The Large Binocular Telescope Project

J. M. Hill & P. Salinari

LBT Project Office/USA
The University of Arizona
Steward Observatory, Tucson AZ 85721-0065, USA

LBT Project Office/Italy
Osservatorio Astrofisico di Arcetri
Largo Enrico Fermi 5, 50125 Firenze, ITALY

TABLE OF CONTENTS

1. Introduction
2. Primary Mirrors
3. Secondary Mirrors and Other Optics
4. Telescope Structure
5. Site and Enclosure
6. Conclusion

Abstract:

The Large Binocular Telescope (LBT) Project is a collaboration between institutions in Arizona, Germany, Italy, and Ohio. With the addition of the partners from Ohio State and Germany in February 1997, the Large Binocular Telescope Corporation has the funding required to build the full telescope populated with both 8.4 meter optical trains. The first of two 8.4 meter borosilicate honeycomb primary mirrors for LBT was cast at the Steward Observatory Mirror Lab in 1997. 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. 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 arms which allow rapid interchange of the various secondary and tertiary mirrors. 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 detailed design of the telescope structure was completed in 1997 by ADS Italia (Lecco) and European Industrial Engineering (Mestre). A series of contracts for the fabrication and machining of the telescope structure had been placed at the end of 1997. The final enclosure design was completed at M3 Engineering & Technology (Tucson), EIE and ADS Italia. During 1997, the telescope pier and the concrete ring wall for the rotating enclosure were completed along with the steel structure of the fixed portion of the enclosure. The erection of the steel structure for the rotating portion of the enclosure will begin in the Spring of 1998.

KEYWORDS: astronomical telescopes, interferometry, honeycomb mirrors

1. INTRODUCTION

1.1 Project Overview

Now that the original Multiple Mirror Telescope (MMT) has been refurbished with a single 6.5 meter mirror (see West et al. 1996), the Large Binocular Telescope (LBT) now represents a new generation of ``multiple mirror'' telescopes -- or more correctly ``multiple telescope'' telescopes. 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 also 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 references therein

1.2 Partners

The international partners in the Large Binocular Telescope Corporation include Arizona (25%), Germany (12.5%), Italy (25%), Ohio State (12.5%) and the 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 and the Research Corporation in Tucson. With the addition of the partners from Ohio State and Germany in February 1997, the Large Binocular Telescope Corporation now has the funding required to build the full telescope populated with both 8.4 meter optical trains. One-eighth of the project remains uncommitted although Germany holds an option on that share.

1.3 Project Management

1.4 Design Drivers

The design drivers for the LBT Project have included: a large total collecting area for sensitivity on faint objects; excellent image quality through passive, active and adaptive control of seeing and other aberrations; a low background derived from a clean thermal infrared design; versatile instrumentation and the possibility of rapid changes in configuration to take maximum advantage of specific observing conditions; a high ultimate spatial resolution achieved by providing a relatively long baseline with the stability and field-of-view provided by a common mounting; and low construction and operations cost.

1.5 Budget and Schedule

The total project budget is $63.5 million ($1989) or about $80 million current dollars. This budget includes approximately $10 million in instruments. The budget is about equally divided between the primary mirrors, the telescope, the enclosure and auxiliary optics plus instrumentation.

The installation of the telescope in the enclosure is scheduled to begin during the summer of 2000. First light, with one primary mirror, is scheduled for spring 2002. Second light, with two primary mirrors, is scheduled for late 2003.

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 to be sure that the telescope images are not blurred by local seeing.

2.1 Casting

The casting of the first 8.4 meter mirror in 1997 is 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 remelt was successful in the summer of 1997 and the mirror blank now has a complete faceplate of appropriate thickness. A photo of the mirror on the furnace is shown in Fig. 1. The mirror blank has been removed from the furnace and now awaits removal of the mold material from the honeycomb cores. The second primary mirror blank for LBT is scheduled to be cast in 1999.

Figure 1. The completed 8.4 meter primary mirror blank on the furnace at the Steward Observatory Mirror Lab in October 1997. Optician Bryan Smith in the center hole contemplates the polishing work that lies ahead. Photo by Dean Ketelsen.

2.2 Polishing

Preliminary finishing work on the first 8.4 meter mirror will begin in 1999 although the final polishing will not be complete until 2001. The Mirror Lab has just finished work on the MMT 6.5 meter F/1.25 mirror. Results of the polishing have been reported by Martin et al. (1998a). The polishing of the fast parabolic surfaces to accuracy is made possible by use of the stressed lap which adjusts its shape to the local curvature of the surface. Integration of the MMT mirror into its telescope cell is reported by Martin et al. (1998b).

2.3 Testing

Naturally, optical testing of an 8.4 meter F/1.142 paraboloid is a nice challenge to go along with polishing. Burge (1998) describes some designs that are under consideration for the LBT null correctors. For the binocular telescope, the optical shop has the additional requirement of making two mirrors as a matched set.

2.4 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.

