COLLIMATION


General

The purpose of collimation is to align all optical axes. This is necessary for an optimal performance of the scope. For slow scopes collimation is just important, for fast scopes it is crucial.

Sometimes the term collimation is used to refer to alignment as well. IMHO this is not correct and should be avoided since it can easily lead to misunderstandings.

 

Optical Axes

In a Newton, used visually, there are two optical axes, one for the (paraboloidal) primary and one for the eyepiece. A spherical mirror does not have an optical axis. The secondary (being flat) does not have an optical axis. (The flat is a sphere with an infinite radius) This is the reason why the collimation of slow mirrors is uncritical, their shape is almost a sphere.

 

The Focuser

The eyepiece is mounted in the focuser. This is the weakest point in collimation, since we just have to assume that the optical axis of the eyepiece lies in the center of the draw tube, and is parallel to its path of travel. Possible errors are an excessive play between the draw tube and the eyepiece. This can lead to a lateral displacement of the optical axis of the eyepiece relative to the center of the draw tube. (This could be solved by using fill material around the eyepiece.) Another possible error is a 'wobble' along the path of travel, this occurs especially with rotational focusers.

Usually the errors described above are too small to notice, but beware of excessive play between the draw tube and the eyepiece.

Another point to keep in mind is sagging. A fully extended focuser with an heavy weight at it's end is bound to sag. (Eyepiece projection imaging!) Sagging from a cheap (light) focuser can easily ruin your images. (Usually fast scopes are used for imaging, these suffer the most from being not collimated)

 

The Secondary

The position of the secondary is not important for collimation, as long as it deflects the optical axis of the primary into the draw tube. (The position of the secondary determines the position of the fully illuminated circle.)

There are however collimation methods that use the position of the secondary. The most know is the 'circles within circles' method. Beware that this method can only be used when the secondary is centered above the primary. I.e. no secondary offset.

 

Laser Collimator

A simple laser collimator collimates the optical axis of the illuminated spot on the primary with the focuser draw tube. This can easily introduce an error for paraboloidal mirrors, since the only the 'center of revolution of the paraboloid' shares it's optical axis with that of the mirror.

A laser collimator can provide a useful instrument if proper care is taken.

For the following, these assumptions are made :

  • The optical center of the mirror is in it's exact geometrical center.
  • The laser collimator is properly adjusted.

If the laser is inserted in the eyepiece tube, and the laser beam is reflected by the secondary to the exact middle of the primary, and the primary reflects the laser beam back into itself, so that the returning beam disappears in the laser, then the telescope is collimated.

BUT : This does not mean that the telescope will perform optimal since this does not take vignetting into account. The primary's optical axis may be misaligned with respect to the telescope tube, and the secondary could be de-centered on the primary's optical axis.

The laser collimator can be adjusted by placing it on a V-block, and projecting the beam on a far (minimum 5 meters) away wall. When the laser is rotated, the spot should remain stationary. (i.e. the spot may not move on the wall)