Binocular Telescope

Achieves First Light

THE LARGE Binocular Telescope (LBT) stands on the threshold of its scientific career as the first of the next generation of extremely large telescopes. The telescope achieved first light on 12 October 2005, by imaging the nearby edge-on spiral galaxy, NGC 891. The light was gathered by the first of two primary mirrors of 8.4-meters (27.6 feet) diameter and focused on a wide-field CCD mosaic camera at prime focus. This image below marked the very first step toward full realization of the power of this massive instrument.

The goal of the astronomers in the LBT partnership is to attack some of today's most fundamental problems in astrophysics. Astronomers intend to exploit the power of the binocular aspect of the telescope in two ways. One is to use the collecting area of the two optical channels in parallel to feed pairs of light analyzing instruments. There will be a pair of wide-field CCD mosaic cameras ('LBC' - the Large Binocular Cameras), one at each prime focus station. Observations through multiple color filters are the keys to distinguishing the temperatures of

H First-light image w/tfi LBT, the edge-on spiral galaxy, *- *

NGC89I. Theimagewastakenthroughabluefilterwith , a total exposure time of 300 seconds. » * •

stars, the redshifts of galaxies, and the presence of objects with markedly different color-color ratios in different wavebands compared with stars, such as quasars. One camera works best in ultraviolet through green, the other in yellow through far red. Multi-color surveys will therefore take half the time by observing the same field simultaneously in two passbands.

There will be two pairs of wide-field spectrographs, one pair for optical light and one pair for the near-infrared. Both will employ custom-drilled focal plane masks, which provide a pattern of slits exactly matched to the distribution of faint targets (stars or galaxies). When the telescope is perfectly aligned on the field, the instrument pairs will record a total of some 40 faint objects simultaneously. With multi-hour exposures and large telescope time at a premium, this multiplex advantage is critical for studies of groups of faint objects. The immense light-gathering power of LBT will allow spectroscopy of proto-galactic clumps that lie near the edge of the visible universe - so far away their starlight is redshifted into the near-infrared. The theory of galaxy formation by coalescence of smaller units will be directly tested, along with measuring the degree to which galaxies were clustered with each other in large-scale structures at early cosmic times. An additional high-resolution, high-stability spectrograph will be on a climate-controlled optical bench, fed by fiber optics from the telescope. The variations of solar activity that impact life on earth will be set in context by its exquisitely detailed study of the magnetic fields and spot activity on other stars.

The unique and powerful mode of LBT will be in combining the two beams of incoming light in phase. This combination, called interferometry, allows the resolution of the telescope to increase until it is limited by diffraction in the telescope aperture. With the two primary mirrors on a common and stiff mounting structure, their light beams can be combined coherently to provide a genuine field of view. This situation is in contrast to combining the light from separate telescopes; the result in that case is yet higher resolution because of the greater separation, but a tiny field of view in the immediate vicinity of the field center.

With diffraction-limited resolution nearly ten times greater than the Hubble Space Telescope, LBT will image the remnant disks of gas and dust in newly formed solar systems, and detect the gaps plowed out by nascent planets. This will contribute to our understanding of how planetary systems formed and how they evolve. The mid-infrared interferometer will make images and can also combine the beams in a way to null out the light from a bright central star, revealing faint details in its surroundings, such as proto-planetary disks or actual planets. The near-infrared interferometer is an ambitious instrument that will realize the full potential of the LBT for dissecting the nuclei of galaxies and stellar nurseries. The nuclear regions of nearby galaxies will be explored with that resolution, revealing the powerful influence of supermassive black holes on the distribution of stars and gas.

The Primary Mirrors

LBT is the pathfinder for the next generation of extremely large telescopes. Every technical aspect of the project is visionary and state-of-the-art. The giant primary mirrors represent the culmination of developments at the University of Arizona's Steward Observatory Mirror Lab to produce borosilicate honeycomb mirrors. To make sharp images, the front surface of a telescope mirror must have a carefully controlled figure and be held within a fraction of a degree of the ambient air temperature, so as not to create thermal disturbances. The design of the 8.4-meter diameter mirror accomplishes both of those goals. The structure is stiff because the mirror is formed as thin front and back plates supported internally by a network of honeycomb cells of thin vertical glass walls. The front surface is 28 millimeters thick (about an inch), and comes to thermal equilibrium with the air very quickly. It is helped along by forced ventilation of thermally controlled air into the back of each of the honeycomb cells.

