The Large Binocular Telescope Project

J. M. Hill

University of Arizona, Large Binocular Telescope Project
Steward Observatory, Tucson, AZ 85721
email: jhill@as.arizona.edu
http://medusa.as.arizona.edu/lbtwww/tech/spielbt.htm

Proceedings of SPIE conference on Optical Telescopes of Today and Tomorrow, 2871, (1996)

Abstract

1. LBT Partners and Funding

2. Design Drivers

3. Optical Configuration

4. Telescope

5. Enclosure

6 Observatory Site

7. Primary Mirrors

8. Conclusions

9. References

Abstract

 The Large Binocular Telescope (LBT) Project has evolved from concepts first proposed in 1985. The present partners involved in the design and construction of this 2 x 8.4 meter binocular telescope are the University of Arizona, Italy represented by the Osservatorio Astrofisico di Arcetri and the Research Corporation based in Tucson. These three partners have committed sufficient funds to build the enclosure and the telescope populated with a single 8.4 meter optical train --- approximately 40 million dollars (1989). Based on this commitment, design and construction activities are now moving forward. Additional partners are being sought. The next mirror to be cast at the Steward Observatory Mirror Lab in the fall of 1996 will be the first borosilicate honeycomb primary for LBT. The baseline optical configuration of LBT includes wide field Cassegrain secondaries with optical foci above the primaries to provide a corrected one degree field at F/4. The infrared F/15 secondaries are a Gregorian design to allow maximum flexibility for adaptive optics. 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 two folded Gregorian focal planes to a central location. The telescope elevation structure accommodates swing arms which allow rapid interchange of the various secondary and tertiary mirrors. Maximum stiffness and minimal thermal disturbance continue to be important drivers for the detailed design of the telescope. 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 will be completed in 1996 by ADS Italia (Lecco) and European Industrial Engineering (Mestre). The final enclosure design is now in progress at M3 Engineering (Tucson), EIE and ADS Italia. Construction activities on the Emerald Peak (Mt. Graham) site have resumed in the summer of 1996.

Keywords: astronomical telescopes, interferometry, honeycomb mirrors

View Figure 1 here

1. LBT Partners and Funding

The total budget for the Large Binocular Telescope Project is $60 million ($1989) or about $75 million in current dollars. This includes about $10 million for an initial complement of instrumentation. Accounting is always referenced to 1989 dollars in order to clearly separate the effects of inflation and fluctuating exchange rates from design changes and uncertain construction costs. The budget includes all the costs of design and technical development and construction, but does not include fundraising costs or legal expenses related to the site. Some savings have been achieved by sharing the costs of developing the honeycomb mirror technology and control systems with other projects such as the MMT Conversion1 and Magellan2

The present consortium partners are: The University of Arizona committing $15 million ($1989) for a 25% share of the total telescope; Italy represented by the Osservatorio Astrofisico di Arcetri also committing a 25% share; The Research Corporation based in Tucson committing a 12.5% share; and The Ohio State University retaining a 4% share after withdrawing as a major partner in 1991. The present partners have already committed sufficient funds to assure the completion of the telescope, the enclosure and a single 8.4 meter optical train. The budget for this so-called ``one-eyed telescope'' is approximately $40 million ($1989). It now appears likely that Germany will join as an additional partner for up to a 25% share. This will assure sufficient funds for the procurement of the second optical train and full binocular operation. An additional partner is still being sought to fill out the remaining 8-12% of the baseline project.

2. Design Drivers

2.1 Observational Priorities

As the conceptual design of the Large Binocular Telescope (LBT) developed over the years3,4, the Scientific Advisory Committee (SAC) set the following observational priorities which were used to guide the philosophy and optimization of the telescope and its capabilities. These priorities were evolved based on assessments of which astrophysical problems were most interesting, and on which observations were tractable with ambitious but practical technology. These priorities are: interferometric imaging over wavelengths from 0.4 to 400µ m; infrared imaging and photometry over wavelengths from 2.0 to 30µ m; wide field multiobject spectroscopy over wavelengths from 0.3 to 1.6µ m; faint object, long slit and high resolution spectroscopy over wavelengths from 0.3 to 30µ m.

2.2 Optical and Mechanical Design Drivers

Many of the priorities listed above are common to nearly all of the current large telescope projects. After all, these are the principal reasons for building a large optical/infrared telescope. Perhaps the desire for exploiting diffraction-limited imaging over a baseline of more than twenty meters and the desire for a wide field-of-view are the features that distinguish LBT from the various other 6 -- 12 meter telescopes now under construction. The following principles were the guidance for developing the conceptual design and its refinement into the detailed design described below.

