Possible applications of a novel type of photon counting astronomical 
instrument for Intensity Interferometry observations

Giampiero Naletto (University of Padova)

During the last years we realized in Padova two very similar instruments, 
namely AquEYE and IquEYE: the Asiago Quantum EYE, from the name of the 
telescope in Italy to which it has been applied, and the Italian Quantum EYE, 
that will be applied at the NTT on January 2009. These instruments collect 
the visible light of a star at the telescope focus and send it on four SPADs 
by means of a suitable optical system that splits the telescope pupil in four 
parts: essentially, each SPAD sees the star and time tags all the collected 
photons with a 50 ps time accuracy. All the timing data are stored in a mass 
memory and are then fully available for any type of post processing analysis. 
As an example, the analysis of a 30 minutes acquisition data of the Crab 
pulsar at the Asiago telescope (182 cm) has allowed to measure its period 
with an accuracy better than 100 ns.

We are planning to further develop this type of instrument for possible 
applications that can range from -Y´quantum¡ observations with future ELTs, 
as measurement of second and higher order correlation functions from remote 
light sources, to intensity interferometry with existing telescopes as VLT 
and Keck. Two are the present limitations in these instruments: the first is 
the available SPADs that are limited to 3 MHz bandpass if the NIM output is 
used (10 MHz with TTL, which however gives a less accurate time tag); the 
second is the CAEN electronics board that presently limits the total output 
to 8 MHz. However, these limitations can be overcome by selecting more 
performing and expensive devices.

This type of instrument is really versatile because it allows to operate 
independently with distant telescopes if a suitable clock synchronization 
can be obtained. On this respect, an experiment has run on last November to 
simultaneously observe in photon counting regime the Crab pulsar with the 
telescopes in Asiago and Ljubljana (Slovenia) which are about 250 km apart, 
using a prototype high precision common clock. We are presently analyzing the 
data, and the preliminary results show a good agreement between the two data 
sets: this could be the first demonstration of the feasibility of realizing 
synchronized observations with a network of distant telescopes.