Very fast photon counting photometers for astronomical applications

Very fast photon counting photometers for astronomical applications: from QuantEYE to AquEYE Giampiero Nalettoa,b , Cesare Barbieric , Tommaso Occhipintia , Fabrizio Tamburinic , Sergio Billottad , Silvio Cocuzzae , Dainis Dravinsf a

Department of Information Engineering, University of Padova, Via Gradenigo 6/B, I-35131 Padova, Italy b CNR-INFM-LUXOR, c/o Department of Information Engineering, University of Padova, Via Gradenigo 6/B, I-35131 Padova, Italy c Department of Astronomy, University of Padova, Vicolo dell’Osservatorio 2, I-35122 Padova, Italy d INAF- Astrophysics Observatory of Catania, Via S. Sofia 78, I-95123 Catania Italy e CISAS - Center of Studies and Activities for Space “G. Colombo”, Via Venezia 15 - 35131 Padova (Italy) f Lund Observatory, Box 43, SE-22100 Lund, Sweden ABSTRACT In the great majority of the cases, present astronomical observations are realized analyzing only first order spatial or temporal coherence properties of the collected photon stream. However, a lot of information is “hidden” in the second and higher order coherence terms, as details about a possible stimulated emission mechanism or about photon scattering along the travel from the emitter to the telescope. The Extremely Large Telescopes of the future could provide the high photon flux needed to extract this information. To this aim we have recently studied a possible focal plane instrument, named QuantEYE, for the 100 m OverWhelmingly Large Telescope of the European Southern Observatory. This instrument is the fastest photon counting photometer ever conceived, with an array of 100 parallel channels operating simultaneously, to push the time tagging capabilities toward the pico-second region. To acquire some experience with this novel type of instrumentation, we are now in the process of realizing a small instrument prototype (AquEYE) for the Asiago 182 cm telescope, for then building a larger instrument for one of the existing 8-10 m class telescopes. We hope that the results we will obtain by these instruments will open a new frontier in the astronomical observations. Keywords: high-speed photometry, extremely-large telescope, quantum optics, avalanche photodiodes

1. INTRODUCTION In the past, the frontiers of astronomy have expanded adding new spectral regions to the most classical visible one, and today almost all the electromagnetic spectrum is accessible from ground and/or space telescopes. However, essentially all the present astronomical instruments are measuring either the directions of photon arrival, as for the cameras, or the energy of the arriving photons, as for the spectrometers, or some combination of these properties. In practice, current astronomy exploits only the first order spatial and temporal coherence properties of light. It is evident that this type of instrumentation does not allow to distinguish the physical processes at the base of the radiation source emission, for example if thermal effects, or stimulated emission, or synchrotron emission. However, as shown by Glauber,1 beyond the first-order coherence, and encoded in the photon arrival times, information can be found about the physics of the light emission or propagation (as for example whether photons have arrived directly from the source, or have undergone scattering on their way2–4 ) (see Fig. 1). Further author information: (Send correspondence to G.N.) G.N.: E-mail: [email protected]

Figure 1. Statistics of photon arrival times in light beams with different entropies (different degree or “ordering”): these can be random, as in maximum-entropy black-body radiation (Bose-Einstein distribution with a certain bunching in time), or may be quite different if the radiation deviates from thermodynamic equilibrium. (Adapted from Ref. [3])

