The photon-counting camera

  Finally L₂ focuses the beam onto the camera with an $8\times$ magnification, bringing the aperture to f/976.
The detector is a cooled CP20 photon-counting camera ([Vakili 1990]) having a first-generation electrostatic intensifier coupled to a second-generation microchannel intensifier and a fiber optics taper, feeding a $384\times 288$ CCD. The resulting amplification is about 10⁵. Such a device allows single photon detection and a very low dark count, less than 10 photons per 20ms exposure for $50\mu m$ pixels at $-20\degr$C (about 0.0045 photons/s/pixel). A low dark noise is necessary to detect accurately any ``filling'' of the dark speckles, which could reveal the presence of a faint companion. The drawbacks of such photon-counting cameras are their low quantum efficiency ($<10\%$ at 700nm) and their low saturation level of 50000 ph/s, limited by the acquisition system.
As mentioned above, the coronagraph forms a f/976 focus on the detector in order to achieve the dense sampling required by the dark-speckle technique. For a central wavelenght of 635nm, it represents 150 pixels/speckle area or 144 pixels/arcsecond. The field of view is limited to a diameter of 250 pixels (about $1.8\arcsec$) by the Wynne corrector. In addition to the Wynne corrector, spectral filters can be inserted in front of the photocathode to select different narrow bandwidths.