2.3 Binocular Interferometry

In addition to providing the collecting area equivalent to an 11.8 meter circular aperture, the two 8.4 meter mirrors on a common mount allow us to make interferometric observations over an extended field-of-view (several arcminutes). To carry out the best possible scientific program and to take advantage of the diffraction limit of the 22.8 meter baseline, we have set ourselves the following goals for interferometry: produce a stable phased focal plane; phase the focal plane over the entire isoplanatic patch; operate from 0.5 to 30µ m; make the individual telescopes diffraction-limited; use adaptive correction when needed; match scale, distortion and focus for both telescopes; achieve minimum obscuration and low emissivity; allow a rapid switch from other focal stations to take advantage of the best conditions; and fit the optical systems inside the envelope of the telescope structure. Additional discussion and details of the interferometric configuration are provided by Salinari5, Hill6, and Byard & Bonaccini7.

3. Optical Configuration

3.1. Baseline Focal Stations

The principal optical focal stations on the LBT are twin trapped Cassegrain foci at F/4. With 3-element refractive correctors these focal stations provide a 1 degree field-of-view. The main uses of this trapped position will be for imaging and multiobject spectroscopy. Space is allowed for instruments up to 2 meters in diameter.

Twin Gregorian F/15 focal stations are optimized for infrared performance over a 4 arcminute unvignetted field-of-view with the aperture stop at the undersized secondary mirror. Instruments up to 3 meters diameter and 4 meters length can be mounted at the direct Gregorian foci behind the primary mirror. For observations which will take advantage of the phased combined focal plane of the binocular telescope a pair of tertiary mirrors can redirect the Gregorian focal planes to any of three focal stations in the center of the telescope between the two primary mirrors. At these locations, the Gregorian focal planes can be reimaged into the beam combination optics as shown in Figure 2. The middle of these three central focal stations can also be used as a pseudo-Nasmyth focus with the instrument derotated into a gravity invariant orientation.

Other focal stations which are possible on the telescope structure, but which are not included in the baseline configuration are: optical F/15 Gregorian, chopping F/25 Cassegrain, combined F/33, prime and a phased coud\`e beam which could also feed an interferometric array.

3.2 Secondary Complement

The optical Cassegrain secondaries produce a ~ F/3.8 beam naked which is slowed to F/4.0 by a 3-element refractive corrector to give a field-of-view near 1 degree. The optical design of this configuration has not been completely finalized, but the secondaries will be between 1.2 and 1.3 meters diameter with 2.8 meter diameter baffles. The back focal distance from the secondary is approximately 3.6 meters. The secondary, the corrector and the instrument all swing in and out of the beam together on a swing arm to leave a very clean telescope for infrared use.

The infrared Gregorian F/15.0 secondaries are 871 mm in diameter. These concave mirrors are undersized sections of an F/14.7 parent telescope to place the aperture stop at the secondary mirror. The infrared field-of-view without vignetting is 4 arcminutes diameter. Optical instruments willing to tolerate a small amount of vignetting can use a field up to 10 arcminutes in diameter. The focal planes are 2500 mm behind the primary vertices with classical Gregorian optics (primary asphere: --1.0000; secondary asphere: --0.7326) It is very probable that these F/15 secondaries will be fully adaptive from the start of telescope operation. Using the secondary as the deformable element provides adaptive correction without any additional optics to add infrared emissivity to the telescope system. Lloyd-Hart et. al 8 and Bruns et. al 9 describe a similar adaptive mirror for the MMT Conversion and some results of testing a prototype. Salinari et. al 5,10 describe the philosophy of the adaptive secondaries in greater detail. The F/15 swing arms also include provision for a 500 mm flat mirror above the secondary to project a laser beacon from the center of each telescope. Gray, West & Gallieni11describe themounting and actuation scheme for the secondaries in greater detail.

View Figure 2 here

3.3 Error Budget and Improvements for Infrared Imaging

The error budget specifies that the telescope and its optics will produce images to match an r0 = 45 cm atmosphere. Hill3 reviews the error budget and detailed performance specifications. This specification produces an image which is 0.22 arcsec FWHM in the visible without including any active or adaptive correction beyond active focus and alignment at frequencies less than 0.01 Hz. By including rapid tip-tilt guiding, active figure correction and adaptive wavefront correction, the detected image can be improved by a factor of two or more for wavelengths around 2µ m.

4. Telescope

4.1 Telescope Design

The detailed mechanical design of the telescope is being done by the Italian engineering firms ADS Italia (Lecco) and European Industrial Engineering (Mestre). This work should be mostly completed by July 1996. Already over 300 MB of Autocad drawings have been generated.