In fact, the statistics of photon arrival times gives a measurement of the time ordering within the photonstream, and of its possible deviation from randomness. To quantify the amount of “order” in the photon stream arrival time, second and higher order coherence of light have to be measured: this means to measure the time correlations between two (or more) photons over timescales of the order of the first order coherence time. For astronomically realistic passbands (∆λ ≈ 1 nm), this time is of the order of picoseconds, much shorter than actual photometric resolutions. Acting on a more realistic nanosecond scale, the effects are still measurable even if not so strong, as demonstrated already years ago by the intensity interferometer,5 so far the only astronomical instrument that studied the second-order coherence of light. To perform this type of measurements, the time resolution and time tagging capabilities of astronomical instruments must be pushed well beyond their current capabilities. Even if with present instrumentation numerous discoveries have been made about the source timing characteristics (as, for example, optical pulsars, lunar and planetary-ring occultations, rotation of cometary nuclei, pulsating white dwarfs, X-ray binaries, gamma-ray burst afterglows, and many others), all these measurements are not really suitable for observing very fast phenomena. In fact, on one side the imaging systems are usually limited either by the camera electronics, which does not allow frame rates shorter than 1-10 ms, or by the low total count rate if photon counting devices are used; on the other the non-imaging systems are limited either by the low efficiency or by the low count rate. So, presently, no real “quantum effect” has been measured yet. With modern technology, mainly in the field of very fast photon counting detector, we would like to go well beyond these present achievements. To this end, we have carried out a conceptual study for QuantEYE, the fastest time resolution instrument for astronomical applications. QuantEYE was in theory capable to enter the domains of micro- and nano-second variability and beyond, with GHz photon counting rates to match. These would enable detailed studies of extremely fast phenomena as for example variability close to black holes, nonradial oscillation spectra in neutron stars, fine structures on neutron-star surfaces, and possible free-electron lasers in the magnetic fields around magnetars. A more complete description of the astrophysical problems that can be tackled by QuantEYE can be found in references.6–8 QuantEYE was designed9 taking into account the characteristics foreseen in 2005 for the 100 meter OverWhelmingly Large (OWL) telescope of the European Southern Observatory (ESO). However, after the exploration of the 100 m OWL concept and the production of the OWL Blue-Book,10 the International Review Panel concluded that while a viable concept with many strong points had been established, its likely overall cost, time

frame and level of complexity were uncomfortably high. So it recommended proceeding immediately with a more “modest” size telescope in the 30 to 60 m diameter range. In this paper we are going to describe what is the status of our activities in the framework of the new realm of quantum astronomy. In particular, although the final OWL design will differ from the one adopted in the QuantEYE definition, the proposed concept maintains its full scientific appeal, and can be easily adapted to a different extremely large telescope (ELT). So we will report in Sect. 2 what had been the results of the study performed for defining the QuantEYE characteristics, showing that the instrument modularity allows a very simple adaptation to any other existing telescope. However, this instrument, or an equivalent one that hopefully will be installed in one of the next generation ELTs, will not collect its first photons until rather far in the future. In the meantime, also to acquire some experience with this novel type of instrumentation, we are in the process of realizing a small instrument prototype, AquEYE (the Asiago Quantum Eye), for the Asiago (Italy) 182 cm telescope. This instrument and its characteristics will be described in Sect. 3. Owing to the peculiar characteristics of the used detection system, Sect. 4 has been dedicated to its description and characterization, together with the indication of how the electronics is able to manage the huge amount of data that will be collected and stored. Finally, Sect. 5 is dedicated to a brief description of what will be the future activities we are planning with this type of instrumentation, and of what could be the expected technological developments to improve the system performance.

2. QUANTEYE CHARACTERISTICS AND DESIGN The design of QuantEYE was driven mainly by two factors. The first was obviously the OWL telescope characteristics and performance. OWL, in its 2005 design, had a 100 m aperture with an f /6 focal ratio; moreover, it was fully corrected for geometric aberrations, and limited by seeing in the absence of an adaptive optics (AO) system. Since QuantEYE is essentially a very fast photometer, it did not need an AO corrector if the detector was sufficiently large: so, on the basis of nowadays rather long experience with large telescopes in “normal” seeing conditions, we had assumed that OWL would concentrate a large fraction of the light coming from a point-like source at infinity within a 1 arcsec image over a fairly satisfactory percentage of the observing time. Taking in account the 600 m telescope focal length, this means that one star-like object would normally fill a 3 mm diameter spot at the OWL focus. The second factor driving the QuantEYE design was the limited selection of presently available very fast photon counting detectors. Actually, the variety of photon counting detectors is not so restricted, but essentially all of them are not really suitable to this specific application. For example, image intensifiers coupled either with CCD or CMOS sensors do not allow a fast time tagging of the detected events. MCP based detectors,11–13 which have an extremely good temporal resolution down to a few tens of picoseconds, are limited by a rather low maximum count rate of a few kHz and by relatively low efficient photocathode in the visible range. Even if the development of the second generation H33D MCP based photon counting detector14, 15 seems very promising (expected time resolution of the order of 250 ps, global count rate of the order of 20 MHz), this detector has not yet the characteristics to satisfy the performance required for QuantEYE. Unfortunately, not even the state-of-the-art non-imaging photon counting detectors are able to satisfy the required performance. In practice the only possibilities are photomultipliers, which in any case have the drawback of needing high voltage, specific timing electronics, and have a rather low quantum efficiency; or the single photon avalanche photodiode (SPAD). At present, the latter has the best performance: with the most recent techniques of realizing SPAD’s,16 a time resolution as low as 50 ps, with count rates as high as 10 MHz can be obtained. Even if the time resolution is acceptable, for the specific application remains still unsolved the problem of the total count rate that is at least two orders of magnitude smaller than what is necessary. For what concerns the possibility of realizing SPAD arrays (SPADA), there is presently a large effort; however, until now only small SPADA’s have been produced (for example, up to 60 pixels17 ), and typically with separated pixels to limit the crosstalk. Unfortunately, also these SPADA are not useful for this application because the preferred solution would be an array of “contiguous” pixels; but it is reasonable to expect that the latter will be available in the near future.

