Vibration
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Now let's consider vibration of the scope and pier. Vibration is a cyclic or repetitive movement that would cause the telescope to tilt (rotate) and cause the image to move back and forth. If we push and release the pier once a second, we will be causing the telescope to vibrate. Vibrations occur with a specific frequency (cycles per second or Hertz) and with a particular amplitude (measured by the amount of movement - inches for translation and degrees for a rotation).

A simple device such as a pendulum will vibrate with a single frequency and amplitude. However, a complex object, such as a telescope, mount, and pier, may vibrate simultaneously in many ways or modes, i.e., its different parts can vibrate with different frequencies and amplitudes. These various vibration modes will involve different interactions and movements of the telescope and mount, and will cause different mixtures of translation and rotation. Vibration of one part can even cause other parts to vibrate.

Vibration results from a disturbance or perturbation of a system. You can excite different vibrations, depending on how you perturb the system. You can have a single push (e.g., touch the telescope), or you can have a repetitive or cyclic force (e.g., an applied vibration). After the excitation is removed, the induced vibration in the scope/pier/mount will die out as friction removes energy from the movement. How fast this decay or damping occurs is measured by the Q (quality factor) of the system. Roughly, the Q equals the number of cycles of vibration that occur before the amplitude drops to about 1/3 the starting value. A large Q means the damping is small, and the vibration continues for a relatively large number of cycles. The Qs for typical mechanical systems range from 1-3 up to perhaps 100. The Q of the suspension of your car is about 1 when the shock absorber is working properly; i.e., vibration is damped quickly. A hacksaw blade vibrating in a vise will show a Q of about 20-30.

How does all this affect telescopes? Movements of a telescope occur when the system (pier, mount, or scope) are moved by some force. If the force occurs and then is steady and unchanging, the scope will move and then be still. However, if the force is applied, then removed, vibration(s) will occur, which will then die out according to the Q of the system. If the force is applied and/or released very quickly rather than slowly and gently, a wider variety of vibrations will be excited.

If the exciting force is repeatedly applied in a cyclic manner and occurs at a frequency matching one of the natural frequencies of the system, large vibrations can occur, even from small excitations. The greater the rotation component of a vibration, the greater the shifting back and forth of the image. The lower the Q (the greater the damping), the faster the vibration will die out after the excitation is removed.

Figure 6.2 PIER/MOUNTING CHARACTERISTICS

In summary, our ideal telescope mount and pier will be designed to have high natural frequencies of vibration (giving low amplitudes), and a low Q (to dampen vibrations quickly). Even better, no vibration inducing forces will get to it, i.e., it will be perfectly isolated from outside forces.

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