Very fast photon counting photometers for astronomical applications: IquEYE for the ESO 3.5m New Technology Telescope

Almost all astronomical instruments detect and analyze the first order spatial and/or temporal coherence properties of the photon stream coming from celestial sources. Additional information might be hidden in the second and higher order coherence terms, as shown long ago by Hanbury-Brown and Twiss with the Narrabri Intensity Interferometer.1 The future Extremely Large Telescopes and in particular the 42 m telescope of the European Southern Observatory (ESO) could provide the high photon flux needed to extract this additional information. To put these expectations (which we had already developed at the conceptual level in the QuantEYE study for the 100 m OverWhelmingly Large Telescope2, 6 to experimental test in the real astronomical environment, we realized a small prototype (Aqueye) for the Asiago 182 cm telescope.10 This instrument is the fastest photon counting photometer ever built. It has 4 parallel channels operating simultaneously, feeding 4 Single Photon-Avalanche Diodes (SPADs), with the ability to push the time tagging capabilities below the nano-second region for hours of continuous operation. Aqueye has been extensively used to acquire photons from a variety of variable stars, in particular from the pulsar in the Crab Nebula. Following this successful realization, a larger version, named Iqueye, has been built for the 3.5 m New Technology Telescope (NTT) of ESO. Iqueye follows the same optical solution of dividing the telescope pupil in 4 sub-pupils, imaged on new generation SPADs having useful diameters of 100 micrometers, time jitter less than 50 picoseconds and dark-count noise less than 50 counts/second. The spectral efficiency of the system peaks in the visible region of the spectrum. Iqueye operated very successfully at the NTT in January 2009. The present paper describes the main features of the two photometers and present some of the astronomical results already obtained.