Given all these limitations, considering that the present imaging photon counting detectors do not reach the required performance, and that a photomultiplier offers worse performance than a SPAD, we decided to adopt an optical solution in which the light collected by the telescope was falling on a “distributed” SPAD array, that is a sparse array of single SPADs. Even if with this solution QuantEYE was a “simple” photometer, that is it had no imaging capability so limiting the possible science, this was the only reasonable solution we could devise with the available technology. The QuantEYE wavelength range is set by that of the selected photon counting SPAD’s, that is 400-900 nm, where the quantum efficiency is better than 40%. Actually, SPAD’s based on germanium and other suitable materials for operating in the near infrared (1.0-1.8 µm) are being developed by industry. However, they have not been considered for our application for two reasons. First, because their dark count rate is relatively high; second, because a very wide spectral range makes extremely difficult to realize a lens system not seriously affected by chromatic aberration.

2.1. QuantEYE Optical Design The optical design of QuantEYE can be thought as divided in two parts: at first the beam is collimated after the OWL focus; then the beam section is subdivided into N × N sub-pupils, each of them suitably focused on a single SPAD (so giving a total of N 2 SPADs). In this way, since the detector “distributed array” is essentially sampling the telescope pupil, a system of N 2 parallel smaller telescopes is realized, each one acting as a fast photometer. For the specific case, we have selected N = 10. A schematic of the optical design of QuantEYE is shown in Fig. 2 and its 3D cut-view is shown in Fig. 3. QuantEYE consists of an inverse Cassegrain telescope with 600 mm focal length and 100 mm diameter which collimates the light beam after the focus of the OWL telescope. After collimation, where the radiation beam has an annular shape of about 100 mm maximum radius, it is foreseen the possibility of inserting filters and/or polarizers. The filtered beam is then collected by a 10 × 10 lenslet array sampling the instrument pupil. Each lenslet is an ad-hoc compound system of two doublets with a 10 × 10 mm2 section and 10 mm focal length. The 1/60 demagnification of this system is such to reduce the 3 mm diameter focus of OWL to 50 µm, corresponding to the baseline SPAD active area. To direct the lens focus on the SPADs, an optical fiber link has been assumed. This optical solution has two main advantages. The first is that in this way the global count rate is statistically increased by a factor N 2 = 100 with respect to the maximum count rate of a single SPAD: so, the global count rate becomes as high as 1 GHz. The second advantage is the possibility of obtaining a rather simple design for the lenses in the array, which greatly simplifies the optical design. In fact, it derives from basic optics that

Figure 2. Schematic of the optical solution for the conceptual design of QuantEYE . From left to right: from the OWL focus, the beam is collimated by an inverse Cassegrain, filtered, and then collected by a lens array (for simplicity only one lens has been drawn). At the lens focus, the beam is collected by a fiber and sent to the SPAD detector.

Figure 3. Tridimensional sketch of the optical concept of QuantEYE. The optical baffle is visible on the right, followed by the telescope and the lens array. The cylinder on the side locates the stack of insertable filters.

one arcsec 3 mm spot at the OWL focus can be focused on a 50 µm SPAD only if the beam aperture on the detector is so fast as f /0.1. This can be extremely complex to be realized taking in account the need of very small aberrations. However, if the beam size is divided in 10 × 10 portions, the beam aperture of each subpupil that has to be focused on the detector becomes f /1, which is a much more manageable value. To have an idea about the optical performance of this instrument, in Fig. 4 the plot of the encircled energy distribution due to a 1 arcsec extended source is shown. It is possible to see that essentially 100% of the 1 arcsec source energy falls within the 50 µm diameter of the fiber core.

