The Foundation Structure
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Foundation structure refers to the underlying building structure that will support the dome, floor, and pier.

In Section 3, we discussed constructing a pad and how to install a pier (or at least, prepare for one) at the same time. Figure 3.2 showed a typical pad and pier setup. In this case, the concrete footing is the foundation of the pier, while the soil under the pad is the foundation for the pad (observing floor). The only significant way for vibration to be transmitted between the pier and the pad is via the soil. Depending on the soil types, this coupling may or may not be significant. For example, we visited the Visitor's Center at McDonald Observatory in Texas, where piers for amateur observing are kindly provided. When we put our hands on a four inch steel pier, we could easily feel the vibration from striking a second pier more than twenty feet away! In contrast, in the otherwise awful clay soils in our native Maryland, jumping up and down next to a pier may be barely detectable.

If the observatory is on a house or building, rather than a concrete pad, the foundation structure is usually the frame of a house, garage, or small building. The building structure will have some flexure, i.e., it will move in response to exciting forces.

If the pier rests on the floor, then movement of the astronomer on the floor may tilt the telescope resulting in an image shift. For this reason, most people want the pier isolated from the observatory floor/dome. At one extreme, the pier can be separated from the entire building structure by sinking the pier to bedrock. Or, the pier may be partially isolated, i.e., it may use some of the same support or foundation structure as the floor. Partial, rather than complete isolation may be all you need to reduce sufficiently the problems generated by movements of persons. However, wind pressing against the side of a house, or moisture and temperature changes, may still produce movements of the scope that adversely affect some observations.

Remember, the degree of isolation needed depends on the type of observing you do. If you do relatively casual visual observing, then the floor (even in an on-home installation) may be sufficiently rigid to allow placing the pier (or tripod) directly on the floor. However, if you do high magnification visual observing, have frequent visitors who may move around, or perform moderately high magnification photographic or CCD observing, then a more stable pier is likely needed. There is much mythology about this issue, so don't hesitate to test the structure with a telescope, and see for yourself what is needed.

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