Iqueye, a single photon counting very high speed photometer for the ESO 3.5m NTT

Iqueye is a single photon counting very high speed photometer built for the ESO 3.5m New Technology Telescope (NTT) in La Silla (Chile) as prototype of a ‘quantum’ photometer for the 42m European Extremely Large Telescope (E-ELT). The optics of Iqueye splits the telescope pupil into four portions, each feeding a Single Photon Avalanche Diode (SPAD) operated in Geiger mode. The SPADs sensitive area has a diameter of 100 μm, with a quantum efficiency better than 55% at 500 nm, and a dark count less than 50 Hz. The quenching circuit and temperature control are integrated in each module. A time-to-digital converter (TDC) board, controlled by a rubidium oscillator plus a GPS receiver, time tags the pulses from the 4 channels. The individual times are stored in a 2 TeraByte memory. Iqueye can run continuously for hours, handling count rates up to 8 MHz, with a final absolute accuracy of each time tag better that 0.5 ns. A first very successful run was performed in Jan 2009; both very faint and very bright stars were observed, demonstrating the high photometric quality of the instrument. The first run allowed also to identify some opto-mechanical improvements, which have been implemented for a second run performed in Dec 2009. The present paper will describe the first version, the improvements implemented in the second one, and some of the obtained astronomical results.