The production of the two 8.4-meter primary mirrors is a remarkable technical achievement. Custom-cut refractory cores are bolted down to the floor of the furnace to create the mold to yield the honeycomb pattern and the 'deep dish' (f/1.14) shape of the mirror. The borosilicate glass is type E6 from Ohara glass works in Japan. Mirror Lab team members load 18,600 kg (41,000 pounds) of that glass into the furnace on top of the refractory mold. The entire furnace is spun at 6.8 revolutions per minute to maintain a uniform thickness of the parabolic front surface as the glass melts. The sides of the mold must be restrained by strong metal bands (with a high melting temperature) to resist the centrifugal force of the load of melting glass. At 1,150

degrees Celsius (2,100°F), the glass becomes viscous like honey, and flows around the cores. It is then slowly annealed by gradually reducing the heat in the furnace. After 3 months, the top of the furnace is lifted off, and the resulting glass blank can be inspected.

foot) Emerald Peak summit on Mt. Graham in southeastern Arizona. The mirror in its shipping container sat flat on its trailer, taking up two lanes and requiring a uniformed escort for its 200-kilometer (124-mile) trip to the base camp in Safford, Arizona, over interstate and state highways. The container was then lifted by crane onto the special, tipped frame custom-built for transport up the 48 kilometers (30 miles) of steep and treacherous mountain road to the observatory facility at a typical speed of 2 km per hour (1.2 miles per hour). Precision Heavy Haul of Phoenix won the national award for 2004 for the most challenging delivery in the country. On site, the mirror was removed from its container by a vacuum lifting fixture, and

Handling an 8.4-meter blank is not a trivial activity. The mirror must be hung vertically for a power wash to clean out all the refractory mold material in each honeycomb cell. To get it there, the walls of the furnace are removed, and 36 steel pads are glued to the front surface of the mirror blank. The mirror is lifted with a star-shaped fixture and then attached to a large ring that allows it to be turned on edge. After clean-out, the mirror undergoes its lengthy polishing process. The University of Arizona Mirror Lab devised precision polishing techniques based on computer control of the stresses applied to the polishing tool, or lap. The resulting figure is so smooth that no point across the final surface of the mirror varies from the desired shape by more than 25 nanometers - a millionth of an inch.

That excellent surface figure is maintained in operation by coupling the mirror to a steel cell through a network of 160 pneumatic actuators and a spacing reference system of six adjustable 'hard points'. The actuators compensate for the changing gravity vector of supporting the glass as the telescope moves from zenith to horizon, and for natural bending of the mirror plus cell structure that would distort an image from near-perfect quality.

The mirrors had to be shipped from their place of production under the stadium in Tucson to the Observatory site on the 3,190-meter (10,470-

I Top: The second 8.4-meter primary ascends the mountain grade in its shipping container in September, 2005.

I Above: The network of 160 actuators attached to the rear ofthe 8.4-meter mirror seen as it is removed from its shipping crate.

I Top: The second 8.4-meter primary ascends the mountain grade in its shipping container in September, 2005.

I Above: The network of 160 actuators attached to the rear ofthe 8.4-meter mirror seen as it is removed from its shipping crate.

M Below: Moving the 8.4-m primary w/th the vacuum lifting fixture from its shipping container into its cell in the mountain high bay staging area. The belljar for aluminizing the mirror is the darker red structure hanging vertically in the upper right.

M Below: Moving the 8.4-m primary w/th the vacuum lifting fixture from its shipping container into its cell in the mountain high bay staging area. The belljar for aluminizing the mirror is the darker red structure hanging vertically in the upper right.

Ring Azimuth Frame Primary Mirror

placed in its steel cell. The large overhead crane then lifted the mirror and cell from the high bay on the ground floor up to the telescope structure, where the mountain crew bolted the cell in place on the telescope structure. At the time of first light last October, only the 'left-hand' mirror was in place. Just afterwards, the second primary was installed, making the telescope truly binocular. The 'right-hand' mirror was aluminized in January of 2006; 'second light', imagingwithboth channels and two LBC cameras is anticipated in winter of 2006.

I Above: Both primary mirrors have been installed on the telescope structure and aluminized. The telescope is pointing at the horizon in this view.

a fraction of a single wave of light, at the level of 1/10 of a micrometer. Disturbances by wind cannot shake the telescope structure and optics mountings beyond the interferometer's ability to compensate by adjusting a miniature 'slide trombone' to match (i.e., phase) the light waves reflected from each side.

