in B at the TBL.

During the observations, a specially designed program (already mentionned in §2.1) computes the elevation of the star and the corresponding atmospheric dispersion using J.C. Owens’ model of atmosphere (Owens, 1967, formulae 29–31). The Risley prisms are then dynamically rotated during data acquisition to compensate for the atmospheric dispersion.

[**3. The observation modes**]{}

10000 PISCO can be used in various modes which are selected during the observations. Switching from one mode to another takes a few mouse clicks and less than one minute for the motors to set the wheels.

[*3.1. Full pupil speckle imaging*]{}

10000 In this mode, no pupil masks are used and a high magnification is selected with wheel GR (Fig. ). It corresponds to the “conventional” way of observing in speckle interferometry. Most speckle cameras (Breckinridge [*et al.*]{} 1979, Strittmater, 1980, Foy, 1988a). offer only this possibility – or would require many optical changes with a full re-calibration of the instrument for other operating modes –. By applying bispectral techniques (§5.2), we have obtained images with an angular resolution close to the diffraction limit of the telescope (Fig. 4b).

to to The full pupil is used and the optical transfer function (OTF) 