The LBT (Large Binocular Telescope) is an unusual project, consisting of two 8.4-m telescopes on the same mount. The two primary mirrors are separated by 14.4 m (Fig. 8.23). Such an arrangement, where all spatial frequencies are measured with a pupil equivalent to 20 m in diameter, has several recombination modes, namely:

• a direct Fizeau imaging mode (the LINC-NIRVANA instrument). Here, the PSF is not an Airy disk but patch of interference fringes caused by interferometer recombination. In this mode, and thanks to the fringe structure of the PSF, it is possible to measure the relative positions of the stars in the field, with an accuracy of about 100 |as.

• A dark-fringe interferometer mode. With a universal recombiner, two instruments may be used: NUL (Nulling Interferometer for the LBT), a dark-fringe interferometer with a typical rejection of 104, and the NOMIC camera (Nulling Optimized Mid-Infrared Camera), capable of operating in the N band (at 10 |m), among others.

Fig. 8.23 The LBT and the UBC universal recombiner for interferometry

The unique feature of the LBT is that is specifically optimized for dark-fringe interferometry in the N (10 |m) band. Unlike the VLT-I and other classical interferometers, the optical path between the point at which it enters the telescope and the instrumentation is reduced, and the number ofmirrors is low. Given the distance between the two primary mirrors (14.4 m, centre to centre), and the spatial resolution that results, this instrument is specifically suitable for observation of stellar environments (at least for nearby stars), and of debris disks out to distances of several AU from their central stars. This instrument is equally well-adapted to the observation and spectroscopy of giant planets.

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