The Large Binocular Telescope Project
J. M. Hilla & P. Salinarib
a LBT Project Office/USA
b LBT Project Office/Italy
The University of Arizona
Steward Observatory, Tucson AZ 85721-0065, USA
Osservatorio Astrofisico di Arcetri
Largo Enrico Fermi 5, 50125 Firenze, ITALY
Abstract:
The Large Binocular Telescope (LBT) Project is a collaboration between institutions in Arizona, Germany, Italy, and Ohio. The telescope will have two 8.4 meter diameter primary mirrors phased on a common mounting with a 22.8 meter baseline. The second of two borosilicate honeycomb primary mirrors for LBT is being cast at the Steward Observatory Mirror Lab this year. The baseline optical configuration of LBT includes adaptive infrared secondaries of a Gregorian design. The F/15 secondaries are undersized to provide a low thermal background focal plane which is unvignetted over a 4 arcminute diameter field-of-view. The interferometric focus combining the light from the two 8.4 meter primaries will reimage the two folded Gregorian focal planes to three central locations. The telescope elevation structure accommodates swing arm spiders which allow rapid interchange of the various secondary and tertiary mirrors as well as prime focus cameras. Maximum stiffness and minimal thermal disturbance were important drivers for the design of the telescope in order to provide the best possible images for interferometric observations. The telescope structure accommodates installation of a vacuum bell jar for aluminizing the primary mirrors in-situ on the telescope. The telescope structure is being fabricated in Italy by Ansaldo Energia S.p.A. in Milan. After pre-erection in the factory, the telescope will be shipped to Arizona in early 2001. The enclosure is being built on Mt. Graham under the auspices of Hart Construction Management Services of Safford, Arizona. The enclosure will be completed by late 2000 and ready for telescope installation.
Keywords: astronomical telescopes, interferometry, honeycomb mirrors
1 INTRODUCTION
1.1 Project Overview
All parts of the Large Binocular Telescope (LBT) are now under construction. The binocular design of LBT has two 8.4 meter telescopes mounted side-by-side on a common altitude-azimuth mounting. This provides LBT with a collecting area of 110 square meters -- equivalent to an 11.8 meter circular aperture. The two mirrors on a common mounting with a 14.4 meter center-center distance provide an interferometric baseline of 22.8 meters for diffraction-limited observations. By using fast focal ratio primary mirrors, the entire telescope and enclosure can be compact. The compact structure helps to increase stiffness and decrease the total project cost. The LBT Project has previously been described by Hill (1996) and by Hill & Salinari (1998) and references therein.
1.2 Partners
The international partners in the Large Binocular Telescope Corporation include Arizona (25%), Germany (25%), Italy (25%), Ohio State (12.5%) and Research Corporation (12.5%). The Arizona portion of the project includes astronomers from The University of Arizona, Arizona State University and Northern Arizona University. The German portion is represented by the LBT Beteiligungsgesellschaft which is composed of Max-Planck-Institut für Astronomie in Heidelberg, Max-Planck-Institut für Radioastronomie in Bonn, Max-Planck-Institut für extraterrestrische Physik in Munich and Astrophysikalisches Institut Potsdam. The Italian astronomical community is represented by the Osservatorio Astrofisico di Arcetri in Florence. Partners at individual institutions include The Ohio State University in Columbus, Research Corporation in Tucson and The University of Notre Dame.
1.3 Project Management
The LBT project is managed by two project offices which work closely together. LBT Project Office/USA is located in Tucson, Arizona overseen by project director, Dr. John Hill. LBT Project Office/Italy is located in Florence, Italy and overseen by deputy director, Dr. Piero Salinari. Dr. James Slagle manages the construction and administrative aspects of the project. Mr. Luciano Miglietta is the engineering coordinator in Italy. Mr. Scott DeRigne and Mr. Warren Davison are the chief electrical and mechanical engineers in Arizona, respectively. Contractual and financial functions are coordinated by the project office in Tucson. Mr. Sam Clapp is the contracts officer and Mr. Don Ferris is the program coordinator. Mr. John Waack is the mountain operations manager.
1.4 Budget and Schedule
The total project budget is $83.5 million ($1998) This budget includes approximately $5 million in instruments. The budget is about equally divided between the primary mirrors, the telescope, the enclosure and auxiliary optics. Additional instrumentation is funded by the partners and other agencies independent of the capital budget of the telescope.
