Completion of the Large Binocular Telescope Enclosure
José Terán U.a, James H. Slagle b, John M. Hill b, Daniel H. Neff a,
a M3 Engineering & Technology Corporation
2440 W. Ruthrauff, Suite 170 Tucson, AZ 85705, USA
b The University of Arizona, Large Binocular Telescope Observatory
Tucson, AZ 85721-0065, USA
ABSTRACT
The Large Binocular Telescope (LBT) under construction on Mount Graham, Arizona is a unique instrument, which supports two 8.4-meter primary mirrors on the same mount. The telescope mirrors will provide a collecting area equivalent to an 11.8 circular aperture plus a diffraction baseline of 22.8 meters. This unique instrument presented new enclosure challenges and configurations in order to accommodate the Owner’s design, telescope operating criteria and budget.
The LBT enclosure completed in the summer of 2002 provides useful information on the planning, designing and construction of a telescope enclosure. The use of a team approach by the contractors, engineers, and the project office has been successful in maintaining quality construction at a reasonable price. This paper discusses the various systems implemented on the LBT enclosure and the lessons learned during the course of the design and construction.
Keywords: Co-rotating enclosure, enclosure design, enclosure construction
1. INTRODUCTION
The Large Binocular Telescope (LBT) under construction on Mount Graham, Arizona is a unique instrument that supports two 8.4-meter primary mirrors on the same mount. This unique optical support structure configuration presented a challenge for a new design and construction techniques to complete the telescope enclosure. Because the telescope incorporates mirrors that have been spin cast, the resulting F/1.14 focal ratio of the primary mirrors allows the construction of a relatively compact telescope enclosure.
The general site selected is on Emerald Peak, the third highest peak in the Pinaleno Mountains of southeastern Arizona. The construction of two other telescopes, the Vatican Observatory 1.8 Lennon Telescope and the Heinrich Hertz Sub-Millimeter Telescope, were successfully completed in 1993, but the start of construction of the LBT was delayed due to legal concerns until July of 1996. The LBT site is located within an endangered species refugium area and is only 1.2 acres. Due to the physical constraints of the site, the enclosure developed into a unique facility with efficient use of space. The LBT Observatory located at the University of Arizona manages the LBT.
The telescope enclosure was originally planned to take a quarter of the total budget. Because of the site, the unique specifications and requirements of the building, and the extended construction schedule due to weather, the building eventually required 30 million US dollars, a third of the planned budget. Earlier estimates were made prior to the availability of the design drawings, and were based on two telescope enclosures that had been built on the mountain. The magnitude and scope of the LBT enclosure, construction specifications, and rising construction costs all resulted in an enclosure that would exceed earlier expectations. An earlier SPIE article (March 1998 – “Management and construction of the Large Binocular Telescope…”) outlines the strategy taken to keep construction costs under control.
2. PROJECT DESIGN CRITERIA
The basic enclosure design criteria are as follows:
Wind Speed Operational: 45 mph
Wind Speed with Shutters Open: 90 mph
Wind Speed Survival: 125 mph
Base Elevation: 10,470 feet above sea level
Snow Load: 90 psf
Ice Load: 60 psf
3. ENCLOSURE design
The enclosure is designed to fit the site and the telescope. Erection of the enclosure required that careful attention be paid to critical tolerances. With only 1.2 acres available for both the enclosure and lay down area, the design resulted in a co-rotating type enclosure. This design requires less volume, which is important since the site is very restrictive and difficult to access with large equipment. Since there is less overall volume, the floor area is less expensive; however, a steel floor system acts as a stiffener, reducing the amount of steel required overall. The interior design allows for easy movement of large pieces of equipment (10-meter diameter). Roller chain drives for the shutter and ventilation doors allow for a higher tolerance for structural deflections during the operation of the doors with the added benefit of requiring less machined steel. The shutter doors were also designed to act as stiffeners for the rotating portion of the building during survival conditions. This also eliminates the need for additional steel while providing an internal structural system. The design takes advantage of conventional framing and emphasizes the use of bolted connections instead of welded connections, which was important to reducing the fire risk in the sensitive forest environment.
The LBT enclosure consists of three separate structures consisting of the telescope pier, fixed building, and rotating enclosure. Two uniquely separated structures were built to support the telescope and its enclosure. At the very center is the pier. The 46-foot diameter pier is completely isolated from the other surrounding structures by a 2-inch air space. The pier is uniquely bonded to the granite of the mountain thirteen feet below ground level. The pier then rises 70 feet above this foundation providing a stiff telescope base avoiding the transfer of vibration from its neighboring surroundings. Thus, while the telescope rotates using the pier as its isolated base, the co-rotating building and its mechanisms will use a separate conical, but isolated concrete base.
The fixed building (non-rotating and not in contact with the pier) contains areas for important offices, living areas, control rooms and equipment rooms. This area is directly connected to a high bay auxiliary area, which will house support equipment needed for the telescope and its instrumentation. Due to the limited site area, the enclosure design took a vertical configuration with the fixed building directly below the rotating enclosure. Not only the size of the telescope site but the need to accommodate vehicular traffic, both in construction and receiving the large telescope pieces, was a large factor in site design.
The confined area at the site, as well as the need to keep as much of the heat away from the observing chamber, required a decision to move the LBT chillers and boiler to the MGIO utility building. The existing utility building had to be redesigned to accommodate upgrades to the electric equipment and to house the chillers, boilers, and pumps required for the LBT operations. The circulating chilled water pipes, heating water pipes, and other critical utility lines are buried beneath the access road between the LBT and the MGIO utility building. A nine-foot deep trench was constructed for over 600 feet to connect the buildings.
