Designing a Dome that Turns
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Having designed the shutter and shutter opening, we still must aim the aperture around the sky. The primary design challenge for a rotating roof observatory is to assure easy turning of the dome. As noted above, the equator of the dome is a circle that is relatively easy to deform. In fact, the very forces used to turn the dome deform the dome and introduce resistance to the rotation that can further increase deformation.

To illustrate this, imagine yourself inside a small dome to be rotated by hand. As you push tangentially on the inside of the dome, you are also, in part, applying a radial force that tends to shift the dome off center. This causes the portion of the equator behind you to contact the wall (or other surface). As you push the dome, any drag behind you will increase that contact pressure, thus further increasing the drag. And as the drag increases, the dome equator can distort in such a way as to further increase the degree of contact. The result can be a dome that is difficult to turn.

Figure 1.8 DOME SHIFT DUE TO PUSHING FORCE

The HOME-DOME solves this problem in several ways. A radially rigid equator flange and easy rolling wheels reduce initial drag. When the dome is being pushed at one point on the interior (as is typical), the dome moves off center a maximum of about 1/4 inch and any sliding contact area will be between the inside of the dome support ring and the outer base ring. Since both of these are smooth, gel coated surfaces, friction is low and the dome remains easy to turn. In larger domes, horizontal wheels may also be used to reduce drag. In observatories using electric drives, except for the smallest HOME-DOMES, two or more motors are symmetrically spaced around the circumference. This minimizes drag by balancing the radial forces acting to move the dome off center.

Figure 1.9 HOME-DOME CROSS-SECTION

Metal or wood domes are usually very much heavier than the fiberglass HOME-DOME. In these domes, the designer usually installs a track or pair of rails on the underside of the dome equator (and or on the top of the wall) to guide the wheels and reduce drag. Such tracks must be accurately circular and rigid. The tracks increase the overall dome weight, and often impose requirements for rigid and expensive walls.

With the shutter and dome rotation design challenges solved, a dome observatory has the advantages of all-sky viewing while providing excellent weather and stray light protection. If provided in modular components, the dome is fast to install, and requires the minimum space --especially important for on-structure installations. The disadvantages of a dome observatory are that its construction does involve use of compound curves which require precise manufacturing and good quality control. Inadequate designs for the shutter and dome rotation, or poor material choice for the dome, will produce a dome that is at best awkward to use, and at worst, unusable.

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