The telescope dome as constructed has the appearance of a truncated sphere 56 feet in diameter and 48 feet high. The unusual space-frame structure of the dome, with aluminum panels, was the result of a visit by Bob Leighton to the "Spruce Goose" at Long Beach, which was housed in a massive dome. The exceptionally wide aperture slot - actually 63% of the dome diameter - posed significant problems of structural design, particularly with respect to survival in extreme wind storms and operability after wind-driven freezing rain and snow. On the other hand, the structural problem was turned around to an advantage in that the dome was designed to include within it at least six good-sized rooms as part of the intrinsic framework. The rooms provide all the necessary space needed to operate the telescope, including a control room, laboratories, workshop, lounge, kitchen, toilets, and storage areas. The final structure was the compromise result of a protracted argument between Phillips and Leighton, Phillips arguing for removal of cross girders to allow room for the astronomers to work, but Leighton arguing for the extra strength provided by the internal girders.
The design concept features a modular, bolt-together approach, with relatively few different types of components. During 1984 the dome was initially assembled, partially outfitted and tested at Caltech in Pasadena. Then it was disassembled, shipped to Hawaii along with the disassembled telescope, and reassembled on Mauna Kea during 1985 to 1987. A critical part of the dome is the system of shutters that cover the aperture and protect the telescope during daylight and bad weather. The final design consists of two doors that move tangentially along tracks on either side of the aperture. The upper door covers the zenith opening and slides backward over the rear of the dome. The lower door slides up and over the zenith, going underneath the upper door. Since it has twice as far to go, it was designed to move at twice the speed of the upper door. The movement of the doors is imparted by a pulley and cable system. A set of steel cables is on either side of the aperture and each set is operated by a hydraulic piston. The alignment of the two sets of tracks and the corresponding rollers on each door is very critical and has been the source of some operational problems. It was primarily for this reason that the practice assembly was carried out at the Pasadena site. Lowering the shutter doors from a crane onto the tracks was found to be impractical when a slight breeze kept the load swinging about. Consequently, on Mauna Kea, the shutter doors were assembled in place. (See Figure 19 in the Photo Gallery section.)
The entire dome rotates on a circular track at ground level, driven by four motors. Although totally separate from the telescope and its foundation, the dome tracks the telescope in azimuth so that its opening is always centered on the telescope. Should this tracking fail, limiting switches will stop the telescope and prevent it from colliding with the dome. Along the base of the dome is a heavy rubber apron that contacts the concrete slab surrounding the dome. This prevents the wind, dirt, and snow from getting under the dome. During storms, the snow will pile up against this apron and must be removed as soon as possible, otherwise it will begin to melt and refreeze, sealing the dome to the ground. Before this takes place, some quick snow-removal action must be taken. Snow blowers are not readily available in Hawaii, so on a trip to the mainland a snow blower was purchased for shipment to Hawaii, to the amazement of the clerks!
Figure 6. Power cables to the dish are wrapped around the base of the telescope so that they wrap or unwrap as the telescope rotates.
Because the dome rotates, there is a bit of a problem with the utilities. Power to the telescope comes via cables wrapped around the base of the telescope. Power to the dome comes through some very large slip-rings concentric with the base of the telescope. Water is supplied from a holding tank on the 4th level of the dome. Water from a large underground tank outside the dome is periodically pumped into the holding tank, as needed. The outside storage tank is supplied by water brought up the mountain by commercial suppliers. For sewage, there is a holding tank under the dome not unlike that of a mobile home. Once a week, or as needed, this is connected to an external cesspool. These inconveniences are a nuisance but minor compared to the functionality and efficiency of the compact rotating dome.
Figure 7. At this stage of the construction, the space frame design is clearly evident. Triangular aluminum panels form the skin of the dome.
The skin of the dome is just that - it has no structural function except to enclose the building in a spherical shape to make it weatherproof.