The telescope structure is an altitude-azimuth platform design developed initially by Davison and refined over the years. The azimuth bearing is a 14 meter diameter steel track attached to the top of the pier. The telescope rides on this track with four hydrostatic bearings on the lower corners of the azimuth platform. The azimuth track and azimuth platform can be seen in Figure 1. On the upper corners of the azimuth platform, four additional hydrostatic bearings ride against two curved sectors on the elevation structure of the telescope. These large ``C-rings'' which are also 14 meters in diameter form the elevation bearings of the telescope. The azimuth platform and the elevation sectors also include the mountings for gear segments and Farrand strip encoders. Eight DC torque motors on the azimuth platform directly drive pinions against the gear segments for both axes. The platform geometry of the telescope provides a direct structural path from the mirror cells and elevation sectors down through the azimuth platform to the track and the pier. This direct load path allows us to achieve a lowest eigenfrequency of 8 Hz for the structure with a relatively modest weight of 520 tons total moving mass including mirrors and instruments. Del Vecchio et. al 12,13report on the latest finite element optimizations of the telescope structure.

4.2 Telescope Fabrication

Once the design drawings of the telescope are completed in the summer of 1996, the project will solicit bids world-wide to select the company or companies to fabricate and assemble the telescope structure. We expect to send detailed specifications for a number of work packages out to bid in the fall of 1996 and to begin placing orders in early 1997. Some smaller parts of the telescope have already been fabricated as prototypes as can be seen in Figure 3.

View Figure 3 here

5. Enclosure

5.1 Enclosure Design

The detailed architectural design of the enclosure is being done by a consortium of companies headed by M3 Engineering & Technology (Tucson) and including ADS (Lecco) and EIE (Mestre). The enclosure design is now in the ``design development'' phase. Major construction of the enclosure and telescope pier should begin during the 1997 construction season.

As described previously by Salinari & Hill14, the enclosure is a corotating box design with two sliding shutters over the observing apertures. In addition to the observing apertures on the front and top of the rotating section of the building, there are large ventilation doors on the sides and back of the observing chamber to promote wind flushing and thermal equilibration. The rotating building has a pair of 32 metric ton cranes on a common bridge for handling telescope parts. 5 ton hooks are available for handling instruments and smaller equipment. Equipment and instruments are raised from the fixed part of the enclosure through a 4 x 10 meter floor hatch. The hatch provides access to a ground-level high bay area where instruments and the aluminizing bell jar are stored. The telescope elevation axis is located 30 meters above the ground. The roofline of the enclosure is 50 meters above the ground. Support equipment which rotates with the telescope is located in an insulated floor beneath the observing chamber. Ambient temperature air circulated through the telescope and these equipment areas is exhausted from the rotating section through whichever of the 2.5 meter diameter tubes is facing downwind. The total rotating mass is about 1500 tons supported on a circular steel track with four sets of bogies. Steel framing in the observing chamber is designed with cross-sections smaller than 15 mm whenever possible to keep the thermal time constant of the structure short. The control room, offices and living areas are located on two fixed floors in the lower 12 meters of the enclosure. Snow is removed from the roof and shutters by melting it with hot air circulated inside the roof sections. A design drawing of the enclosure is shown in Figure 4. The enclosure is designed to permit a maximum observing windspeed of 80 km/hour and to have a maximum survival windspeed of 225 km/hour.

View Figure 4 here

6 Observatory Site

6.1 Site Delays

The site of LBT is located on Mt. Graham in the Pinaleno Mountains in southeastern Arizona. The specific peak selected is Emerald Peak at an elevation of 10477 feet (3194 meters). Trees on the site (Emerald Peak) were cut in December 1993. Geology testing was done in January 1994. An aerial view of the site is shown in Figure~5. Including 0.3 acres of trees remaining to be cut around the LBT site, the entire Mt. Graham International Observatory (3 telescopes, utility building and 2 mile access road) occupies a total of 8.6 acres inside a 150 acre astrophysical preserve. Leveling of the site and completion of the access road were delayed by a lawsuit against the U. S. Forest Service in July 1994. Telescope opponents objected that the site for LBT had been moved approximately 400 meters in an attempt to minimize the impact of construction on some red squirrels now living on a site which had been considered for the Columbus Project prior to 1987. In 1995, the 9th Circuit Court in San Francisco ruled that the Emerald Peak site was improperly approved by the Forest Service --- i.e. that it was not included in the approval of three telescopes granted by the Arizona-Idaho Conservation Act passed by the U. S. Congress in 1988. A significant delay of two construction seasons resulted because the site issues were unresolved. In April 1996, the U. S. Congress once again passed legislation indicating that LBT should be allowed to proceed with construction on Emerald Peak. As of June 14, 1996, all the legal obstacles have been cleared away and we are about to begin the construction of the telescope and enclosure foundations.

