interferometry observations

Possible applications of a novel type of photon counting instrument for

Intensity Interferometry observationsIntensity Interferometry observations

Giampiero NalettoUniversity of Padova

Workshop on Stellar Intensity Interferometry p y ySalt Lake City

29‐30 January 2009

IntroductionDuring the last years we realized in Padova two similar instruments, AquEYE and IquEYE, for astronomical applications .

They are essentially extremely fast photon counters, with the capability of time tagging the collected photons with a 50 ps time accuracy and storing all the timing data in a mass memory.

This type of instrument is really versatile because it allows to t i d d tl ith di t t t l if it bl l koperate independently with distant telescopes if a suitable clock

synchronization can be obtained.

We are planning to further develop this type of instruments forWe are planning to further develop this type of instruments for possible applications that can range from “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.

Giampiero Naletto SLC Workshop on SII

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

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The team

Many people are participating to the realization of this project:realization of this project:

Univ. Padova: C. Barbieri, I. Capraro, G. Naletto, T. Occhipinti, E. Verroi, P. , p , ,Zoccarato, V. Da Deppo, C. Facchinetti, C. Germanà, E. Giro, M. Parrozzani, F. Tamburini, M. Zaccariotto, L. Zampieri

INAF R A Di P lINAF Rome: A. Di Paola,

INAF Cagliari: P. Bolli, C. Pernechele

llINAF Catania: S. Billotta, G. Bonanno,

Collaborations: D. Dravins (Lund), A. C d (Lj blj )Giampiero Naletto SLC Workshop on SII


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Cadez (Ljubljana)

Outline

• Some history: QuantEYE• Some history: QuantEYE

• Description of AquEYE and of some of the obtained results

• Description of Iq EYE and of some of the obtained res lts• Description of IquEYE and of some of the obtained results (very preliminary)

• Results of the Joint Asiago Ljubljana Crab pulsar observation• Results of the Joint Asiago‐Ljubljana Crab pulsar observation (preliminary)

• Instrument present limitations and possible ways to overcomeInstrument present limitations and possible ways to overcome them

• Future applicationspp


4Possible applications of a novel type of photon counting astronomical instrument for Intensity Interferometry observations

QuantEYE proposalIn Sept. 2005, we completed a study (QuantEYE, the ESO Quantum Eye) in the frame of the studies for the 100 m OWL telescope.

The main goal of the study was to demonstrate the possibility to reach the ps time resolution needed to bringthe ps time resolution needed to bring quantum optics concepts into the astronomical domain, with two main ,scientific aims in mind:

‐Measure the entropy of the light through the statistics of the photon time of arrival (TOA)

‐ Demonstrate the feasibility of HBTIIGiampiero Naletto SLC Workshop on SII


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Why studying the photon time statistics ?



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Why Extremely Large Telescopes?

The above mentioned quantum correlations are fully developed on time scales of the order of the inverse optical bandwidth Foron time scales of the order of the inverse optical bandwidth. For instance, with the very narrow band pass Δλ = 0.1 nm in the visible, through a definite polarization state, typical time scales are 10 ps.

However, the photonflux is very weak evenfrom bright stars, sothat only Extremelythat only ExtremelyLarge Telescopes (ELTs)can bring Quantumg QOptical effects in theastronomical reaches.



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QuantEYEQuantEYE was conceived for measuring second‐ and higher‐order correlation functions in the collected photon stream (up to 1 GHz) from OWL with the highest time resolution (better than 0 1 ns)time resolution (better than 0.1 ns).



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Key limitation: the detector

The most critical point, and driver for the possible optical designs of QuantEYE was the availability of very fast and accurateof QuantEYE, was the availability of very fast and accurate photon counting detectors.

