UA-95-01: The Large Binocular Telescope Project

The Large Binocular Telescope Project

J. M. Hill, Steward Observatory

Large Binocular Telescope Project

Technical Memo

UA-95-01
February 23, 1995
presented at January 1995 AAS meeting as poster 10.06
http://medusa.as.arizona.edu/lbtwww/tech/ua9501.htm

Figure 1: Conceptual rotating enclosure for the LBT.

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 Astronomico 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 early 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 1995 by ADS Italia (Lecco) and European Industrial Engineering (Mestre). The final enclosure design is now in progress at M3 Engineering (Tucson) and ADS Italia. Construction activities on the Emerald Peak (Mt. Graham) site should begin in the spring of 1995.

LBT Summary

optical configuration ------------- binocular 8.4 meter apertures
equivalent aperture ------------ 11.8 meters (2 * 55 m ²)
interferometric baseline ------------ 22.8 meters
resolution (/ D) in the visible ------------ 5 milliarcsec
primary mirrors ------------ 8.4 meter F/1.14 borosilicate honeycomb
world's most powerful common mount telescope
consortium partners:
- University of Arizona (25%)
- Italy --- Osservatorio Astrofisico di Arcetri (25%)
- Research Corporation (12.5%)
- Ohio State University (4%)
mounting type ------------ dual ring platform (altitude -- azimuth)
moving mass ------------ 420 metric tons
site ------------ Mt. Graham, Arizona (Emerald Peak, 3190 m)

LBT Project Status

Telescope Design
The detailed mechanical design of the telescope is being done by the Italian engineering firms ADS (Lecco) and EIE (Venice). As of December 1994, the detailed design work is about 40% complete. Already over 100 MB of Autocad drawings have been generated. The design of the control system is being done by Steward Observatory.
Enclosure Design
The detailed architectural design of the enclosure is being done by a consortium of companies headed by M3 Engineering & Technology Corp. (Tucson). Major construction (concrete) should begin during the 1996 construction season.
Site Work
The LBT will be part of the Mt. Graham International Observatory near Safford, Arizona. The construction of the observatory with three telescopes was approved by Congress in November 1988. The site for the LBT Project approved by the Forest Service in 1993 is shown in the drawing below together with the locations of the Lennon (1.8 m F/1.0 optical) and Hertz (10 m sub-millimeter) telescopes which are now operational. Trees on the site (Emerald Peak) were cut in December 1993. Geology testing was done in January 1994 to select the precise pier location. Leveling of the site and completion of the access road were delayed by a lawsuit filed against the Forest Service in July 1994. We are now awaiting a ruling on an appeal by the 9th Circuit Court in San Francisco.
Telescope Fabrication
Once the design drawings of the telescope are completed in the Summer of 1995, the project will solicit bids world-wide to select the companies to fabricate and assemble the telescope.
Mirror Fabrication
The Steward Observatory Mirror Lab has now cast two 6.5 m honeycomb mirrors. The first of these F/1.25 parabolic mirrors is now being ground and polished. The fabrication and assembly of the mold for the first LBT 8.4 m F/1.14 mirror is now underway. The actual casting of this mirror should occur in early 1996. The polishing of this mirror should be completed by 1998.
Funding
The existing partners Arizona, Italy and Research Corporation have committed sufficient funds, $40M, to permit construction of the complete telescope and enclosure with only a single set of optics. The project is proceding with construction on that basis. Additional partners are being sought to complete the $60M (1989) budget and permit the second primary mirror and instrumentation to be added.
First Light
Current schedules for the telescope, mirror and enclosure suggest that first light will occur in Fall 1999.

Optical Design Drivers

Large total collecting area for sensitivity on faint objects.
Excellent image quality through passive, active and adaptive control of seeing and other aberrations.
Low background from a clean thermal infrared design.
Versatile instrumentation and rapid changes in configuration to take maximum advantage of specific observing conditions.
High ultimate spatial resolution by providing a relatively long baseline with the stability and field-of-view provided by a common mounting.
Low construction and operations costs.

Baseline Focal Stations

trapped Cassegrain, optical, dual F/4.0
Gregorian, infrared, dual F/15
phased combined, reimaged F/15, center
bent or pseudo-Nasmyth, F/15

Figure 2

Figure 2: This figure shows what LBT has come to call ``Beam Combiner Design 3''. Collimator and camera lenses are used to reimage a pair of bent 8.4 m F/15 Gregorian focal planes to the combined focus. The reimaging lenses have not been optimized so the design should be considered illustrative of the concept.