Dynamic Collimation, The Andrew Angle and the Seven Ways

by Mel Bartels

Dynamic Collimation

How many of you check your collimation with the telescope aimed vertically then with the telescope aimed horizontally? That’s dynamic collimation, demanded by fast F5 and even faster F3 telescopes.

Notice a difference? That’s bad news. The best one can do is to stiffen the telescope and aim it partway up the sky so that any drift in collimation is split between vertical and horizontal orientations. But what angle should be used? Andrew Bell has derived a formula for collimation error as a function of collimation angle that takes into account better seeing when the scope is aimed higher and proportion of sky that’s observable from horizon to zenith. Mathematically, he used his formula to find a "best" collimation angle that will minimize RMS collimation error as your telescope is pointed across the whole sky.

The Andrew Angle

Andrew found that the optimal angle to collimate at is 48 degrees up from the horizon. He’s also calculated that this is a broad answer in that a few degrees higher or lower gives very close to the same results.

In dynamic collimation, the scope is aimed at this angle (use an angle finder available from hardware stores) and collimated. Then check the collimation when the scope is aimed vertically and again when aimed horizontally. Verify that any dreaded collimation change is split between the two orientations. That’s the best that can be done for the moment. If noticeable movement is found then stiffen the telescope structure including the diagonal holder and spider.

Do you know that there are seven combinations of optical parts (mirror, diagonal, focuser) that can be adjusted to achieve collimation?

No wonder so many telescopes are not properly collimated; no wonder many amateurs are confused. All the collimation gadgetry in the universe will not help if you jump back and forth between combinations of optical parts.

If you have a fast scope, F5, or a super-fast scope F3, or a binocular telescope then it’s important to understand these seven combinations, selecting one as your method de jour. This analysis will be of particular interest to binocular telescope designers since these telescopes require two collimated telescopes mounted in parallel such that the two eyepieces fit squarely in front of the eyes.

Collimating is the act of aligning all the axes of the optical elements. That means that the eyepiece axis as set by the focuser and the primary mirror axis are coincident or on top of each other.

The diagonal has no axis but does need to be positioned so that all the light from the primary mirror reaches the eyepiece. For visual observations, in order to keep the diagonal reasonably sized, the diagonal often does not provide full illumination across the full field of view of low power wide angle eyepieces. In this case, the diagonal needs to be positioned so that the center of full illumination as determined by the diagonal is centered on the eyepiece’s field stop. This is sometimes called centering the illumination cone.

For those using setting circles or computer control, the optical axis needs to be coincident or collimated with the mechanical axis, otherwise pointing and tracking errors occur.

The Seven Ways

What are the seven combinations, what are their advantages and disadvantages and how are they used to achieve collimation?

1. Primary mirror and diagonal. The most common is a combination of primary mirror and diagonal. The diagonal’s angle is adjusted to aim the eyepiece/focuser’s axis at the primary mirror’s center then the primary mirror’s pointing angle is adjusted to bring its axis in line with the eyepiece/focuser while.

Here we see the situation prior to collimating where both the diagonal and the primary mirror are out of adjustment.

First the diagonal is brought into adjustment by making the laser beam point to the exact center of the primary.

Finally we adjust the primary mirror's collimation screws so as to aim the laser beam's return upon itself.

2. Primary mirror, diagonal and focuser. Sometimes the eyepiece/focuser’s angle is adjusted with shims so that the focuser aims at the diagonal’s center, thus all three elements are in play.

3. Primary mirror and focuser+diagonal as a unit. One option with a binocular telescope is to treat the eyepiece/focuser and diagonal as a unit that is tipped such that the eyepiece/focuser axis is aimed at the primary.

4. Primary mirror+diagonal as a unit. Another option with a binocular telescope is to adjust both the angle and position of the primary mirror and diagonal as an integral unit.

5. Primary mirror+diagonal as a unit and focuser. Sometimes the eyepiece/focuser is adjusted in angle and position as well as the primary mirror and diagonal in a binocular telescope.

6. Primary mirror alone. It is also possible to achieve collimation by adjusting the primary mirror in both angle and position. The primary mirror is first positioned such that its center is under the eyepiece/focuser axis, then the primary mirror is angled so that its axis is aligned with the eyepiece/focuser.

7. Eyepiece/focuser alone. By the same reason it is possible to achieve collimation by moving the eyepiece/focuser to and fro to center it on the primary mirror’s axis then tipping the eyepiece/focuser so that its axis is aimed at the primary mirror’s center.

The last two combinations are of particular interest in that the number of elements to adjust is but one, either adjust solely the primary mirror or adjust solely the eyepiece/focuser. This could prove conceptually and mechanically easier to achieve.

Imagine a focuser mounted on a plate that can slide up and down plus left to right. Many focusers have tip or angle adjustment screws but if not the movement can be incorporated into this focuser mounting plate. How convenient would that be – all the collimation screws right there together at the focuser.  The diagonal can be coarsely adjusted initially in order to center the illumination cone or be attached to the focuser mounting plate so that it moves in concert with the focuser always keeping the illumination cone centered.

Or imagine the primary mirror in the standard cell that provides for tip or angle adjustments where the cell can be moved side to side and up and down. For example, the two point mirror edge support can be built on a plate with slow motion controls such that the mirror slides to and fro. Then all the adjustments are made right there at the primary mirror. The adjustments can be motorized too by adding small servo motors to the threaded rod adjustment screws.

Here we see the situation prior to collimating where both the diagonal and the primary mirror are out of adjustment.

First the primary mirror is centered such that the laser collimator's beam points to the exact center of the primary.

Finally we adjust the primary mirror's collimation screws so as to aim the laser beam's return upon itself.

This is all done with adjusting the primary mirror only.