Both the 6.5 meter and 8.4 meter mirrors are supported in the telescope cell by active adjustable pneumatic supports. The 8.4 meter mirror is 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. LBT plans also to add some active vibration damping to these hardpoint units as described by Miglietta et al. (1997).

2.5 Aluminizing

The primary mirrors of LBT will be aluminized in-place on the telescope structure. A portable bell jar can be positioned with the crane and sealed against either of the mirror cells. The bell jar will contain liquid nitrogen cryopumps and crucibles for evaporating the aluminum. The internal components of the aluminizing bell jar will be designed and integrated at Ohio State. To strip the old coating off the mirror, a washing station is being designed to spray the appropriate chemicals to dissolve the previous coating. The spray stripping concept has been described by Zitelli & Ciattaglia (1997).

3. SECONDARY MIRRORS AND OTHER OPTICS

3.1 F/15 Gregorian Focal Stations

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. 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. These alternate configurations might include oversized F/15 secondaries or wide-field F/4 optical secondaries.

3.2 Adaptive Secondaries

Because of the recent successes in building prototype adaptive secondaries as reported by Salinari & Sandler (1998) and Brusa et al. (1998), it appears very likely that LBT will go into operation with two fully adaptive secondary mirrors. These mirrors have 2 mm thin meniscus shells whose shape is controlled by 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.

Figure 2. The Large Binocular Telescope features two 8.4 meter diameter primary mirrors. The structure includes two F/15 Gregorian foci. Tertiary mirrors provide access to several central F/15 focal stations for interferometric use of the two halves of the telescope. (Drawing by ADS Italia, rendering by MPIA Heidelberg).

3.3 Tertiary Mirrors

Two 63 x 50 cm tertiary 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 main purpose of these 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 Angel et al. (1998), Salinari (1996), Byard & Bonaccini (1994) and Hill (1994).

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 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 was completed in 1997 by ADS Italia (Lecco) and European Industrial Engineering (Mestre) 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.

4.2 Fabrication

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 early 2000. Additional details on the telescope structure are provided by Marchiori et al. (1998)

The hydrostatic supports pads and pumping system are being built by Tomelleri S.r.l. of Verona, Italy. We will be contracting for some smaller components of the telescope such as spiders during 1998.

4.3 Thermal Control

Active thermal control of the primary mirrors is provided by forced circulation of air in each cell of the honeycomb. The adaptive secondary mirror units, where a considerable power is dissipated, are controlled by circulation of fluid. Cooling fluid is provided also for instruments at each of the focal stations. The parts of the telescope structure that have thermal time constant shorter than about one hour are passively controlled by natural air ventilation and are covered with low emissivity Aluminum to reduce radiative exchange with the cold sky. Massive parts of the telescope, such as the elevation rolling sectors and the azimuth platform are shielded from direct exchange with ambient air. A flow of air of about 50 between the shields and the structural parts is used to reduce temperature differences between the shielded and non-shielded structural elements and to remove heat produced by telescope drives and other heat sources within the shielded volumes.

5. SITE AND ENCLOSURE

5.1 Site

The site for LBT is Emerald Peak at an altitude of 10470 feet (3191 meters) 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 consists of three telescopes, a utility building and the 2-mile access road, and occupies 8.6 acres of national forest land. The LBT site occupies 1.2 acres out of the 8.6 acre total.

Figure 3. This figure shows the optical layout of the LBT bent Gregorian focal stations. These foci can be used for independent instruments or for various kinds of beam combination to realize the 22.8 meter diffraction baseline.

5.2 Enclosure Design

The enclosure for LBT was designed by the architectural and engineering firm M3 Engineering & Technology of Tucson, Arizona in cooperation with ADS Italia of Lecco, Italy and European Industrial Engineering of Mestre, Italy. The basic design is a corotating box structure with two sliding shutters. This rotating box is positioned over a fixed building. The telescope elevation axis is located 98 feet (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 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. For thermal reasons and because of limited acreage on the site, the fixed building is fairly compact. It 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 are being fabricated by Lario Impianti S.r.l. in Lecco, Italy.

5.3 Past Enclosure Construction

After some delays due to legal and environmental issues, construction on the LBT site got underway in July 1996. During the fall of 1996, the site was cleared, a retaining wall was constructed, loose rock was excavated down to bedrock, and concrete for the pier foundation was poured. The general contractor / construction manager is Mr. Wood Hart of Hart Construction Management Services of Safford, Arizona. Additional details of the enclosure construction are provided by Slagle et al. (1998).

In the spring of 1997 the concrete pier of the telescope was completed along with the circular concrete ring wall which supports the rotating building. The concrete work was contracted to Peterson-Weaver Concrete of Mesa, Arizona. In the fall of 1997, the structural steel frame for the fixed portion of the enclosure was erected. The structural steel is being fabricated and erected by Schuff Steel of Phoenix, Arizona. The fixed enclosure as of December 1997 is shown in Fig. 4.

