IQUEYE: Italian Quantum Eye

Iqueye (Naletto et al. 2009, A&A 508, 531), as its precursor Aqueye (Barbieri et al. 2009, Journal of Modern Optics, 56, 261), is a conceptually simple fixed aperture photometer that collects light within a field of view (FoV) of only a few arcseconds (selectable from 1 to 6), divides the telescope light beam into four equal parts, and focuses each sub-beam on an independent single photon-counting diode. Two wheels allow to insert suitable filters and polarizers along the optical path. Since the instrument has no imaging capability, a field camera visualizes the portion of the sky under investigation. The most innovative part of Iqueye consists of a data acquisition system that, coupled to ultrafast detectors plus a rubidium oscillator and a GPS receiver, allows us to time tag the detected photons with a final absolute UTC referenced rms time accuracy superior to 0.5 ns over one hour of observation. The optical design of Iqueye is fairly simple. At the NTT Nasmyth focus, where Iqueye interfaces the telescope, a holed folding mirror deflects the light from the telescope by 90°, sending, by means of a suitable objective, the field around the star to be studied in the field camera focal plane. The light from the target object instead passes through the hole and is collected by a collimating refracting system. Two filter wheels located after the first lens allow the selection of different filters or polarizers. The light then reaches a focusing lens system, which, together with the preceding lenses, (de)magnifies the collected telescope image by a 1/3.25 factor. Along each arm, the sub-pupil light is first collimated and then refocused by a suitable lens system, which further (de)magnifies the image by an additional 1/3.5 factor, over the SPAD. The signals from the 4 SPADs is transferred, by means of calibrated equal electrical length coaxial cables to ensure the same electrical delay, to one of the input channels of a time to digital converter (TDC) board (CAEN V1290N, originally developed for high energy physics applications). This TDC board is nominally able to time-tag the voltage pulses on its inputs with a 24.4 ps time resolution: this very high sampling is obtained by means of a 40 MHz internal oscillator whose frequency is multiplied by 1024. Unfortunately, the quality of the internal oscillator is insufficient to satisfy the extremely severe stability requirement of our applications, for which it is necessary to obtain simultaneously both the short-term stability typical of a quartz oscillator and the long-term one assured by a primary time reference. This task could be reached by using primary time references such as hydrogen-maser clocks, which however are really expensive, difficult to handle and not available at NTT. To obtain a nearly optimal and affordable performance, we therefore focused our attention on a combination of a rubidium oscillator, a GPS receiver, and a post-processing algorithm. This unit corrects the long-term drift of the reference frequency provided by the rubidium oscillator by means of a post-processing algorithm that uses the pulse-per-second (PPS) signal provided by a mini-T Trimble GPS receiver. The GPS also provides the synchronization to UTC. A dedicated server manages all the data exiting the TDC. This server is connected to the VME bus inside the CAEN crate. From here, through a bridge and an optical fiber, the data are sent to the acquisition server, with an approximate maximum speed of 60 Mb/s. The user interface, which is implemented by means of a second server, has been developed as a Java multitasking code: it allows the acquisition control and the data storage, the control of each instrument subsystem and in particular of all the mechanisms, it can perform several real-time quick-look statistical analyses of the acquired data, and allows us to read and store the images of the stellar field around the selected object acquired by the field camera. These images can be used for an a-posteriori analysis of guiding errors and sky conditions.