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J. M. Hill, J. R. P. Angel, P. M. Hinz, N. J. Woolf (U. Arizona)
A key project for the Large Binocular Telescope (LBT) will be its use as a Bracewell nulling interferometer to study thermal emission from extra-solar planets and dust. With two 8.4 m telescopes rigidly co-mounted side-by-side, it is ideally suited for nulling. The beams will be combined in a cryostat so that the stellar emission is destructively interfered, allowing detection of close-by, much fainter sources that would otherwise be overwhelmed. The separation of 14.4 m between telescope centers results in interference fringes spaced by 140 msec of arc at 10 micron wavelength. Such fringes are optimally spaced, being both broad enough to suppress the centered 1 mas disc of a sun at 10 pc by < 10-4, but fine enough so that a planet in an orbit as close as 0.7 AU will be constructively reinforced. The method was recently demonstrated with two mirrors of the old MMT configured on a common mount like the LBT. Direct images of the 10 micron dust cloud around Betelgeuse were obtained with the star nulled out (Hinz et al. 1998). The LBT is being made with adaptive correction built-in to the secondary mirrors, for lowest thermal background while correction of atmospheric aberration is made to the level needed for nulling at the 10-4 level. The enclosure, telescope structure and mirrors are all now under construction, and operation with both telescopes projected for 2003.