Foundation Structure Tests
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We performed a series of tests on several foundation structures to measure the effects of personal movement on image stability.

We performed several measurements on a concrete pad constructed as described in Chapter 3. We found a sag of less than .0005 inch under a person, with the pad more-or-less rising back to its original position when the person moved away. We also used a small telescope on a tripod resting on the pad to measure the direct image shift arising from tilting the telescope as a person moved back and forth. We observed an image shift of about 5 arc seconds. In most amateur observing situations, a slow or occasional shift of this magnitude would not cause a problem. Of course, a continuing vibration of this magnitude would very seriously degrade the image: no Mars canals for you! A telescope mounted on a pier isolated from the pad would have had undetectable tilt in this test.

Figure 6.3 PAD AND PIER TILT FROM PERSON

A more interesting set of tests involved placing a telescope on various roof and floor structures of a frame house. In theory, if a floor supporting a tripod or pier sinks 0.1 inch in a 10 foot span from the weight of a person, the image shift could be as high as 30' arc - equal to the apparent diameter of the moon. However, as shown in Figure 6.4 on the next page, if the telescope and the observer were at the exact center of the roof, the image shift would be zero, since the scope only moves up and down, and does not tilt (rotate).

To observe this effect in the real world, we tested a telescope resting at the center of a 16 foot porch roof span made of 2x6s on 16 inch centers - a very marginal roof that bounces noticeably when walking on it. Although the roof sags about 3/16 inch when a person moved to its center, we found that the test telescope tilted only 10' arc. If we sat reasonably still, then the image would remain steady within about 20" arc. Obviously, this is a terrible roof to use for supporting the pier or tripod of a telescope. However, even in this case, it could be usable for some observations, especially if the observer were to sit still while using the telescope.

We also tested much stronger interior floors in the same frame house, using a telescope placed in the middle of the room. There we found only 1.2' arc tilt from a person moving from one side of the telescope to another. This was much better than on the porch roof, and stable enough for many non-critical uses. We could not detect any movement while sitting still for many minutes at a time.

Figure 6.4 FLOOR/ROOF TILT FROM PERSON

The point of showing the stability of typical structures is not to dissuade you from constructing the highest quality pier that is feasible. Rather, the measurements show the potential for using existing structures for at least a part of your pier support. With care, and with careful attention to how the pier responds to movement of the foundation structure, you may even find that your pier does not necessarily have to be isolated from the floor/dome support structure to the degree you might have expected.

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