View Figure 5 here

6.2 Summary of Mt. Graham Fire

Those of us with forested mountains in southern Arizona often look enviously at the photographs of treeless Cerro Paranal shown by Massimo Tarenghi in his presentation on the VLT. The trees have the effect of raising the turbulent boundary layer from the ground to the top of the tree canopy. This means that telescopes must be placed higher above the ground to avoid the ground-layer seeing --- the elevation axis of LBT is 30 meters above the ground. I joke that we have found the solution to this problem of trees when the Clark Peak forest fire started on Mt. Graham. The fire was about 6 km from the observatory when it started on April 15, 1996. (The fire was human-caused, but was not started by astronomers nor by opponents of the observatory as far as I know.) The fire spread quickly due to the unusually dry conditions exacerbated by minimal snowfall over the past winter. By May 3, the main body of the fire approached the observatory site. Flames reached within 200 meters of the existing telescopes, but neither the Vatican Observatory15 1.8 meter optical telescope nor the Hertz 10 meter submillimeter radio telescope (SMT) were damaged by the fire. Thanks to a large protection effort mounted by the U. S. Forest Service, local fire companies and the Mt. Graham International Observatory staff. Some flames seen near the observatoy are shown in Figure 6. The fire was finally contained on May 8, 1996. Approximately 6500 acres were burned in total and 1100 persons were eventually assigned to the fire fighting efforts.

View Figure 6 here

7. Primary Mirrors

7.1 Honeycomb Mirrors

The primary mirror blanks for LBT are of borosilicate honeycomb construction developed by Angel and Hill. These blanks are being produced at the Steward Observatory Mirror Lab at the University of Arizona. Two 6.5 meter mirrors have previously been produced. The details of the casting process have been described most recently by Olbert et. al 16 . The mold is now under construction for the first of the two 8.4 meter mirrors for LBT as shown in Figure 7. The casting of the 8.4 meter blank is scheduled for the fall of 1996. Each mirror has a finished diameter of 8417 mm and a clear optical aperture of 8408 mm. The mirrors are parabolic with a focal length of 9600 mm to give a focal ratio of F/1.142. These will be the most aspheric large telescope mirrors made to date. The mirrors have a central hole of 889 mm or an aperture around the hole of 898 mm. The cast honeycomb structure has a 28 mm faceplate thickness and a 25 mm backplate thickness with a total thickness at the edge of the plano-concave blank of 894 mm. The mirrors weigh almost 16 metric tons each with the internal honeycomb structure making them just over 20% of solid density. Parodi et. al 17,18 describe the finite element calculations used to design the support and handling systems of the honeycomb mirrors.

These primary mirrors will be polished at the Steward Observatory Mirror Lab using the techniques described by Martin et. al19. The polishing of the first mirror should be completed in early 2000. After the mirrors have been polished, they will be installed in the mirror cells using a vacuum lifting system. The secondary mirrors will also be polished at the Steward Observatory Mirror Lab using the facilities described by Burge20.

7.2 Mirror Cells and Supports

The mirror cells protect the mirrors from harm and they support the mirror against external forces using an active pnuematic support system. The 8.4 meter mirrors have 160 axial, 104 lateral and 4 cross-lateral pnuematic supports. The position of the mirrors in the telescope is controlled by a system of 6 fixed points. These so-called ``hard points'' provide an adjustable kinematic mount for each mirror while the weight of the mirror is supported by the pnuematic force actuators. The hard points include breakaway mechanisms so that they cannot apply excessive forces to the mirror if the pnuematic system fails. The mirror cells also contain a series of nozzles used to ventilate the borosilicate honeycomb structure with air in order to maintain temperature equilibrium near ambient temperature. This ventilation air is circulated in the cells by air entrainment devices which we have come to call ``jet ejectors''. Miglietta et. al 21 and Gray et. al22 describe the details of the mirror cells and support systems in more detail. The mirror cells also contribute mechanically to the telescope structure and provide the lower section of a vacuum chamber used to aluminize the mirrors in-situ on the telescope structure.

8. Conclusions

The Large Binocular Telescope Project has survived the onslaught of:

And yet the project continues to survive and prosper:

At the present pace, first light with a single optical train is scheduled for the year 2001. Second light with the full binocular optics should happen one or two years later.