Figure 4. Extended source encircled energy plot: percentage of energy falling within a circle of given radius for a uniformly illuminated 1 arcsec extended circular source. Note that the 25 µm limit corresponds to the radius of the fiber core.

It is clear from the just given description of this instrument, that this design is extremely versatile, and can be very easily adapted to any of the existing telescopes. What is necessary is only to have a collimating system, that can be either an all-reflecting, as proposed for QuantEYE, or a refracting one, and then a system selecting smaller portions of the instrument aperture. Each of these “sub-telescopes” will then focus the collected radiation on an extremely fast photon counting detector, so the instrument count rate will linearly depend on the single detector capability multiplied by the number of detectors.

2.2. Data Handling System Design The fundamental information that has to be recorded with QuantEYE is the arrival time of each detected photon. Taking in account the maximum possible data rate of 1 GHz, that is one photon every nanosecond, it is clear that the time tag associated to each event has to be accurate at sub-nanosecond precision for all the time of integration. Assuming integration times of the order of tens of minutes, a couple of critical points are immediately evident: the first is the need of having a sufficiently stable and accurate time-tagging capability over such a long time; the second is the capability of storing all the acquired data at their “production rate” and of performing a suitable data analysis, both in real time (to have a preliminary analysis) and in post processing. We have foreseen to solve the first problem using a Global Positioning System (GPS), or the future Global Navigation Satellite System (GNSS) Galileo, satellite receiver for providing an initial absolute time reference. This would also enable the possibility of having time coordinated observations with other instruments on ground and in space. Then, an exact differential time-tagging would be provided by a hydrogen maser clock, which assures a precision better than 100 ps for several hours. To store and analyze all the collected data, it was foreseen that QuantEYE had a central storage unit with a minimum capacity of 50 Terabytes connected to an a-posteriori analysis system through a high bandwidth transport channel. The arrival time of each photon is given as input to an asynchronous post processor (a cluster of CPU’s) which guarantees data integrity for the following scientific investigation. Specific parallel algorithms should be working together, optimizing the computations of the high order correlation functions between the time tagged photons.

3. AQUEYE DESIGN Owing to the versatility of the optical design of QuantEYE, we decided to apply the same concept to realize a much smaller version of the instrument, compatibly also with the available funds, for an application to the Asiago-Cima Ekar (Italy) 182 cm telescope: we called this prototype AquEYE, the Asiago Quantum Eye. This telescope can offer a good availability of observing time and an excellent environment. Moreover, one of the focal plane instruments of this telescope, AFOSC (Asiago Faint Object Spectrograph and Camera), can be easily adapted to our goals. AFOSC (see Figure 5) can be of great utility as it can be easily mounted on the telescope, and can take care of many observational needs, from pointing and guiding, to field vision and rotation, etc. A very simple way of realizing this small prototype is to consider an optical configuration in which the telescope pupil is divided in four parts only (N = 2). Under this assumption, an easy way to obtain four independent optical paths is to insert a pyramidal mirror at the exit of AFOSC, where the radiation beam is diverging (see Fig. 6). The beams reflected by the pyramid are independently sent along four perpendicular direction and each of them can be collimated by a suitable lens. At this position, a filter/polarizer can be inserted, and then the beam is focused by means of a second lens on a SPAD.

Figure 5. Left panel: the side view of AFOSC mounted at the Cassegrain focus of the 182-cm telescope in Asiago. Right panel: the exit lens of AFOSC at the center of the mounting flange of AquEYE.

Figure 6. Left panel: following the last lens of AFOSC shown at the left, a pyramid splits the light to four separate channels to the SPADs. Right panel: mechanical details of the pyramid.

