The art of collimation

Well, it took me a while, but I finally figured out what all this buzz is around “collimation.” I am no expert on this, but I wanted to record my discoveries for future reference.

Simply put, collimation means making sure that you primary mirror is pointed at the center of the eyepiece, and that your eyepiece is pointed at the center of the primary mirror. This means that the focal plane of the primary mirror can meet the focal plane of the eyepiece, and the view can be perfectly focused.

This diagram shows a telescope that is out of collimation. All you want to do is to get the blue and red lines to lie on top of each other. That means that the focal points of the primary and the eyepiece can meet at the same point. If they do not, you will never be able to achieve focus.

The Secondary

A lot of confusion came from how the secondary mirror affects any of this. As I finally learned, it doesn’t affect anything. The secondary mirror is “optically inactive,” which means it’s orientation doesn’t affect the perfomance of the optical system as a whole. The only point of the secondary is to reflect all light from the primary into the eyepiece.

As I read about collimation, the term “offset” came up and greatly added to my confusion. Offset is a positioning of the secondary mirror away from the geometric, or mechanical, center of the telescope. The offset is used to keep the reflection of the primary centered on the eyepiece, and it also keeps the view of the secondary circular in the eyepiece. My SkyWatcher telescope has the offset B pictured below. As the image demonstrates, this allows the entire view of the primary to be reflected and seen by the eyepiece. You can easily tell if your secondary is offset by looking to see if its mount is geometrically centered on the back of the mirror.

The offset causes the optical axis of your eyepiece to not point at the geometric center of the secondary mirror. This seemed really wrong to me at first, but made more sense when I considered the perspective of the view of the secondary from the eyepiece.

To begin collimation, you will want to have the secondary mirror centered underneath the focuser. This can be done in several dimensions:

  1. The secondary can be moved inward and outward in the OTA by its mounting screw.
  2. The secondary can be rotated on its mounting screw to achieve a perfectly circular shape.
  3. At least on my SkyWatcher, the focuser can be tilted with three screw pairs in its base.

What I’ve found works best for me is the following:

  1. Place a long sight tube in the focuser.
  2. Move the focuser in or out until the secondary mirror almost takes up all of the view in the bottom of the sight tube.
  3. Try to center the secondary lengthwise in the OTA using its mounting screw.
  4. Rotate the secondary on its mounting screw so the reflection of the primary looks centered with respect to the axis of rotation of the secondary. That is, the center of the primary should lie on the long axis of the secondary.
  5. Even the tilting set screws of the secondary so it does not have any tilt relative to its mount.
  6. Adjust the set screws on the base of the focuser so that the secondary mirror is centered and concentric with the bottom of the sight tube.

This should give a good base for collimating the eyepiece. To do that, you then adjust the tilt screws of the secondary so the center donut of the primary mirror aligns with the crosshairs of the sight tube. This aligns the red line from the first image with the center of the primary.

The Primary

To finish collimation, you want the primary optical axis aimed at the center of the eyepiece. This can be done many different ways, but I prefer the “laser Barlow” method. You place a laser collimator in a Barlow lens, and place that in the focuser. You will then get a reflection of the silhouette of the donut on the primary in the laser collimator. The primary can be adjusted to center this silhouette in the target of the laser collimator.

In the picture above, you can somewhat see the dark ring around the center of the laser target.

When you are finished, you should see something like the following image. The small, bright red dot is the donut on the primary illuminated with a red flashlight. It is centered in the sight tube. The big, bright white area is the reflection of the primary in the secondary. The black ring around that is the back of the tube behind the primary. You can see that this is perfectly centered in the sight tube, the inside of which is the beige halo around the outside of the image.