* Apologies to the vast majority of environmentalists, lawyers, Apaches, conservatives, liberals and administrators that we consider to be our friends.

View Figure 7 here

9. References

  1. West, S. C.et. al, 1996, ``Toward First Light for the MMT 6.5-m telescope'', S.P.I.E., 2871, (These Proceedings).
  2. Johns, M. 1996, ``Magellan 6.5 m Telescopes Project: status report'', S.P.I.E., 2871, (These Proceedings).
  3. Hill, J. M. 1990, ``Optical design, error budget and specifications for the Columbus Project Telescope'', S.P.I.E., 1236, pp. 86-107.
  4. Hill, J. M. and Salinari, P. 1994, ``Optomechanics of the Large Binocular Telescope'', S.P.I.E., 2199, pp. 64-75.
  5. Salinari, P. 1996, ``The Large Binocular Telescope interferometer'', S.P.I.E., 2871, (These Proceedings).
  6. Hill, J. M. 1994, ``Strategy for interferometry with the Large Binocular Telescope'', S.P.I.E., 2200, pp. 248-259.
  7. Byard, P. and Bonaccini, D. 1994, ``Optical design for interferometry with the Large Binocular Telescope'', S.P.I.E., 2200, pp. 446-457.
  8. Lloyd-Hart, M.,et. al 1996, ``Design of the 6.5m MMT adaptive optics system, and results from its prototype system FASTTRAC II'', S.P.I.E., 2871, (These Proceedings).
  9. Bruns, D. G., Barrett, T. K., Sandler, D. G., Martin, H. M. and Brusa, G.et. al 1996, ``MMT adaptive secondary mirror concave prototype'', S.P.I.E., 2871, (These Proceedings).
  10. Salinari, P., Del~Vecchio, C. and Biliotti, V. 1993, "A study of an adaptive secondary mirror", {Proc. ICO-16 Conference on Active and Adaptive Optics, ed. F. Merkle, (Garching), p. 247.
  11. Gray, P. M., West, S. C. and Gallieni, W. 1996, ``Support and actuation of six secondaries for the 6.5m MMT and 8.4m LBT'', S.P.I.E., 2871, (These Proceedings).
  12. Del Vecchio, C., Davison, W. B., Gallieni, W., Rigato, G. and Miglietta, L. 1996, ``The mechanical structure of the Large Binocular Telescope'', S.P.I.E., 2871, (These Proceedings).
  13. Del Vecchio, C. 1996, ``Optimization of the elevation structure of the Large Binocular Telescope'', S.P.I.E., 2871, (These Proceedings).
  14. Salinari, P. and Hill, J. M. 1994, ``Enclosure of the Large Binocular Telescope'', S.P.I.E., 2199, pp. 442-451.
  15. West, S. C., et. al1996, ``Progress at the Vatican Advanced Technology Telescope'', S.P.I.E., 2871, (These Proceedings).
  16. Olbert, B., Angel, J. R. P., Hill, J. M. and Hinman, S. F. 1994, ``Casting 6.5 meter mirrors for the MMT Conversion and Magellan'', S.P.I.E., 2199, pp. 144-155.
  17. Parodi G., Hill J. M. and Salinari P. 1992, ``Supporting the 8.4m honeycomb mirrors of Columbus'', Proceedings of the ESO Conference on Progress in Telescope and Instrumentation Technologies, ed. M.-H. Ulrich, (Garching:ESO), pp. 301-306.
  18. Parodi, G., Cerra, G. C., Hill, J. M., Davison, W. B. and Salinari, P. 1996, ``LBT primary mirrors: the final design of the supporting system'', S.P.I.E., 2871, (These Proceedings). %19
  19. Martin, H. M., Burge, J. H., Ketelsen, D. A. and West, S. C. 1996, ``Fabrication of the 6.5m primary mirror for the Multiple Mirror Telescope Conversion'', S.P.I.E., 2871, (These Proceedings).
  20. Burge, J. H. 1996, ``Measurement of large convex secondary mirrors'', S.P.I.E., 2871, (These Proceedings).
  21. Miglietta, L., Gray, P., Gallieni, W. and Del~Vecchio, C. 1996, ``The final design of the Large Binocular Telescope M1 cells'', S.P.I.E., 2871, (These Proceedings).
  22. Gray, P. M., Hill, J. M., Davison, W. B., Callahan, S. P. and Williams, J. T. 1994, ``Support of large borosilicate honeycomb mirrors'', S.P.I.E., 2199, pp. 691-702.