Fabry‐Pérot Observations Using a New GaAs Photon‐counting System

A third‐generation image photon‐counting system (IPCS) camera is presented, based on a GaAs photocathode that can achieve a quantum efficiency of up to 23%, which is comparable to a thick CCD but without readout noise. This system is 10 times more sensitive at Hα than previous photon‐counting cameras. In terms of signal‐to‐noise ratio, the system outperforms CCDs for extremely faint fluxes, including antireflection‐coated, low‐noise, thin CCDs. This system, with up to 1K × 1K pixels, is one of the largest monolithic IPCSs. A unique cooling system, based on a Ranque‐Hilsh vortex tube, is used for this camera. Real‐time centering is done by a scalable digital signal processor board. Astrophysical projects and preliminary results obtained with this new camera coupled with a scanning Fabry‐Pérot interferometer at the Cassegrain focus of the 3.6 m ESO telescope, the 1.93 m Observatoire de Haute Provence telescope, and the 1.6 m Observatoire du Mont Mégantic telescope are presented.