Obviously, the AquEYE instrument specifications are much less stringent than the QuantEYE ones. In fact, the Asiago telescope focal length is only 16.1 m, and a 3 arcsec extended source (the average size of a point-like star, due to the limited seeing) gives a spot size at the telescope focus of about 0.23 mm. In addition, AFOSC introduces an almost 1/2 demagnification factor, bringing the size of the spot at the AFOSC output at about 130 µm. So, the lens train after the pyramid has to further demagnify the spot of only a factor 1/4 to have a final spot size of the order of 40 µm: it is because of this rather relaxed specification that it has been possible to design the system with only commercial lenses. As detectors, we have selected and acquired the 50 µm SPADs produced by the MPD (Micro Photon Devices, Bolzano, Italy) company. According to the data sheet, their quantum efficiency in the visible band is better than 45%, the dead time is around 70 nanoseconds, the time tagging capability is better than 50 picoseconds, and the afterpulsing probability is less than 1%. The internal thermoelectric cooling produces a dark count lower than 50 s−1 at −20◦ C. In Sect. 4 we will give the results of the detector quantum efficiency measurements performed at the Catania Observatory. An important simplification of the AquEYE optical design with respect to the QuantEYE one is the possibility of directly feeding the SPADs without the need of the optical fibers. In fact, in QuantEYE the fibers were necessay owing to the difficulty of accommodating in “parallel” the relatively large detector boxes at the lenslet array focal positions; in this case, since the four beams are directed along radial directions, there is no mechanical interference, and the SPADs can be directly mounted on the lens foci (see Fig. 7). The optical performance of the designed system, as estimated by a ray-tracing simulation, is excellent over the 50 µm size active area, from the blue (420 nm) to the red (720 nm), as shown in Figures 8 and 9. Here both the extended source focal spot, obtained considering the central and four extreme fields of view, and the corresponding encircled energy are shown. It is evident the instrument capability of focusing more than 90% of the incoming flux on the detector active area. Other system losses (of the order of less than 15%) are due to light scattering at the edges of the pyramid were the radiation beam is splitted and to lens backreflections. The surface characteristics of the optics have been measured by means of interferometric techniques, showing that all the optical elements behave nominally. At present we are performing the alignment of the four optical paths, using a CCD lab camera for monitoring the spot quality during the alignment activity. Only at the end of this activity, the SPADs will be integrated. A view of the AquEYE instrument, with the structure still open is shown in Fig. 10.

Figure 7. Opto-mechanical sketch of one radial optical path of AquEYE.

Figure 8. The energy concentration between 420 and 700 nm, for five positions (at the center and at the edges) over the assumed 3 arcsec field of view. The circle represents the sensitive area of the SPAD.

Figure 9. Extended source encircled energy plot: percentage of energy falling within a circle of given radius for a uniformly illuminated 3 arcsec extended circular source. The 25 µm limit corresponds to the SPAD sensitive area.

Figure 10. Top view of AquEYE with no cover. The pyramidal mirror is evident at the center of the structure. The SPAD detectors have still to be integrated.

Figure 11. Schematic diagram of the AquEYE detection system.

4. AQUEYE DETECTION SYSTEM The electronics of AquEYE can be divided into four sub-blocks with the following functionalities (see Fig. 11): (1) detection, which consists essentially of the four SPADS that convert the single photon detection events in NIM pulses; (2) acquisition, performed by a Time to Digital Converter (TDC) capable of time tagging the arrival of each NIM pulse from the detectors; (3) pre-processing and storing, where a computer reads the data given by the TDC board, performs in real time some statistics and finally writes all the acquired data into an internal and successively into an external storage mass; (4) time and frequency reference, which has to assure a stable and accurate temporal reference to the time tags for both short and long time. The used TDC acquisition board is a CAEN (Costruzioni Apparecchiature Elettroniche Nucleari) V1290N. This TDC is able to time tag the voltage pulses on its inputs with a 25 ps resolution. It is equipped with an internal counter that has a roll-over time of 52 µs which can be improved by means of an external trigger reference, reaching the possibility of performing an indefinitely long measurement. The time reference for the CAEN TDC board must permit a long (even several hours) time acquisition of data with a precision better than 100 ps. To obtain this extremely severe performance, we have analyzed the scientific

background and the presently available commercial solutions, finding that we need both a short term stability typical of a quartz oscillator and a long term stability assured by a primary time reference. Then, excluding too expensive solutions like Hydrogen-Maser or Caesium clocks, we focused on a GPS disciplined clock. We are presently finding the best trade-off between short and long term stability given by an OCXO (Oven Controlled X-tal (Crystal) Oscillator) or a Rubidium oscillator both giving an Allan deviation of approximately 10−13 at 1 second (under investigation). This primary reference system will be disciplined by a GPS receiver, at least until the arrival on the market of a good GNSS Galileo receiver, assuring also an accuracy of ≈ 10−14 for long time. This reference, at 40 MHz, is given in input to the CAEN TDC board to achieve the 25 ps time tagging precision. In addition, the same reference system generates a secondary signal trigger (1 to 10 kHz). The latter is necessary because the TDC can operate in two different modes: continuous storage (CS), in which the absolute time is rolled-over every 52 µs, or in trigger matching (TM), in which this secondary trigger is given in input to give a relative time reference. Since rather long exposure (several hours) measurements could be realized with not very bright sources, it is reasonable that this last operative mode will be the most used one. The overall system has been designed with a modular approach. The acquisition block can reach with no further additional hardware the number of 16 inputs in parallel. Moreover, in the future application of an instrument like AquEYE to a larger telescope, it will be possible to increase the number on SPAD channels simply adding in parallel other TDC boards all connected to the same VME bus.