The installation of the telescope in the enclosure is scheduled to begin during the spring of 2001. First light, with one primary mirror, is scheduled for early 2003. Second light, with two primary mirrors, is scheduled for mid 2004.
1.5 Instruments
Current plans for LBT facility instruments include a pair of optical/ultraviolet spectrographs described by Osmer et al. (2000) and a pair of near-infrared spectrographs described by Mandel et al. (2000). Several interferometric instruments are also under development. Ragazzoni et al. (2000) describe a pair of wide-field prime focus imagers.
2 PRIMARY MIRRORS
The primary mirrors for LBT are being fabricated by the Steward Observatory Mirror Lab at the University of Arizona. Each 8.4 meter diameter mirror will be a spincast borosilicate honeycomb structure weighing 16 metric tons. Each primary mirror has a clear aperture of 8.408 meters and has a paraboloidal surface with a focal length of 9.600 meters (F/1.142). The honeycomb structures provide light weight and good stiffness against wind forces. They also provide a short thermal time constant (45 minutes with ventilation) to be sure that the telescope images are not blurred by local mirror seeing.
2.1 Casting
The casting of the first 8.4 meter mirror in 1997 has been described by Hill et al. (1998). The ``E6'' borosilicate glass for these mirrors is made by Ohara in Japan. A leak from the mold during the initial casting resulted in the need to remelt an additional two tons of glass onto the faceplate. The cause of the leak was unexpected friction between the interwoven sets of Inconel 601 bands that support the mold against the hydrostatic pressure of the molten glass. The remelt was successful in the summer of 1997, and the mirror blank now has a complete faceplate of appropriate thickness. The mirror blank has been removed from the furnace, and the mold material has been removed from the honeycomb cores. A photo of the mirror hanging in the vertical position is shown in Figure 1. The mold for the second primary mirror blank for LBT has been completed and it is scheduled to be cast in the summer of 2000. Figure 2 shows the installation of the last of the 1662 honeycomb cores into the mold for the second 8.4 meter casting.
2.2 Polishing
Preliminary finishing work on the first 8.4 meter mirror has been completed in 1999. The back surface of the mirror has been machined to the final dimensions and the surface treated by polishing. The figuring of the front surface will begin in 2001. Results of the recent figuring and integration of the Magellan 6.5 meter mirror are reported by Martin et al. (2000).
Figure 1. The completed 8.4 meter primary mirror blank hanging in the
handling ring at the Steward Observatory Mirror Lab. This view shows the
flat back side of the borosilicate honeycomb blank. Photo by Lori Stiles.
Figure 2.
The installation of the last aluminosilicate fiber core into the
mold for the second 8.4 meter primary blank in January 2000.
Randy Lutz and John Martin are installing the cores. A bolt made of
silicon carbide is used to keep each core from floating in the molten glass.
Cross pins of aluminosilicate fiber connect between the cores to keep them
from tipping. The structure to the left is a bridge used to access the
mold from above. Photo by Ray Bertram.
2.3 Handling and Support
Davison, Williams & Hill (1998) describe the fixtures and procedures that are used for handling and transportation of 6.5 meter and 8.4 meter mirrors. The mirrors are lifted from the furnace and turned using a fixture which has 36 steel pads glued to the faceplate of the mirror with silicone rubber. After optical finishing begins, the mirrors are lifted with a 36-pad vacuum system. The mirrors will be shipped to the mountain in a double-frame vibration-isolated box. The 8.4 meter mirrors are supported in the telescope cell by active adjustable pneumatic supports. The mirrors are supported at 158 axial support positions. The lateral supports are applied to the backplate of the mirror with the overturning moment compensated by the axial supports. The mirrors are positioned in the cells by rigid hardpoints which kinematically position the mirror. The entire weight of the mirror cannot be safely supported on the hardpoints so they must be engineered to collapse above a certain load. The pneumatic support system will be built by Steward Observatory. The hardpoints are being built by Max-Planck-Institut für extraterrestrische Physik.
Figure 3.
The 9 meter diameter aluminizing bell jar in fabrication at
Ansaldo Energia in Milan.
The primary mirrors will be aluminized in-place on the telescope
structure. This portable bell jar can be positioned with the overhead
crane and sealed against either of the two primary mirror cells. The bell jar
will contain liquid nitrogen cryopumps and crucibles for evaporating
the aluminum. The internal components of the aluminizing bell jar are
being designed and integrated at Ohio State University. To strip the old coating
off
the mirror, a washing station has been designed to spray the
appropriate chemicals to dissolve the previous coating.
