film can works great for this: just cut off the bottom of the can, and drill or cleanly cut a small round hole in the center of the cap. An old high power eyepiece with the lenses removed makes a nice one too! This is really the only collimation tool you will probably ever need for a well-made telescope.

Start the collimation process with the mirror closest to your eye; the outside edge of the secondary should be brought concentric with the end of the focuser tube. With a well made telescope, this step should only be required the first time the secondary mirror is installed, and can be subsequently skipped.

Illustration A shows a view of a secondary mirror that is woefully out of alignment.

This is evidence of either the spider/secondary holder not being centered in the main tube, or a seriously tilted focuser! It also shows that the secondary holder is not extended far enough down the main tube toward the primary mirror to be centered under the focuser.

The most common method of construction is such that the secondary holder is mounted to a threaded rod that passes through a central hole in the spider, and is held captive by a pair of nuts that can be loosened to provide axial as well as rotational adjustment for the secondary holder. After the axial (up and down the axis of the main tube) adjustment has been made, and the nuts are still only finger tight, make the rotational adjustment, then finish tightening the nuts. It may be helpful to tighten these nuts in stages, checking your alignment as you go.

The next figure (B) shows that the axial alignment is good, and the secondary holder is well centered under the focuser, but the rotational adjustment is still a bit off.

A slight loosening of one of the two nuts mentioned above will allow you to rotate the secondary holder a bit to bring the reflection of the primary mirror to the center of the secondary.

A quick look at the next view (C) shows us that the axial and rotational adjustments look good, however the reflection of the primary mirror is still not centered in the diagonal.

At this point we need to adjust the deflection, or tilt, of the diagonal. Your secondary holder may have three screws that are equally spaced around the back of it that can be manipulated to bring about this adjustment; loosen two to enable you to tighten the third, or loosen one to tighten the other two. Many secondary holders employ a hinge, the angle of which can be adjusted with one spring-loaded screw.

In view D we see that the reflection of the primary mirror is now centered in the diagonal, and all that remains is to center the tiny image of the diagonal and it's spider vanes in the reflection of the primary. This is accomplished by adjusting the three wing-nuts (usually) at the back of the primary mirror cell.

With most styles of mirror cell construction, the image of the secondary will move away from the nut that is tightened. The cell is usually oriented to the main tube in such a way that by adjusting two of the nuts, a predominantly "left and right" motion is effected, leaving the third nut to subsequently provide an easy "up and down" movement.

When the image of the secondary mirror has been centered in the reflection of the primary, we can consider rough collimation to be complete. At this point, it is a good idea to double-check our work, and make certain that all reflections are still as concentric as we can make them.

Now is a good time to align the finder scope and/or reflex sight with the main instrument.

When the stars come out, and it is too dark to see inside the tube, alignment of the primary mirror can be checked as follows: Point the telescope at a fairly bright star (2nd or 3rd magnitude) that is near the zenith to maximize the chances for steady air. Put a medium to high power eyepiece in the focuser, and center the star in the field of view. Rack the eyepiece out of focus (beyond the focal plane), and the star will look something like the illustration below.

The shadow of the secondary mirror and it's spider vanes appear superimposed on the out-of-focus star image. Be sure to keep the star centered in the eyepiece's field of view; if it is not, the shadow will falsely appear to be off-center in the star. This effect is much more pronounced in short focus telescopes. If the shadow of the secondary is not perfectly centered in the defocused star image, use the three adjusting nuts on the back of the primary mirror cell to make it so. As you make these adjustments, the star will move away from the center of the field of view. It will be necessary to re-center the star after each touch of the adjustment nuts.

When this step is complete, the telescope is very nearly collimated as well as it can be. If the atmospheric seeing is poor, finer collimation is unnecessary. If the seeing is in an excellent state, we can proceed with fine collimation: With the star still centered in the field of view of a high power eyepiece, rack the eyepiece back in until it is just outside best focus by a couple of millimeters. The slightly de-focused star will look something like the illustration below.

Any small misalignment of the primary mirror will be evidenced by non-concentric interference rings in the image. This is the effect of coma. Short focus Newtonians suffer greatly from this aberration, and it is why they are especially sensitive to being out of collimation. Telescopes of about f/7 and longer will probably need no further adjustment beyond what we have already done. For the shorties, there will probably be some adjustment yet to do.

To proceed with bringing the rings concentric: the central dot points the way toward the optical axis of the primary mirror, and also to which of the adjusting nuts that should be loosened to bring the axis to the center of your eyepiece. As in the rough alignment procedure above, we need to re-center the star in the field of view after each touch of the adjusters. You will find that this time, the image is very sensitive to very small amounts of adjustment.

The advantages of a solid equatorial mount with clock drive become very obvious in this exercise!

After some tweaking, we should wind up with an image where the rings are concentric, and the in-focus view through the high power eyepiece looks like the illustration below, with a bright central Airy disk, surrounded by a couple of faint diffraction rings (seeing conditions permitting, of course!).

Re-check the finder scope alignment, and have an enjoyable night!

Steady Skies!

- Jim Sapp
Autumn 1998
Re-edited and HTML-ized Summer 2010