4.1. Detector Characterization At the COLD laboratory of the INAF - Catania Astrophysical Observatory we have characterized one of the four MPD modules that will be installed in AquEYE. The module is characterized by a 50 µm cooled SPAD, a patented integrated Active Quenching, a TTL output pulse, a fast-timing NIM output pulse. For the SPAD characterization we have used one of the available facilities at the COLD laboratory.18 This facility consists of a Xenon lamp radiation source, a system to select the wavelength of interest (a first module that acts as a pre-filter, and a Czerny-Turner monochromator), a beam splitter to direct the monochromatic radiation towards an integrating sphere hosting a reference detector and the detector to be characterized (see Fig. 12). The optical apparatus heart is the vacuum monochromator that can select a wavelength of a Xenon lamp source with a bandwidth better than 1 nm; however, for this specific measurement we set the bandwidth to 6 nm to have a larger signal. In fact, owing to the very small sensitive area of the SPAD, and the consequent impossibility of focusing the radiation beam on it, we performed this measurement using the integrating sphere. The function of the integrating sphere is to spatially integrate radiant flux, so that, using the reference photodiode, we can know the number of photons per area unit. To have both detectors, photodiode and SPAD, having the same radiant flux, we designed and realized an appropriate mechanical flange allowing to connect the SPAD module and the photodiode at the same distance from the center of the sphere. The three elements (photodiode, sphere and SPAD module), before assembling, are shown in Fig. 13. The SPAD device has been characterized in the 350-1050 nm spectral range at the operating conditions settled by the electronics inside the module. We have measured a dark count of about 25 ± 15 counts/s, and the quantum efficiency values reported in Table 1 with relative errors, also shown in Fig. 14.

4.2. Data Analysis and Storage The CAEN board is mounted on a VME crate together with an optical bridge (extender). The latter is able to transmit all the VME data traffic in the crate toward a standard PCI board inside the controlling personal computer (PC) with a maximum data-rate of 33 Mbytes/s. We are presently developing the software for the PC in C++ language using the CAENVME libraries for the low level acquisition programs. In the near future we will develop a statistics engine, to be run in parallel to the acquisition program, adopting a multi-thread programming style for a multi-core platform. The acquired data will be temporary stored on the internal hard drives of the PC, for the real time statistical analysis, and then it will be placed on the external 1 TByte hard disk for a permanent storing. At this point the data can be moved to any other high performance processing workstation. To have the highest computational power to perform the necessary correlation analysis, it will be necessary to use a cluster of computing systems.

Figure 12. Schematic diagram of the facility of the COLD laboratory used for characterizing one of the available SPADs.

Figure 13. The photodiode, the integrating sphere and the MPD module utilized during the test before to be placed on the characterization system.

5. FUTURE DEVELOPMENTS After the completion of the AquEYE instrument and the analysis of the obtained results, the subsequent goal will be to define an upgraded version of this very fast photometer to be brought to one of the existing 8-10 m telescopes, such as the Very Large Telescope (Cerro Paranal, Chile), or the Large Binocular Telescope (LBT) (Tucson, AZ, USA). In particular, the two parallel telescopes of LBT would allow to perform a modern version of the Hanbury Brown - Twiss Intensity Interferometer,5 with a 100-fold improved sensitivity thanks to the augmented quantum efficiency, the much higher electrical bandwidth, the higher collecting area, and the much superior optical quality of the telescopes. Finally, consideration will be given to the possibility of mounting a quantum detector in the central pixel of the Cherenkov light collector MAGIC (Major Atmospheric Gamma Imaging Cherenkov) (Roque de los Muchachos, Canarias, Spain) as a precursor for an ELT. Some considerations, moreover, can be done thinking to the possible technological developments. In fact, as already mentioned, the QuantEYE design and the derived AquEYE one have been conceived on the basis of present technologies, mainly in the field of very fast photon counting detectors. However, this field is relatively young and a lot of development can be expected in the next years. So, in the perspective of realizing in the future an instrument for measuring the second and higher order correlation function of a photon stream coming from an astrophysical source, for application to an ELT, it is reasonable to expect that new and better detector will be available. Under this hypothesis we have studied some possible variations to the instrument designs previously described, and in particular to the QuantEYE one.

