Shutter Design
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Another design challenge for astronomy domes is the shutter opening. The most obvious issue is how the shutter will open and close, including the ability to operate it either manually or by an electric motor. A less obvious problem is how to construct the opening itself so that it will have sufficient rigidity: recall that the opening is in a spherical shape without much resistance to deformation.

When we try to design a shutter that opens past the zenith, we immediately run into another challenge: where does the opened shutter get stored? After all, if the dome is a hemisphere, a shutter opening that goes past the zenith requires a shutter that is longer than the storage space available on the back side of the dome. Will we just let the shutter hang down the back and perhaps flap in the wind? This design problem has had several different solutions:

• Provide one or more shutter sections that can be lifted from the dome and stored elsewhere. This solution will function for small domes, but in larger domes, the weight of the heavier shutters, and the effect of wind on their larger area, can cause major problems in handling.
• Use a flexible shutter that rolls or folds (in accordion pleats) out of the way. The problems are how to assure smooth operation under cold and ice conditions, obtaining a long life for the shutter, and providing weather tight operation along the shutter edges.

Figure 1.7 ALTERNATIVE SHUTTER DESIGNS

• Use sideways shutters. This design is used in many large domes. However, it does require coordinated movement of the top and bottom of the shutters on one or both sides of the opening. A strong frame must be attached to the dome to hold the shutters in both open and closed positions, and must function under all wind conditions. Sideways shutters have long joints susceptible to water leakage.
• Use a two piece non-nesting shutter. The shutter is in two pieces, but cannot be stored out of the way. In one design, the upper shutter is moved up and over the dome, and the lower shutter is flopped down (open). In an alternative design, the upper shutter alone is moved up, thus allowing use of the upper part of the shutter opening. If one desires to use the lower part of the shutter opening, the lower shutter is attached to the upper shutter which can pull it partway up. The major disadvantage is the inability to use the entire shutter opening, thus complicating the sequencing of observations at different altitudes, and reducing the amount of sky that is visible.
• Use a two piece nesting shutter. Both the front and top piece move together, with automatic disconnecting and nested storage on the back of the dome. This is the system (patent pending) used in HOME-DOME observatories, and provides for a fully open shutter, convenient storage of the shutters, and low wind resistance in both open and closed positions. And since the shutters straddle the shutter opening flanges, the system is weather tight without gaskets.

These are alternative designs for the mechanical design aspects of the shutter. But in addition to the physical opening and closing, the size of the shutter (i.e., the width of the slot) is of great importance. In general, you will want as wide a slot as feasible. A wider view of the sky allows more enjoyment of the night sky, and easier orientation for the observer. In addition, a wide slot allows you to use the telescope for longer periods, without turning the dome. However, as you might expect, the wider the slot, the tougher the design challenge!

No matter how this design problem is solved, a shutter opening represents a cut into the hemisphere. There is no physical "center" at the top of the dome (since the zenith is open), so all forces in the dome have to "go around" the shutter opening. This requires that the opening be strongly framed, so that neither the dome nor shutter opening will distort under load. The HOME-DOME solves this challenge with wide flanges along the shutter opening (used also to guide the shutter and to provide the weather tight operation) and with cross bracing at the top rear of the shutter opening.

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