3 SECONDARY MIRRORS AND OTHER OPTICS
3.1 F/15 Gregorian Adaptive Secondaries
The baseline telescope is designed with a pair of F/15 Gregorian secondary mirrors. The design is optimized for minimum thermal background, so the secondary mirrors are undersized to serve as the aperture stops of the system with a clean pupil projected against the sky. The field-of-view is unvignetted over a 4 arcminute diameter. Each secondary is 91 cm in diameter and places the Gregorian focal planes 305 cm behind the primary vertices. The LBT will go into operation with two fully adaptive secondary mirrors. These mirrors have 1.6 mm thin meniscus shells whose shape is controlled by 918 voice coil actuators and magnets servoed to a reference surface with capacitive distance sensors. An advantage of the Gregorian optics is that the adaptive secondaries can be tested independently of the sky from real conjugate points. By making the secondary mirrors the adaptive element, we can feed the light into a cooled infrared interferometric instrument with only three warm reflections. The design of the secondary mirrors is reported by Gallieni & Del Vecchio (2000) and Del Vecchio & Gallieni (2000). Each of the main Gregorian focal stations has a 3 meter diameter instrument rotator bearing and can accommodate instruments up to 4.4 meters in length. Swing arm spiders allow the secondary mirrors to be exchanged with other secondary configurations in less than 30 minutes.
Figure 4.
This figure shows the three dimensional engineering layout of the
LBT telescope structure. This view shows the back side of the
telescope with the swing arms for the primary mirror covers, for the
tertiary mirrors, for the prime focus cameras, and for the adaptive
secondary mirrors.
3.2 Tertiary Mirrors and Combined Foci for Interferometry
Two 64 x 54 cm tertiary flat mirrors also on swing arm spiders can redirect the Gregorian focal planes to any of three pairs of focal stations in the center of the telescope as shown in Fig. 3. The tertiary mirrors will be fused borosilicate honeycomb blanks made by Hextek in Tucson. The design of the tertiary mirror mounting is described by Gallieni et al. (2000).
The main purpose of these bent focal stations is for combining the beams in interferometric mode, but the middle position also provides a quasi-Nasmyth gravity-invariant instrument mounting. These bent Gregorian or combined focal stations each have 1.8 meter diameter rotator bearings. Additional details of the plans for interferometric beam combination have been provided by McCarthy et al. (2000), Herbst et al. (2000) and Correia et al. (2000) along with previous papers by Angel et al. (1998), Salinari (1996), Byard & Bonaccini (1994) and Hill (1994). Within the first few years of operation, we expect that LBT will be producing diffraction-limited images from the thermal infrared down to visible wavelengths. Walker et al. (2000) report also on plans for using LBT at submillimeter wavelengths.
3.3 Acquisition, Guiding and Wavefront Sensing
To support both active and adaptive optics on LBT, acquisition, guiding, and wavefront (AGW) units are being built above the instruments by Astrophysikalisches Institut Potsdam. These units provide conventional off-axis guiding plus wavefront sensing for active optics and an acquisition imager. These units also provide for fast wavefront and tip-tilt sensing for adaptive optics using both natural and artificial stars. The details of the AGW units are provided by Storm et al. (2000).
3.4 Laser Guide Star Projectors
Flat mirrors above the F/15 adaptive secondaries will be used for projecting sodium laser guide stars for use as the wavefront reference for adaptive optics. Beam expanders will be located on the telescope elevation structure while the sodium laser(s) will be located in a quasi-coudé position inside the telescope pier.
4 TELESCOPE STRUCTURE
4.1 Design
The detailed design of the telescope structure has been done by European Industrial Engineering (Mestre) and ADS International (Lecco) in cooperation with Steward Observatory (Tucson) and Arcetri Observatory (Florence). These two companies also act as LBT technical representatives during the manufacturing and assembling of the telescope. Including the optics and instruments, the total moving mass of the telescope is approaching 600 metric tons. The predicted lowest resonant frequency is near 8 Hertz. The telescope is driven by four motors on each axis driving pinions against a rim gear with a diameter of 13 meters.
