Translation and Rotation
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Now for a short physics lesson! It is important to understand how the various movements of the pier can affect the image seen in the telescope. While some of these effects are obvious, others are not. With a good understanding of the physics of the pier, you will be able to understand more clearly why certain pier designs function better than others, and how you can design one that gives the performance you want within your constraints.

Assume a telescope is aimed in a particular direction, say at the exact center of a star. Let's look at the three different kinds of movements that can happen to a pier and its attached telescope.

You can move the telescope in translation. That is, in your mind's eye, move the telescope (and pier) sideways without changing its direction in space. If you translate the telescope ten feet, you will now be aimed at a point ten feet off the center of the star. Obviously, if you are observing the star, you will not notice any change in the location of the image in your eyepiece! That is, translation, or lateral movement of the telescope, does not affect the image.

You can also move the telescope in rotation, i.e., keep the telescope at the same location, but change the direction of its optical axis. If you rotate the telescope 20 seconds of arc (equivalent to shifting a one foot long telescope by .001 inch), you will see the star shift away from the center of the field of view. That is, rotation of the telescope does affect the image.

Figure 6.1 IMAGE EFFECT OF TILTING PIER

You can also move a telescope BOTH in translation and rotation. Let us consider a telescope on a simple pier (a pipe in the ground) as shown in Figure 6.1. If you push the top of the pier with your hand (or if the wind pushes the telescope), the top of the pier and the attached telescope will move laterally (perhaps .001 inch). This causes no effect on the image. However, because the base of the pier is fixed, the force will also make the pier bend, thus tilting (rotating) the head of the pier and the attached telescope. Depending on the pier dimensions, and the flexibility of the pier and footing, the attached telescope will rotate - in this case, some tens of seconds of arc, normally an unacceptable amount! No wonder astronomers want solid piers!

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