• Imaging PC detectors (ICCD, ICMOS, MCP) either do not allow g g ( , , )fast time tagging of the detected events, or have a rather low maximum total count rate

• Non‐imaging PC detectors (PMT, SPADs) either have a relatively low QE, or have a small sensitive area

SPADs are preferable: a 50 ps time resolution with count rates as high as 10 MHz can be obtained, with standard voltages and QE.

f h l ld b bl f hHowever, even if the time resolution could be acceptable for this application, the total count rate was still two orders of magnitude smaller than what was necessary !magnitude smaller than what was necessary !Giampiero Naletto SLC Workshop on SII


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Solution: splitting the problems …

To suitably design the system and to overcome both the SPAD limitations and the difficulties of a reasonable optical designlimitations and the difficulties of a reasonable optical design (coupling the 100 m pupil / 600 m focal length of OWL with a single 50 μm detector !), we decided to split the problems.In practice, we designed QuantEYE subdividing the system pupil into N × N sub‐pupils, each of them focused on a single SPAD (so giving a total of N2 distributed SPAD's).

In such a way, a “sparse” SPAD array (SSPADA) coping with the i d hi h ld b b i drequired very high count rate could be obtained.

The SSPADA is sampling the telescope pupil, so a system of N2

parallel smaller telescopes is realized each one acting as a fastparallel smaller telescopes is realized, each one acting as a fast photometer.



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QuantEYE optical design

Schematic view of the telescope pupil subdivision



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Advantages of this optical design

Th l b l t t i t ti ti ll i d b f t N2• The global count rate is statistically increased by a factor N2

with respect to the maximum count rate of a single SPAD. In the assumption of having N = 10 (100 SPAD's), the global countthe assumption of having N 10 (100 SPAD s), the global count rate becomes 1 GHz (one photon every 100 ns on each SPAD)

• Simpler optical designSimpler optical design

• Detector redundancy

• By suitable cross correlations of the detected signal a digital• By suitable cross‐correlations of the detected signal, a digital HBT intensity interferometer is realized among a large number of different sub‐apertures across the full OWL pupilp p p



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Overall QuantEYE block diagram

The overall system: two heads controls storage time unitThe overall system: two heads, controls, storage, time unit.



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AquEYE

While expecting the realization of the future E‐ELT we decidedof the future E ELT, we decided to apply the described concept to realize a much smaller version of the instrument, compatibly also with thef il bl f dfew available funds.We named this instrumentAquEYE the Asiago quantumAquEYE, the Asiago quantum eye: it has been applied to the AFOSC camera of the Asiago‐Cima Ekar (Italy) 182 cm Telescope.



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AquEYE optomechanical design

A simple way of realizing this small prototype was to consider an optical configuration in which the telescope pupil is divided inoptical configuration in which the telescope pupil is divided in four parts only by means of a pyramidal mirror.



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AquEYE subsystemsAFOSC focusAFOSC focus

Pyramidy

Focusing lensesg

Filters

SPAD



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Selected detectors

As best compromise, the selected detectors are SPADs produced by Italian company MPDby Italian company MPD.

Their main drawbacks are the small sensitive area (50 µm diameter) and a ≈70 ns dead time.diameter) and a 70 ns dead time.



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Advantages of multiple detectors

Differences between the photon times of arrival for 1 or 4 SPADs.

(some MHz total rate)(some MHz total rate)



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AquEYE electronics schematics



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Time referencing and taggingThe CAEN TDC board samples the collected events at 40 GHz (25 ps time resolution), multiplying a reference frequency at 40 MHz.

To maintain the desired 100 ps time accuracy over hours of observation avoiding too expensive solutions like Hydrogen‐maser or Cesium clock a rubidium oscillator coupled to a Trimble Mini Tor Cesium clock, a rubidium oscillator coupled to a Trimble Mini‐T GPS disciplined OCXO (Oven Controlled X‐tal Oscillator) has been used as external reference frequency to the CAEN TDC board.used as external reference frequency to the CAEN TDC board.

This clock is extremely accurate on short term, but has a drift for long periods. To remove this drift, the PPS signal from GPSDO g p g(GPS Disciplined oscillator, which is synchronized within 25 ns rms to UTC) is given in input to the CAEN board and time tagged

h i h h h li fi l i ftogether with the events. Then a post‐process linear fit analysis of the collected PPS allows to estimate the rubidium drift, and to remove itremove it.Giampiero Naletto SLC Workshop on SII


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Data handlingObviously, all the data have to be stored and preliminary analyzed at their “production rate”.

To store and analyze all the collected data a central storage unit with a capacity of 1 TB has been used.

