A First attempt to Intensity Interferometry with IQUEYE

Intensity Interferometry is a technique for the calculation of stars angular diameters that could in principle overcome some major disadvantages of traditional amplitude interferometry. This technique was first applied in 1956 by Hanbury-Brown and Twiss (HBT) in Narrabri (Australia), when they measured the diameter of several bright blue stars. Since then, the HBT technique nearly disappeared from astrophysics. Nowadays theory and technology are mature enough to guarantee a renewed exploitation of this effect in astronomy. Our group in Padua has built Iqueye, a high speed astronomical photon counting device, first tested at the ESO 3.5m New Technology Telescope (La Silla, Chile) in January 2009. With the collected data, a posteriori we perform the calculation of g2(x,0) as a first step to verify the zero-baseline correlation coefficient in a HBT intensity interferometer. In a similar way, g2(0,t) could be calculated. To perform such calculations, a dedicated software correlator has been designed, implemented and optimized and will be described in depth in the present paper. Although the size of the NTT does not allow an acceptable signal to noise ratio for calculating second order effects, our experiments demonstrate the value of an Iqueye-like instrument for HBT like experiments. Hopefully an improved version of IQuEye applied to larger telescopes, like the four 8m VLTs and even to the future 42m Extremely Large Telescope will be the perfect instrument for a novel exploitation of the photon stream from celestial objects.