The resulting structure is very stiff, and to get that way, it is massive. The base is a concrete pier 20 meters (66 feet) high and 14 meters (46 feet) in diameter, resting on the mountain bedrock. The telescope is an altitude-azimuth design (like a gun turret on a battleship). It addresses any point on the sky by tipping to the correct angle between zenith and horizon, and rotating around its vertical axis to the correct heading. The altitude bearing is comprised of a pair of giant C-rings that project the 500 tons of moving weight of the structure directly to the 150-ton azimuth frame and onto the pier. The C-ring bearings and azimuth bearings float on a film of oil pumped to 120 atmospheres pressure, allowing the telescope to be driven by four 3-horsepower motors on each axis.

The structure's towering center section provides the pivot points for swing arms that move auxiliary optics into and out of the main

The Telescope Structure

The mount for the Large Binocular Telescope is unique among the modern giant telescopes. All other facilities support a single large primary. The LBT mount was conceived and designed explicitly to support two enormous primary mirrors and auxiliary optics stiffly. The coherent combination of the two telescope beams into one image requires maintaining the difference in path length that the light travels from one side versus the other to be constant to beams. Two swing arms support the prime focus cameras for direct imaging. They can be retracted and secondary mirrors put into place to form images at the straight-through Gregorian foci below the primary mirrors. A pair of flat tertiary mirrors can be swung into place to divert the light to bent Gregorian foci with the permanently mounted large instruments in the very center of the telescope structure. The primary mirrors will be protected by mirror covers that swing into place (just below the tertiaries) and that open like an oriental fan around a central hub.

Adaptive Optics

Modern large telescopes achieve their sharpest imaging by passing the light through a system to compensate for the blur of the Earth's atmosphere. Such systems are called adaptive optics (AO), with the central element being a thin, rapidly deformable mirror. Turbulent layers in the atmosphere high above the telescope produce cells in the air with very slight temperature differences from the surroundings. Just as in the case of waves in water on the surface of a pond, light waves that start out as spheres emanating from a distant astronomical source have traveled so far that they are nearly perfect plane waves at the top of the atmosphere. The atmospheric cells advance or retard just slightly the propagating light wave, turning a plane into a lumpy bumpy surface, resulting in a blurred star image. The adaptive optics system samples the shape of that surface and commands the deformable mirror to assume the 'inverse' pattern of lumps and bumps, restoring the incoming light pattern to nearly perfect focus. The mirror surface changes shape typically 1000 times a second.

The unique aspect of LBT's AO system is that the deformable mirror is the secondary mirror of the telescope itself. Adaptive optics systems provide the widest field of view and best correction in the infrared. At those wavelengths, room temperature optics and stray radiation from warm surfaces provide unwanted background noise to an already faint signal from the sources to be observed. Conventional AO systems have small deformable mirrors in cameras with a number of warm lenses to shrink the large telescope beam down onto the mirror, then to expand it outward again to the scientific instrument. Having the telescope's secondary mirror serve as the AO deformable mirror avoids the introduction of substantial extra background noise.

The challenge is in the size of the adaptive mirror. The thin shell of glass that makes the deformable reflecting element is 91 cm (36 inches) in diameter and only 1.6 mm (0.063 inches) thick. It is controlled by 672 electromagnetic actuators, with magnets glued directly onto its back surface. The magnets and associated electrical sensors pass through holes in a glass reference plate. The sensors measure the exact distance between the metallized surfaces of the thin shell and the reference plate, allowing control of the shape of the mirror to accuracy measured in nanometers. For initial implementation, the atmospheric blur must be measured with respect to a relatively bright star in the field of view; the corrections inferred from the 'wavefront sensing' camera are computed in a custom-developed processor and commanded to the electromagnets. A prototype system with half the number of actuators is showing great promise in operation at the MMT Observatory on Mt. Hopkins in Arizona.

I Below: Thin glass shell for adaptive optics secondary mirror, seen from the back side, w/th the mask pattern for gluing actuator magnets.

I Below: Thin glass shell for adaptive optics secondary mirror, seen from the back side, w/th the mask pattern for gluing actuator magnets.