Table 1. Quantum efficiency values of the SPAD under test measured at the COLD facility. The third column gives the measurement accuracy in terms of the root mean square (RMS) value.

Wavelength (nm) 350 400 450 500 550 600 650 700 750 800 850 900 950 1000 1050

Quantum Efficiency (%) 19.7 25.5 39.1 53.3 58.3 53.5 47.3 41.6 22.1 13.4 8.2 4.1 1.9 0.9 0.4

RMS (%) 1.5 1.4 1.8 3.2 2.0 2.5 3.3 6.9 1.0 0.6 0.4 0.2 0.1 0.5 0.1

Figure 14. Plot of the measured quantum efficiency values with the associated error bars.

A first possible improvement of the system is the possibility of avoiding the optical fibers from the lens focus to the SPAD, and to couple the array lens foci directly with a SPADA. In fact, the SPAD technology is greatly advancing in the realization of “granular” arrays. One example of this is the SPADA detector under realization for the adaptive optics MACAO system for VLT,19 in which a 60 × 4 SPAD array has been built. From contacts with the MPD company, it seems that realizing a “granular” SPADA with 50 µm size sensitive areas separated from each other by 10 mm, the distance among the lens array foci in the QuantEYE design, is not a real problem. If feasible, this option would greatly simplify the system, making it at the same time more robust and efficient. A second improvement to the system could be possible if more efficient near infrared (NIR) SPADs would become available. This, coupled to the possibility of inserting suitable filters independently in each sub-pupil of the telescope, would allow the simultaneous observation of the same object at different spectral bandpasses, from the visible to the NIR, greatly enhancing the possible science return. Finally, some considerations can also be done in the hypothesis of having a real “imaging” SPADA, that is a detector with an array of contiguous pixels, a total count rate of the order of GHz and no crosstalk among near pixels. When this detector is available, the instrument design will be extremely simplified, because the only necessary thing would be to locate the detector at the telescope focus to obtain the requested scientific return. However, this ideal array seems rather far from being realized, also with the present rate of technology development. In has also to be mentioned the practically sure improvement of the computing systems necessary to perform the analysis of the huge, multidimensional database generated by this type of instruments. The necessary computational power is really large, and new algorithms have to be developed to realize the data analysis in a reasonable amount of time. To this end, we hope not only in the normal increase of the computational power, but also that in the future quantum computing and quantum algorithms might be available, greatly improving the computational capabilities.

6. CONCLUSIONS In this paper we have reviewed the characteristics of QuantEYE, proposed as one of the focal plane instruments for the OWL telescope. This instrument has been designed to reach the highest performance in terms of photon counting capabilities, to measure the second and higher order correlations in a stream of photons from an astrophysical source. The proposed design is modular, and can be easily adapted to any telescope. To acquire some confidence with this type of new instruments, we are now realizing AquEYE, a much smaller version of QuantEYE, and it should be put in operation at the Asiago telescope in June 2007. With this telescope, of modest size, it will not be possible to achieve significant results on the quantum statistics of the photon streams from astrophysical sources. However, we plan to tackle several high time resolution astrophysical problems (occultations, cataclysmic variables, flickering and flare stars, and so on) with it, taking advantage of the possibility of an a posteriori integration of the counts. AquEYE will thus be able to provide a way to test on a telescope the feasibility of quantum optics with future ELTs, and to elucidate some of the problems likely to be encountered. Moreover, with this instrument we will improve the time resolution achieved by the pioneering observations made in the past with fast photometers (such as MANIA20 ), or more recently with micro-channel plates and APDs (e.g. OPTIMA21 ).

ACKNOWLEDGMENTS The QuantEYE project has been partly supported by ESO and partly by the University of Padova. AquEYE has been supported by the University of Padova and enjoys the collaboration of researchers from INAF Observatories in Rome, Cagliari and Catania, from INRIM Torino and Lubljana University in Slovenia.

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