Piers and Pads
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If the observatory is to be on a pad on the ground, the classic approach is to install a pier through a hole in the center of the pad. This type of installation is commonly used for any situation where the pier is to be supported in the ground.

As shown earlier in Figure 3.2, the pier, usually a 4 to 12 inch diameter steel pipe, is usually set in a concrete footing that might extend 3-5 feet down, and be 3-5 feet in diameter. The footing is usually poured directly into the hole, i.e., it is not poured into a form. The concrete footing either directly contacts the soil supporting the pad, or is separated by a cushion material (e.g., cork such as used in sidewalk construction). The result is reasonably good isolation from movements of persons on the pad; i.e., movement on the pad will produce neither tilting nor vibration of the pier and telescope.

But what exactly is the purpose of the footing? And how big should/must it be? As you might expect, the general answer is: "It depends!"

Suppose you install a pipe directly into the soil. You will dig a hole, insert the pipe, and then heavily tamp the soil in around it. The result, in most soils, is a fairly rigid pier. But how deep do we go? Again, "it depends". One reason to go deeper is to get a good portion of the pipe below the frost line. That is, as the soil freezes, it expands, and moves up (and down when it thaws) along the pipe. A short pipe can be forced out of the ground! More likely, the heaving of the soil will loosen the packed dirt around the pipe, thus reducing the resistance to side motion. A typical depth for a pipe to avoid these problems is 3-4 feet. Ironically, when the ground is frozen, the rigidity of the pier will improve, since frozen soil is more rigid than unfrozen. Interestingly, many people forget that if the pipe is left empty and open to the air, the ground around it will freeze deeper and more quickly.

In the absence of freezing, the major reason for a deep pier is to improve resistance to side motion and vibration of the pier pipe. With the pipe directly in the ground, a side force at the top of the pier (as in Figure 6.1) will cause the pipe to bend. The force will concentrate at the point where the pipe enters the soil. However, the soil is relatively soft, and will easily deform and allow the pipe to bend further, and deeper into the ground. Even worse, sustained sideways force - such as exerted by the unbalanced forces from virtually all equatorial mounts - will cause a slow tilting of the pipe and deformation of the soil. When the scope is used, and the side forces vary, the soil support will become even weaker. This continues until the pipe bending is spread out over so much of its underground length that the strength of the soil is finally sufficient to hold it. Soils that creep, such as clay, are especially susceptible to loosening in this manner.

Also, depending on the stiffness of the pipe, you will probably find that a depth of more than about 5 feet will give diminishing benefits because the pipe itself will bend in response to side forces. You will also find that the work of digging the hole past 4-5 feet will seem to increase with the 87th power of the depth!

So far, our discussion assumed a pipe directly in the ground. What, then, is the advantage of a concrete footing?

A concrete footing counteracts these effects in several ways:

• It increases the perimeter of the rigid pipe/footing system so that the forces are spread out over more soil. Thus, even weak soil will be able to resist the initial sideways force.
• The relative effect of the "off balance" equatorial mount is reduced by the mass and size of the concrete footing.
• If you do touch the telescope or pier, the large mass and rigidity of the system will have a higher frequency of vibration, thus reducing the amplitude and duration of any adverse visual effects.
• Vibrations coming through the soil cannot move the large mass of the footing to a significant degree - in effect, isolating the pier from the soil.

Before leaving the pier design, we should mention that a steel pipe is not the only feasible material for a pier. Another option is to construct a pier and footing of concrete block, tied together with reinforcing bar and small amounts of concrete. A more common approach is to pour concrete (or sand) into a tube or pipe made of plastic, cardboard, sheet metal, short lengths of concrete pipe, or other material. Such a pier can be constructed in one or multiple pours. The footing might be poured separately (as described in Chapter 3), and with the addition of reinforcing steel, the pier itself can be poured in a series of small pours to construct the height desired.

You can also use piers made of wood. In most areas, such a pier (even in concrete) should be treated to resist rot. Be aware, however, that because wood is relatively flexible, you will need to use strong woods in large amounts to achieve a quality pier.

All in all, the conventional wisdom is correct: a strong and massive pier and footing will give good results, and is always a desirable alternative. However, if you face constraints of money, access to concrete, or other factors, better understanding of the design tradeoffs will help you design a successful pier.

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