Due to the LBT electrical requirements, the existing electrical infrastructure at MGIO has been upgraded with a new 25KV-power line that begins at Bonita, Arizona and extends up to Mount Graham. The power line is approximately 23-miles long and is designed to provide 1-megawatt of continuous power at the observatory site. This capacity can handle the three existing observatories as well as future facilities. A new 800KW generator as well as the existing 250KW generator provide uninterrupted backup power.
The Large Binocular Telescope (LBT) enclosure with its characteristic rotating structure (silver) above the fixed building (green).
3.1 Fixed Enclosure
The fixed enclosure is a steel structure with metal siding as the exterior surface. All of the conditioned spaces are well insulated in order to keep the heat from dissipating to the observing chamber above. The ground level has a visitors lobby as well as the LBT offices. The electrical and mechanical rooms are also on the ground level with direct access to the road. The second level has the sleeping quarters on the north side and the main control room, conference room, kitchen and lounge on the south end. The third level is the runway for the rotating building bogies.
Also part of this structure is a 7,800 square foot auxiliary building that is uninsulated and used primarily for set up and staging of the telescope and instrumentation. Situated on the down wind side of the site and designed for semi truck access, the front entry façade has a 24-foot by 34-foot roll-up door and a 55-metric ton bridge crane inside. The clear height is 46 feet to the bottom of roof structure with the access hatch to the observing chamber directly above.
The advantages of the auxiliary building are many. The building is designed so that any size primary mover bringing to the site any piece of the telescope can enter the building and be safely unloaded by the bridge crane. If the piece is required in the observing chamber, the crane can move the piece to a point under the hatchway of the rotating building. The auxiliary building crane can be moved away and a second 55metric ton crane suspended from the top of the rotating building is lowered to move the piece into the observing chamber. It is also possible to store a great deal of equipment in the building by lifting the pieces to the third level. Another storage advantage is that the floor space has been designed to safely house the aluminizing equipment, which includes the bell jar and all of the associated vacuum and mirror cleaning equipment. The auxiliary building provides LBT flexibility in having room for assembly, storage and other important support activities.
3.2 Rotating Enclosure
The rotating enclosure, situated directly above the fixed building, is co-rotating, that is a structure that rotates with the telescope. The design allows for a 4-degree rotation between the building and the telescope. Co-rotation minimizes the size and overall weight of the structure required. Due to the unique telescope design, the building is a prism that is 95 feet by 105 feet by 115 feet in height. The total weight of the structure is approximately 2,000 tons.
The entire structure transfers all of the loads through a series of steel trusses and pylons to four bogies. Due to the eccentric load, the two front bogies have six 40-inch diameter wheels while the back bogies have four wheels. In turn, the bogies travel on a flat, hardened steel track and embed beam that is supported by a concrete cylinder. Each bogie has a lateral restraint mechanism centered on the assembly along with two 20 horsepower motors.
3.2.1
Mechanical Equipment Floor
The entire fourth level, which is the first floor of the rotating building, is dedicated to the telescope and dome mechanical and electrical equipment. The mirror support and ventilation mechanical equipment is equally divided into two separate areas with each side supporting one primary mirror. The dome cooling, electrical motor control center, PLC and electrical switchgear make up the rest of the space. Located directly below the observing chamber, this space is well insulated and ventilated. During telescope operation, any collection of warm air that has gathered in this space is exhausted down wind with ambient air from the observing chamber through the telescope structure and four large axial fans. The warm air is directed through one of four 10-foot diameter exhaust tubes on the exterior of the enclosure.
3.2.2 Observing Chamber
The telescope observing chamber is approximately 95 feet by 100 feet by 70 feet clear to the bottom of the 55-ton bridge crane. The observing floor elevation is 74 feet above grade level with the telescope elevation at 98 feet. These elevations are well above the tree height assuring that seeing will not be effected by the air turbulence created by the trees. Four large ventilation doors located on the two sidewalls and the back wall naturally ventilate the chamber. The approximate open area of all ventilation doors is 4,600 square feet with each door 65 feet in height.
Due to the binocular design of the telescope, there are two independent shutter doors with a portal section through the middle from the front of the structure to the back. Each shutter door provides a 32-foot wide opening that traverses from the observing floor in front to the top of the back wall. The back section of the portal provides space for the elevator shaft and a glass-enclosed visitor’s gallery that is approximately 23 feet above the observing floor. The portal roof section contains the enclosure snow melting equipment. This mechanical space as well as the elevator shaft is insulated from the observing chamber.
3.2.3 Shutter Doors
Performance Criteria / Loads:
Shutter Door Weight: 220 Kips
Shutter Speed: 2 Minutes per Cycle (16 Feet per Minute)
Shutter Drive:
Design Load: 17.5 Kips peak
Operating: 7.2 Kips typical
Shutter Bogie:
Design Load 142 Kips static
Operating: 95 Kips maximum
79 Kips typical
Shutter Lateral Restraint:
Design Load: 25 Kips
Max. Static Load: 55 Kips
The two shutter doors, one for each 8.4-meter primary mirror, are “L” shaped. The vertical leg of the shutter door is the front wall of the enclosure with the horizontal section forming the roof. Each shutter door utilizes two upper and lower bogie assemblies, one at each corner of the door. Each bogie assembly consists of tandem, 24-inch diameter idler wheels supported by a steel housing. Each bogie unit is designed to allow adjustment in camber, elevation and steering through a set of steel plates, one mounted to the shutter door structure and the other to the bogie. Four full threaded shanks between the two plates allow steering adjustment while elevation is adjusted through four
