2 Laboratory simulations

 The laboratory tests have been carried out at the ONERA (Office National d'Études et de Recherche Aérospatiales) headquarter. A special device warms up the air confined in a tank and generates the equivalent of a single-layer turbulent atmosphere. Temperature and speed are calibrated to provide an adjustable set of values (D/r₀, $\overline{v}$), where D is the pupil diameter, r₀ is the Fried parameter and $\overline{v}$ is the average wind speed. The disturbed wavefront originating from an artificial light source (a binary star for example) is then restored with the BOA adaptive optics (AO) system (Conan et al. 1998). The 88 actuators of the deformable mirror allow seeing compensation at visible wavelength with a fast time response of 1 ms. The coronagraphic camera was installed downstream the AO bench.

2.1 The coronagraph

The coronagraphic instrument has already been described in detail in Boccaletti et al. (1998b). Here, additional care was taken for the design of the apodizing device, the Lyot stop, located in the relayed pupil. For a more efficient cancellation of the diffracted light, this stop has to overlap the pupil image, including its central obscuration and the supporting spider, having 3 arms at $120^\circ$. Three stops with different sizes and shapes were made by F. Gex and her collaborators, at the Observatoire de Paris. We have selected, with both computer simulations and laboratory tests, the most efficient stop for suppressing diffracted light. With such a Lyot stop, the dark region defined by the mask in the coronagraphic images is no longer visible and an attenuated image of the Airy peak tends to appear at the center, with an intensity similar to the speckle field (see Figs. 2, 3 and 4).

Some of the limiting factors for the Lyot coronagraph (Lyot 1939) come from inaccurate adjustment of the two main instrument components: the Lyot mask and the Lyot stop. Setting up is made even harder when working on faint objects. In order to understand some misadjustement effects such as alignment defects or longitudinal misplacement of the coronograph's elements, one of us (L.A.) has developed a numerical model of the optical set-up including not only the coronagraph itself, but also the partial correction of the AO and the detector. As explained before, the Lyot stop tends to concentrate some of the remaining light into a central peak in the coronagraphic plane. Simulations have shown that a misalignment error for the stop exceeding $15\%$of its diameter results in a nearly total extinction ($90\%$) of the central peak. Some experimental images also provided evidence of alignment drift. Further studies should allow a more accurate testing of such effects, for providing a reproducible setup of the instrument.

The coronagraphic frames are imaged on the detector with a high magnification (f/976) to achieve a fine sampling of 153 pixels per speckle area.

A Wynne device (Wynne 1979) has been included in the coronagraph to compensate the speckle chromatism as needed for dark-speckle observations with enough spectral bandwidth. The Wynne corrector, previously tested in a telescope run (Boccaletti et al. 1998b), provides a quasi-achromatic Airy pattern on a wide spectral band (650-850 nm).

The Lyot mask diameter is $5.16\lambda/D$ (about 3 Airy radii) and the angular distance between the primary and the companion is $7.63\lambda/D$.

2.2 Data acquisition system

The photon-counting camera CP20+ is an updated version of the CP20 (Abe et al. 1998), which was used on a previous observing run in October 1997 (Boccaletti et al. 1998b). This system is divided into three main components: the camera itself, the photon centroiding electronics and the real-time data acquisition computer.

The camera allows single photo-event detection and a very low dark count: less than 10 photons per 20 ms exposure, which corresponds to approximately 5 10^-4photon/s/pixel for $50~\mu$m pixels at $-20^\circ$C.

Even though the centroiding electronics was limited to a reliable limit of 50000 photons/s (1000 photons per 20 ms short exposure), the low flux we were dealing with, allowed us to use the real-time facilities of the system such as live integration and display. The data files, recorded with CP20+, contain space-time coordinates of photo-events.