The arrival time of eachphoton is given as inputt hto an asynchronouspost processor whichguarantees dataguarantees dataintegrity for thefollowing scientificinvestigation.



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The light curve of the Crab pulsar

Average Crab pulse profile from Asiago data (blue) and from 4 m g ( )Kitt Peak telescope data (red; Fordham et all, ApJ. 581, 2002).

The measured period in Asiago was P = 0.03362160125 s, to be compared with the P = 0.03362160253 s extrapolated from Jodrell Bank ephemeridesGiampiero Naletto SLC Workshop on SII


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Jodrell Bank ephemerides.

IquEYEThanks to the positive experience of AquEYE, it has been decided to realize q ,IquEYE, a more complex instrument for applications to larger telescopes, as NTT and TNG.



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IquEYE optical layout



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IquEYE opto‐mechanics



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IquEYE electronics

CAEN b dMonitor

CAEN boardcamera and motor controls

CAEN board(redundant)GPS

Rubidium clock

Acquisition

Control & data analysis &

storage server serverstorage server



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IquEYE block diagram

EQuAATFU OpticsEQuAElectronics for

Quantum Astronomy

ATFUAquEYE Time

and Frequency Unit

OpticsTelescope, Optical

AquEYE and Detectors

Mass Storage

QuASQuantum Astronomy Software

Scientific DATA



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Equa schematics



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The Crab pulsar at NTT



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The PSR B0540‐69

IquEYE @ NTT (2009)

HSP on HST (1993)

CTIO 4 m (1985)



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At the other extreme: Eta Carinae



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The Asiago‐Ljubljana experiment

The Ljubljana telescope (80 cm diameter) is 230 km far from Asiago



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The Ljubljana telescope (80 cm diameter) is 230 km far from Asiago.

Joint observations of the Crab pulsar

On 10‐11 October 2008 we performed joint observations of the Crab pulsar.

Both the observatories were equipped with a breadboard ACTS (Accurate and Certified Time System) clock unit provided by Th l Al i S Thi i i l i lThales Alenia Space. This is an experimental setup to simulate the characteristics of timing of the future Galileo system.

ACTS assures a time accuracy of 25 ns on UTC and certifies theACTS assures a time accuracy of 25 ns on UTC, and certifies the time. These units were used to have a “common” clock, with which we tried to synchronize the two observations.which we tried to synchronize the two observations.



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Obtained resultsThe obtained data have notbeen completely analyzed yet.

The preliminary results(determination of the initialphase of the Crab pulsarperiod) show that the twomeasurements were aboutmeasurements were about100 µs out of phase. This value is much larger than expected and suspected; investigations are going on to understand the reason p ; g g gof this discrepancy.However the pulsar period obtained by this measurement was in

h h h l b h lagreement within 1 ns with the value given by the value obtained by means of Jodrell Bank ephemerides, demonstrating a “perfect” internal clockinga perfect internal clocking.Giampiero Naletto SLC Workshop on SII


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Instrument present limits

• Total count rateThe used SPADs have two outputs: an extreme timing accurate‐ The used SPADs have two outputs: an extreme timing accurate (25 ps) NIM, which limits the linearity range of the detector to about 2 MHz; an about 10 times less accurate TTL, which gives ; , gup to 12 MHz count rate. To have the best timing, we used the NIM output, accepting a “low” count rate.

‐ The used CAEN board limits the total output count rate to 8 MHz

• Detector dead time

The used SPADs have an about 75 ns dead time, limiting the l h l ( f d) b lsingle channel maximum rate (if TTL output is used) but mainly

inhibiting the capability of detecting very time‐close photons



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Possible instrument improvements (I)

• Total count rateCAEN people assured that they will increase the board output‐ CAEN people assured that they will increase the board output band. Anyway, we could simply use more boards in parallel

‐ It is rather difficult to improve the MPD SPAD time accuracy‐ It is rather difficult to improve the MPD SPAD time accuracy performance. However, SPAD technology is fast improving: several companies are now producing them, and SPAD arrays are becoming available. It is reasonable to suppose that in a few years it will be possible to have more performing SPADs

• Detector dead time

The use of multiple detectors statistically allows to greatly d h bl h h h h d b hreduce this problem. The higher the detector number, the

higher the probability of detecting very time‐close photons, substantially reducing to zero the dead timesubstantially reducing to zero the dead time.