4.2 Fabrication and Pre-Erection
The telescope structure is being fabricated and pre-erected in the factory by Ansaldo Energia S.p.A. of Milan, Italy. This fabrication includes the 14 meter diameter track which serves as the azimuth bearing of the telescope; the azimuth frame of the telescope on which the hydrostatic bearings and motors are mounted; the large rolling sectors or C-rings which act as the elevation bearings; the central windbracing elevation structure and the primary mirror cells. Ansaldo is also fabricating the steel structure of the vacuum bell jar for aluminizing. The schedule calls for the LBT structure to be pre-erected in the Ansaldo factory by late 2000. Already the pre-erection of the azimuth ring and the azimuth frame have been completed. The large rolling sectors (C-rings) that form the elevation bearings are now being machined. Additional details on the telescope structure and its pre-erection are provided by Miglietta et al. (2000). The hydrostatic support pads and pumping system have been built by Tomelleri S.r.l. of Verona, Italy. Tomelleri is also the subcontractor for the sectors of the rim gears. The housings for the Kollmorgen drive motors have been made by Castellini Officine Meccaniche in Cazzago S. Martino, Italy. The swing arm spiders are being made by Carpenteria Fratelli Colombo in Monte Marenzo, Italy.
5 SITE AND ENCLOSURE
5.1 Site
The site for LBT is Emerald Peak at an altitude of 3191 meters (10470 feet) in the Pinaleno Mountains of southeastern Arizona. The telescope is part of the Mt. Graham International Observatory (MGIO) operated by the University of Arizona and other partners. MGIO presently consists of three telescopes, a utility building and the 3.2 km access road, and occupies 8.6 acres of national forest land.
Figure 5.
This photo shows the assembling of the azimuth frame over the azimuth ring of t
he LBT
in the Ansaldo Energia factory in Milan. The azimuth frame is divided in
three sections for shipping and the azimuth ring is split into eight sectors.
The azimuth frame corresponds to the yoke of a telescope with more
conventional geometry. The azimuth pads of the hydrostatic bearing system
are positioned on the azimuth ring. Mr. Bortolamei is the floor manager
of the erection process.
5.2 Enclosure Design
The enclosure for LBT was designed by the architectural and engineering firm M3 Engineering & Technology (Tucson) in cooperation with ADS International and European Industrial Engineering. The basic design is a corotating box structure with two laterally sliding shutters. The telescope elevation axis is located 30 meters above the ground to be sure that seeing is not influenced by the turbulent boundary layer at treetop height. The telescope chamber provides for natural ventilation with several large doors in the sides and the back to allow airflow through the enclosure. Air is drawn through the telescope structure and exhausted from the enclosure in the downwind direction to minimize seeing effects from the massive steel structures. The fixed building underneath the rotating building contains the control room, a few offices and limited living quarters. An unheated high bay space is attached to the enclosure in the prevailing downwind direction. This auxiliary building provides space for storage of the aluminizing bell jar, instruments and other equipment.
Handling of heavy instruments, mirror cells and the aluminizing bell jar is accomplished with an overhead crane in the telescope chamber which has two 32-ton hooks. A similar crane is located in the high bay area of the fixed building for unloading trucks. A 4 x 10 meter hatch allows the transfer of equipment between the fixed and rotating buildings. The cranes have been fabricated by Lario Impianti S.r.l. in Lecco, Italy.
Figure 6.
The LBT enclosure on Emerald Peak in January 2000. The insulated metal panels t
hat
form the skin of the rotating building have been installed. This view shows the
ventilation doors on the left side and rear of the
structure, the rails for the sliding shutters, the elevator shaft on the rear of
the structure, and the downwind exhaust ducts.
5.3 Enclosure Construction
The structural steel for the enclosure has been fabricated and erected by Schuff Steel of Phoenix, Arizona. The concrete work on the telescope pier and enclosure ring wall has been contracted to Peterson-Weaver Concrete of Mesa, Arizona. The fixed building and the telescope pier were completed in 1997. The fixed building cladding has been installed by Sure Steel of Salt Lake City, Utah. The general contractor is Hart Construction Management Services of Safford, Arizona. The 23 meter diameter embedded beam and circular rail for the enclosure have been fabricated by Fravit S.r.l. of Lecco, Italy. The four rotation bogies have been made by Costameccanica S.p.A. of Lecco, Italy and SIAG S.r.l of Milan, Italy. Most of the 1998 and 1999 construction seasons have been spent installing the structure of the rotating building. The steel was erected by Schuff Steel. The insulated metal panel siding was installed by The Engineered Metals Company of Sacramento, California. The steel cap on the telescope pier and the shutter latches have been provided by Vertex - New Mexico of Albuquerque, New Mexico. The interior of the telescope enclosure should be completed by the end of 2000. The mechanical systems are being provided by R. E. Lee Mechanical of Tucson, Arizona. The electrical systems are being provided by Gilbert Electric of Tucson, Arizona. The interior finishes are being done by Emerald Peak Enterprises of Mesa, Arizona. Additional details of the enclosure construction and management are provided by Slagle et al. (2000) and Slagle et al. (1998).