Figure 4: The construction of the enclosure for the Large Binocular Telescope on Emerald Peak. The Structural steel for the fixed building has been erected and the concrete pier for the telescope can be seen in the center. This photo from December 1997 represents the completeion of the construction season after the first snowfall.

5.4 Future Enclosure Construction

Even in Arizona, the winters are vigorous at elevations of 3200 meters, so the normal construction season on Mt. Graham runs from April through October. Jim Slagle can be seen in Fig. 5 inspecting the snow on site in March 1998. Plans for the 1998 construction season include placement of the embedded beam, circular rail and bogies for the rotating building, erection of the rotating steel structure, and enclosing the structure with roofing and siding. The 23 meter diameter embedded beam and circular rail have been fabricated by Fravit of Lecco, Italy. A sector of the circular rail is shown in Fig. 6. The four rotation bogies are being made by Costamasnaga S.p.A. of Lecco, Italy and SIAG S.r.l of Milan, Italy. The fixed building cladding will be installed by Sure Steel of Salt Lake City, Utah. The interior of the telescope enclosure should be completed by the end of 1999.

Figure 5: Jim Slagle inspects the snow level inside the LBT high bay structure on Emerald Peak on March 3, 1998. The winter of 1997/98 was fairly typical with about 2 meter accumulation of snow. Photo by Wood Hart.
Figure 6: A sector of the 23 meter diameter circular rail for the LBT enclosure being machined at Fravit S.r.l. in Lecco, Italy. The 9 rail sectors are numerically machined from ASTM A517-64 "T1" steel. Photo by Fravit.

6. CONCLUSION

Construction of the optics, telescope and enclosure of the Large Binocular Telescope are well underway. The first 8.4 meter borosilicate honeycomb mirror blank has been removed from the furnace. The second of the two 8.4 meter primary mirror blanks is scheduled for casting in 1999. The first primary mirror should arrive at the telescope in 2002. The telescope structure is being fabricated in Milan, Italy and will be shipped to Arizona in 2000. The enclosure on Emerald Peak is under construction and should be completed by the end of 1999. First light is expected in 2002 with second light to follow in 18 months.

Up to date images of the LBT construction can be found on the world wide web at URLs

REFERENCES

1. West, S. C., et al. 1996, ``Towards first light for the 6.5-m MMT telescope'', Proc. S.P.I.E., 2871, pp. 38-48.
2. Hill, J. M. 1996, ``The Large Binocular Telescope Project'', Proc. S.P.I.E., 2871, pp. 57-68.
3. 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, (These Proceedings).
4. Martin, H. M., et al. 1998a, ``Fabrication and measured quality of the MMT primary mirror'', Proc. S.P.I.E., 3352, (These Proceedings).
5. Martin, H. M., et al. 1998b, ``Active supports and force optimization for the MMT primary mirror'', Proc. S.P.I.E., 3352, (These Proceedings).
6. Burge, J. H. 1998, ``Design of null test optics for an 8.4-m, f/1.1 paraboloidal mirror'', (in preparation).
7. Davison, W. D., Williams, J. T. & Hill, J. M. 1998, ``Handling up to 20 tons of honeycomb mirror with a very gentle touch'', Proc. S.P.I.E., 3352, (These Proceedings).
8. Miglietta, L. Biliotti, V., Braccesi, C. & Pante, V. 1997, ``Large Binocular Telescope: the control system of the position actuators of the 8.4-m borosilicate honeycomb primary mirrors'', Proc. S.P.I.E., 3112, pp. 132-140.
9. Zitelli, V. & Ciattaglia, S. C. 1997, ``An automatic washing machine to remove aluminum from a very large mirror'', Bologna Technical Report 12-1997-03 (or LBT Technical Memo OAB-97-02).
10. Salinari, P. & Sandler, D. G. 1998, ``High-order adaptive secondary mirrors: where are we?'', Proc. S.P.I.E., 3353, (Companion Proceedings).
11. Brusa, G., et al. 1998, ``Optical testing results of a reduced-size adaptive secondary prototype'', Proc. S.P.I.E., 3353, (Companion Proceedings).
12. Angel, J. R. P., et al. 1998, ``Large Binocular Telescope interferometer'', Proc. S.P.I.E., 3350, (Companion Proceedings).
13. Salinari, P. 1996, ``The Large Binocular Telescope Interferometer'', Proc. S.P.I.E., 2871, pp. 564-574.
14. Byard, P. & Bonaccini, D. 1994, ``Optical design for interferometry with the Large Binocular Telescope'', Proc. S.P.I.E., 2200, pp. 446-457.
15. Hill, J. M. 1994, ``Strategy for interferometry with the Large Binocular Telescope'', Proc. S.P.I.E., 2200, pp. 248-259.
16. Marchiori, G., et al. 1998, ``Mechanical structure of the Large Binocular Telescope'', Proc. S.P.I.E., 3352, (These Proceedings).
17. Slagle, J. H., et al. 1998, ``Management and construction of the Large Binocular Telescope enclosure: meeting unusual challenges with a competetive discipline'', Proc. S.P.I.E., 3352, (These Proceedings).

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