If one wishes to carry out direct observation in the infrared of objects very close to their stars (hot Jupiters for example) or more distant planets, the angular resolution available with a monolithic telescope (which is diffraction limited) is not sufficient to resolve the system. So it is necessary to resort to interferometry (cf. Chap. 2) Ground-Based Observatories


Apart form the astrometric methods and instruments mentioned earlier, classical interferometer arrays can selectively provide observations of extrasolar planets. Here, in the absence of dark-fringe recombination interferometry, measurement of the modulations of visibility caused by the presence of a companion is the method used (cf. Chap. 2). The principal limitation of this method is the dynamical range that results from the star/planet contrast. So this technique is limited to giant planets.

Several instruments (AMBER and MIDI on the VLT-I) are capable of measuring visibility with sufficient accuracy, which is about 0.1 per cent. However, such results are available only at the price of several tens of nights of observation, which limits this method to an extremely limited number of candidates. In practice, it is solely if one wants to carry out spectroscopy of the object that such observations are necessary. It is then essential to have a method of translating the chromatic information on visibility into chromatic information about the companion. In the majority of cases, however, information obtained by this method may also be used by other, less restrictive methods.

A nulling instrument (KIN: Keck Interferometer Nuller) has been installed on the Keck-Interferometer, combining the two 10-m Keck telescopes. The KIN allows interferometric nulling of the on-axis object better than 100 in the N spectral region. This instrument is thus particularly suited to the observation of stellar environments and disks.

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