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Possible instrument improvements (II)

•The optical design can be improved•The optical design can be improved.

In fact, the present design is a consequence of the limited availability of suitable detectors Presently the detectoravailability of suitable detectors. Presently, the detector limitations imposed a multi channel optical design, with all the related complexity.

If SPAD arrays will be available in the future, a much simpler optical design will be possible.

• The timing accuracy can be improved

In future it will be possible to use the better GNSS Galileo receiver with the aim to achieve a better synchronization to UTC.



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Future developments

We are planning to bring IquEYE also to TNG, which is very similar to NTT A hypothesis under investigation is to leavesimilar to NTT. A hypothesis under investigation is to leave IquEYE (upgraded) as a resident instrument for NTT.

The next step will be to realize another version of this pinstrument to be brought to one of the existing 8‐10 m telescopes (for example the Very Large Telescope at Cerro Paranal, Chile, or the Large Binocular Telescope in Tucson, or Keck on Mauna Kea). We have already applied to be funded for this experiment and contacts have already been taken with VLTthis experiment, and contacts have already been taken with VLT.

We are also considering the possibility of mounting a quantum detector in the central pixel of the Cherenkov light collectordetector in the central pixel of the Cherenkov light collector MAGIC (Major Atmospheric Gamma Imaging Cherenkov) (Roquede los Muchachos, Canarias, Spain).



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HBTII possible application

This type of apparatus could be used with a network of telescopes allowing for example multi‐dimensional HBTIItelescopes, allowing for example multi dimensional HBTII performed by means of post‐process data analysis.



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Possible performance of HBTII applications

Simulations have been performed to verify the possibility of realizing HBTII with this type of instrumentrealizing HBTII with this type of instrument.

Test conditions:• λ = 500 nmλ 500 nm• Δλ = 3 nm• QE = 0.7• Losses = 0.3• Detector dead time = 70 ns• Number of detectors = 4• Number of detectors = 4

Two cases have been considered:

• 8 m telescopes 1 ns time accuracy 2 hours integration time• 8 m telescopes, 1 ns time accuracy, 2 hours integration time

• 1.8 m telescopes, 20 ns time accuracy (Tempo2), 4 hours i.t.



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Possible performance of HBTII applications



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Comments on the plot

• Small telescopes are rather inefficient for HBTII applications, but could be used with long exposure timesbut could be used with long exposure times

• To synchronize the observations, the photon TOA’s have to be “homogenized” at the solar system baricentre by suitable s/w, as g y y / ,Tempo2. The time error associated with Tempo2 is 20 ns: this is the error in time that has been considered for the present instrumentation applied to 1.8 m telescope. However it is not clear how it should be considered in these applications.

Th SNR i i i l h f h• The SNR ratio is proportional to the square root of the integration time: very long observations can be done

• Flattening of the lines is due to saturation of the SPAD because• Flattening of the lines is due to saturation of the SPAD because of high rate. If more SPADs can be used, the SNR can linearly increaseincrease



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Another simulation



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Other possible applications

The realized instrument allows to perform measurements of other very fast phenomena:other very fast phenomena:

• Variabilities close to black holes• Variabilities close to black holes• Free electron lasers in magnetars

• Flare stars• Flare stars• Lunar occultations• CV• CV• Exoplanetary transits•• …



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ConclusionsThe characteristics of QuantEYE, AquEYE and IquEYE, the instruments studied, realized and tested have been reviewed.

The proposed designs are very modular, and can be easily adapted to any “optical” telescope.

The performed tests showed that this type of instrument performs very well as extremely fast photon counters / photometers.

The instrument characteristics make it very suitable for HBTII applications also with the present design. It is reasonable to expect that in a few years much better performance can beexpect that in a few years much better performance can be obtained, mainly improving the time tagging accuracy.

The adopted philosophy of storing all the collected data allows theThe adopted philosophy of storing all the collected data allows the possibility of using network of telescopes, also located in different sites.



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interferometry observations

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