6 CONCLUSION
The optics, telescope and enclosure of the Large Binocular Telescope are all under construction. The first 8.4 meter borosilicate honeycomb mirror blank is in optical finishing. The second of the two 8.4 meter primary mirror blanks is scheduled for casting in 2000. The enclosure on Emerald Peak is under construction and should be completed by the end of 2000. The telescope mechanical structure is being fabricated in Milan, Italy and will be shipped to Arizona in early 2001. The first primary mirror should arrive at the telescope in late 2002. First light is expected in early 2003 with second light to follow in 15 months.
Up to date images of the LBT construction can be found on the world wide web at URLs
| Large Binocular Telescope | http://medusa.as.arizona.edu/lbtwww/ |
| Mt. Graham International Observatory | http://medusa.as.arizona.edu/graham/ |
| Steward Observatory Mirror Lab | http://medusa.as.arizona.edu/mlab/ |
REFERENCES
- Hill, J. M. 1996, ``The Large Binocular Telescope Project'', Proc. S.P.I.E., 2871, pp. 57-68.
- Hill, J. M. & Salinari, P. 1998,
``The Large Binocular Telescope Project'',
Proc. S.P.I.E., 3352, pp. 23-33. - Osmer, P., et al. 2000,
``Multiobject double spectrograph for the Large Binocular Telescope'',
Proc. S.P.I.E., 4008, (Companion Proceedings). - Mandel, H., et al. 2000,
``LUCIFER: a NIR spectrograph and imager for the LBT'',
Proc. S.P.I.E., 4008, (Companion Proceedings). - Ragazzoni, R., et al. 2000,
``Double prime focus camera for the F/1.14 Large Binocular Telescope'',
Proc. S.P.I.E., 4008, (Companion Proceedings). - Hill, J. M., et al. 1998, ``Casting the first 8.4 meter borosilicate honeycomb mirror for the Large Binocular Telescope'', Proc. S.P.I.E., 3352, pp. 172-181.
- Martin, H. M., et al. 2000,
``Primary mirror system for the first Magellan telescope'',
Proc. S.P.I.E., 4003, (Companion Proceedings). - Davison, W. D., Williams, J. T. & Hill, J. M. 1998,
``Handling up to 20 tons of honeycomb mirror with a very gentle touch'',
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``Near-infrared image-plane beam combiner for the LBT'',
Proc. S.P.I.E., 4006, (Companion Proceedings). - Correia, S., et al. 2000,
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Proc. S.P.I.E., 4006, (Companion Proceedings). - Angel, J. R. P., et al. 1998, ``Large Binocular Telescope interferometer'', Proc. S.P.I.E., 3350, 881-889.
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Proc. S.P.I.E., 2200, pp. 248-259. - Walker, C., et al. 2000,
``Submillimeter-wave receiver system for the Large Binocular Telescope'',
Proc. S.P.I.E., 4015, (Companion Proceedings). - Storm, J., et al. 2000,
``Wavefront sensing and guiding units for the Large Binocular Telescope'',
Proc. S.P.I.E., 4007, (Companion Proceedings). - Miglietta, L., et al. 2000,
``The Large Binocular Telescope pre-assembly'',
Proc. S.P.I.E., 4004, (These Proceedings). - Slagle, J. H., et al. 1998, ``Management and construction of the Large Binocular Telescope enclosure: meeting unusual challenges with a competitive discipline'', Proc. S.P.I.E., 3352, pp. 334-343.
- Slagle, J. H., et al. 2000,
``Nearly completed Large Binocular Telescope facility yields many lessons learned'',
Proc. S.P.I.E., 4004, (These Proceedings).
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The Large Binocular Telescope Project
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