The Large Binocular Telescope (LBT) is located at Mount Graham International Observatory in the Pinaleno Mountains of Southeastern Arizona at an elevation of 3192 m (10,567 feet). The $120 million, 650 metric ton LBT uses two parabaloid 8.4m (27 feet) diameter borosilicate honeycomb f 1.1 primary mirrors (produced at the Steward Observatory Mirror Laboratory in Tucson. The mirrors are mounted side-by-side on a common mount to produce a collecting area of 110 square meters (361 square feet) which is equivalent to an 11.8m (38.7 feet) circular aperture.
Two ellipsoidal secondary mirrors for the LBT are each 911mm (36 inches) in diameter and 1.6mm (0.063 inches) thick. On the backside of the mirrors there are 672 small magnets that bend the mirrors up to 1000 times/second to compensate for atmospheric turbulence (adaptive optics).
The two primary mirrors and their secondary mirrors can be used together or independently to obtain seeing-limited images over a wide field of view. A unique feature of the LBT is that the light from the two primary mirrors can be combined optically in the center of the telescope (using interferometry). This gives the telescope the diffraction-limited resolution of a 22.65m (74 feet) telescope in one spatial direction, producing images with a resolution of 5 milliarcseconds in visible light and 20 milliarcseconds in the near infrared.
The LBT uses deployable swing arms to support various optics near the foci of the primary mirrors. The two 8.4m diameter telescopes deliver bent Gregorian focal planes at f=15. The diverging light from the f=15 focal planes is then collimated, folded into a common pupil plane, and re-imaged with a single camera onto a common focal plane.
LBT Focal Stations
● Gregorian, infrared, adaptive, dual F/15
● Phased combined, re-imaged F/15, center
A pair of curved secondary mirrors and a pair of tertiary flat mirrors are required to implement those focal stations. These smaller optics are moved in and out of the light path by swing arms. The tertiary mirrors rotate to direct the light to several central instrument locations (Prime focus cameras are not shown on the drawing to the left). The red "T-shaped" instrument in the center of the telescope is an instrument which contains the optics for combining and phasing the beams from the two telescopes.
The following 3D drawings of the Large Binocular Telescope structure were made by European Industrial Engineering in Mestre, Italy. These are vintage February 2000.
LBT Oblique View
LBT Front View
The non-traditional design of the telescope offers many advantages over a conventional Newtonian telescope.
- With the two primary optical paths, one target can be photographed in two different parts of the spectrum at the same time, reducing the amount of observing time needed to collect the images. See Figure 5 for the focal path of the telescope.
- Taking spectrographs will also be quicker because of this arrangement. Each primary will have the same unique mask for the target field being observed - one using visible light, the other infrared.
- The most important mode that can be used is to combine the images from both primary mirrors using a technique called interferometry. This allows the telescope to have the resolving power of a single telescope with a primary mirror 22.8 meters (74.8 feet) in diameter. This resolution is ten times what the Hubble Space Telescope has.
- This design allows different secondary mirrors to be stored on the telescope for quick change over as the observing conditions change.
After more than a decade of preparation, the world’s most powerful telescope achieved first light using it's first primary mirror in October, 2005, and achieved first binocular light (using both primary mirrors) in November, 2007 (Click here to see video: http://lbtwww.arcetri.astro.it/press/LBTVNR.wmv). With it's binocular vision, the LBT provides new and more detailed views of deep space which may help answer some of the fundamental questions about the origins of the universe and mysterious worlds in other planetary systems. Ultimately, the interferometric combination of the light paths of the two primary mirrors will provide a resolution of a 22.8-meter (75 foot) telescope. With its capability, the LBT is the largest single-mount telescope in the world.
Quick Facts about the LBT
Congressional Approval of site in November 1988
Construction began in July, 1996
First Light: October 2005
Second light: January 2008
Telescope Mass: approximately 650 metric tons
Telescope Pier: 14 meters in diameter (46 feet)
Building Pier: 23 meter diameter (75.5 feet)
Mirror Weight: approximately 17.7 metric tons each
Mirror Size: 8.408 meters in diameter (331 inches)
Mirror center hole: 0.889 meters in diameter (35 inches)
Mirror aluminum coating thickness: 100 nanometers (0.00000394 inches)
Observatory Elevation: 3221 meters (10,567 feet)
NGC 891 First Light Primary Mirror 1 October 2005
NGC 891, an edge-on spiral galaxy (type Sb) in the constellation of Andromeda, lies at a distance of 24 million light years (approximately 144 trillion miles away from us). NGC891 is of particular scientific interest because the galaxy-wide burst of star formation. Note that there are numerous smaller and more distant galaxies in the background of the NGC891 field. These are more typical of what a large telescope like LBT will study.
The First Light observation was made through a blue filter (B-Bessel) as a series of ten 30-second exposures. The images were captured through the "Large Binocular Camera" (LBC1) which is mounted high above the primary mirror at the prime focus of the first (left) primary mirror. The camera has four CCD (charge-coupled devices) chips in the focal plane - each with dimensions of 2048x4608 pixels - for a total of 36 megapixels. In front of the CCD array is a set of 6 fused silica corrector lenses that correct the comatic aberration of the fast primary mirror to make an extended field-of-view. The ten exposures were each offset slightly on the sky so that the seams in the CCD array do not appear in the final combined image. After the images were calibrated and registered, they have been combined in the computer to make a single blue image of the galaxy with an exposure time of 300 seconds. The image quality of the individual exposures ranges from 2.9 pixels to 3.6 pixels FWHM where each pixel corresponds to 0.227 arcseconds on the sky. Thus the final stacked image has a resolution of 0.8 arcseconds which is typical of the wide field imaging which will be obtained with this camera. The angular size of the final image on the sky is about 30 arcminutes across -- similar to the angular size of the full moon.
FIRST 'BINOCULAR' LIGHT JANUARY 2008
Using both LBT mirrors, these First Binocular Light images show three false-color renditions of the spiral galaxy, NGC 2770. The galaxy lies 102 million light years from our Milky Way (a relatively close neighbor at approximately 600 million trillion miles away), and has a flat disk of stars and glowing gas, tipped slightly toward our line of sight. The first image combines ultraviolet and green light, and emphasizes the clumpy regions of newly formed hot stars in the spiral arms. The second image combines two deep red colors to highlight the smoother distribution of older, cooler stars. The third image displays ultraviolet, green, and deep red light in the same composite, showing the detailed structure of hot, moderate, and cool stars in the galaxy. The cameras and these images were produced by the LBC team, led by Emanuele Giallongo at the INAF- Rome Observatory (http://lbc.oa-roma.inaf.it/) . Images were obtained by Vincenzo Testa (Rome Observatory) on the nights of 11 and 12 January, 2008.