General view

The dark-speckle coronagraph (DSC) combines features of adaptive optics, Lyot coronagraphy and speckle interferometry. Figure 1 shows the arrangement utilized at the Coudé focus of the 152cm telescope at Observatoire de Haute-Provence. The stellar beam from the telescope enters the adaptive optics bench BOA, of the ONERA (Office National d'Etudes et de Recherches Aérospatiales). It has 88 actuators and responds fast enough for seeing correction in the visible range ([Conan et al. 1998]). Downstream from BOA, the afocal beam reaches a dichroic beamsplitter which transmits the IR (K band) light towards another instrument used simultaneously, the achromatic interfero-coronagraph ([Gay & Rabbia 1996]), while visible light ($650nm<\lambda<850nm$) is reflected to the DSC.

 

Figure: Dark speckle camera used at the Haute-Provence 1.52m telescope (not a scale drawing). The dichroic beam-splitter selects the wavelength range 650-850nm in reflection. The collimated beam received from the telescope with BOA adaptive optics crosses a pair of Risley prisms for atmospheric dispersion compensation and is focussed by concave mirror CM onto the Lyot occulting mask OM. The masked field is re-imaged onto photon-counting camera CP through the pair of Wynne corrector triplets WT which compensate the first-order lateral chromatism of diffraction patterns. A Lyot stop LS removes stellar diffracted light at the edge of the relayed pupil, according to the classical Lyot coronagraph principle.

The Strehl ratio reached 10% to 30% depending on seeing conditions. Although the adaptive optics gain is higher in the IR range, the advantage of working with visible light results from the use of a photon-counting camera. Its low dark count allows short exposures with negligible added noise, as required for speckle observations.
The centering of the star's image on the Lyot occulting mask is critical, and should be part of the adaptive optics loop. This is achievable by collecting light reflected from the mask, but the drift had to be corrected manually at this stage.