The Partners

The successful completion of a visionary project requires creative initiators, solid engineering to bring a concept to reality, and inspired and inspiring entrepreneurs to attract the substantial support needed. University of Arizona astronomers Roger Angel and Neville Woolf conceived the basic LBT approach in the early 1980s. An engineer, Warren Davison, at the university played an important role in conceptualizing the mechanical structure with its C rings. One of the authors (JH) has been involved in leadership of the project for over 20 years, from the earliest days of producing borosilicate honeycomb mirrors. Peter Strittmatter, the Director of Steward Observatory, has been a driving force in forming and leading the consortium that runs the Observatory. The not-for-profit corporation was formed in 1992, and construction began in 1996. The University of Arizona holds a one-quarter share of LBT on behalf of the State University system, and is providing the two primary mirrors and adaptive secondary mirror thin shells. Phil Hinz, a University of Arizona astronomer, is the principal investigator for 'LBTI', the mid-infrared interferometer which has been supported by NASA for precursor observations for the Terrestrial Planet Finder mission. The University of Arizona hosts the LBT Observatory as a division of its College of Science.

Italian astronomers have played a critical role in the development and technical advances of LBT. Piero Salinari of the Arcetri Astrophysical Observatory in Florence led a team of astronomers and engineers in the design of the telescope mount. The telescope mount structure was built and assembled at Ansaldo-Camozzi Energy Special Components in Milan, and completed in 2001. It was then disassembled and shipped to Houston, whence the pieces were trucked to the base camp in Arizona. Salinari and his Arcetri colleagues, Simone Esposito and Armando Riccardi, are leading the scientific and industrial partnership to develop the adaptive secondary mirror systems, along with Daniele Gallieni of ADS and Roberto Biasi of Microgate. The astronomers Franco Pacini and Giancarlo Setti were instrumental in exciting their colleagues and institutions about forming a partnership to produce LBT. Emanuele Giallongo and his colleagues at the Rome Observatory are producing the 36-Megapixel Large Binocular Cameras for prime focus imaging. INAF, the Italian government agency supporting all of astronomy research in the country holds a one-quarter partnership share.

The LBTB is a consortium of German astronomical research institutes that also holds a one-quarter share in the LBT. It includes institutes in Heidelberg, Bonn, Potsdam, and Garching. Through partnerships, they are producing a major suite of instrumentation for the telescope. Holger Mandel and Walter Seifert in Heidelberg are leading the team creating 'LUCIFER', the pair of infrared imagers / spectrographs to be mounted at the outside bent Gregorian foci. Tom Herbst in Heidelberg is the PI of 'LINC-NIRVANA', the ambitious near-infrared interferometer with three levels of adaptive optics correction: ground layer and turbulent layers at two heights above the site. Klaus Strassmeier in Potsdam is producing the high dispersion, high stability spectrograph and spectropolarimeter, 'PEPSI', that will be mounted in its own climate-controlled chamber in the pier of the telescope and fed by optical fibers from the telescope foci.

Arizona-based Research Corporation provided the support for a one-eighth share. That observing opportunity will be divided among Ohio State University, the University of Notre Dame, the University of Minnesota, and the University of Virginia. The last one-eighth share belongs to Ohio State. Darren DePoy is the OSU astronomer leading the team to produce 'MODS', the pair of wide-field optical spectrographs. Bruce Atwood and a team of OSU engineers developed the giant bell jar that is the portable aluminizing facility, mounted directly to the primary mirror cells on the telescope.

Many other astronomers and administrators are involved in the challenge of the technical and financial development of the LBTO. The construction of the Observatory is costing $119 million, plus additional commissioning effort.

Significance of First Light and Path Forward

The stunning first-light image was taken with one primary and one LBC prime focus camera, with an overall telescope control system that was just beginning to function. It marks the first step on the road to cutting-edge science with this unique facility.

The goal for 2006 is achievement of 'second light', the initial imaging with both LBC's, and the commissioning of the telescope as a binocular system to the point that it is ready for

Binocular Concept Architectures

I Above: LBT open under moonlight on Emerald Peak.

prime focus imaging science. The next milestone will be initial commissioning of the Gregorian foci with a simple fixed secondary mirrir in fall of 2007, followed by commissioning of the spectrographs LUCIFER and MODS. LUCIFER is a cryogenic spectrograph and imager designed to work in the near infrared. MODS is a multi-object spectrograph that works from the ultraviolet to the near infrared. Both spectrographs will be able to determine red shifts, chemical composition and physical conditions in many astronomical objects, and in particular, provide valuable follow-up observations to high redshift galaxies identified by the Hubble Space Telescope.

With arrival of the first adaptive secondary mirror system anticipated in early 2008, we will then see the telescope's first diffraction-limited imaging. The interferometers are scheduled for installation in the second half of 2008. The full instrument suite of first generation instruments in planned to be in full operation in 2009. We invite you to follow the progress of this exciting project at http://www.lbto.org.

I Above: LBT open under moonlight on Emerald Peak.

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