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ATMOSPHERIC MONITORING FOR CHERENKOV TELESCOPE ARRAY CTA: DEVELOPMENT OF A HSRL PROTOTYPE FOR SYNERGY WITH RAMAN LIDAR. NTUA. E. Fokitis S. Maltezos A. Papayannis. P. Fetfatzis N. Maragos Y. Manthos. A. Aravantinos V. Gika M. Kompitsas. National Technical University of Athens.

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E. Fokitis

S. Maltezos

A. Papayannis

P. Fetfatzis

N. Maragos

Y. Manthos

A. Aravantinos

V. Gika

M. Kompitsas

National Technical University of Athens


  • VHE gamma ray astronomy & CTA observatory

  • Atmospheric monitoring for CTA

  • Design of a HSRL and synergy with RAMAN Lidar

  • Performance evaluation of a laser source

  • Optical characterization of F-P etalon receivers

  • Conclusions and Prospects

VHE g-ray astronomy

The VHE photons travelling through large distances are powerful probe of fundamental physics under extreme conditions.

Some of scientific issues:

  • Understand the High Energy cosmic phenomena and the relevant objects.

  • Identify the main cosmic accelerators, both galactic and extra galactic.

  • Search in extreme conditions for exotic phenomena implying new physics.

Possible mechanisms producing γ-rays:

Electromagnetic Processes

Synchrotron emission

Inverse Compton scattering (IC)


Hadronic Cascades

Decay of neutral pions produced by CRs interacting with the ambient gas.

p+p p± + p0+… e± + v + g +…

Ground based observatories

Gamma-ray fluxes for E>1 TeV are typically of the order of 2x10-7 m-2s-1, and thus, large detection area (>105 m2) is required.

Some contemporary observatories using IACT

[ Alessandro De Angelis, INAF INFN/Univ. Udine & LIP/IST

ECRS, Turku 2010 ]

The Imaging Atmospheric-Cherenkov Technique

[ Alessandro De Angelis, INAF INFN/Univ. Udine & LIP/IST

ECRS, Turku 2010 ]

  • Using an array of telescopes we can accomplish better:

  • background reduction of CRs

  • angular resolution (~arcmin at T1 eV)

  • energy resolution (~15 %)

Cherenkov angle (θc): ~10

Energy threshold: 21 MeV in NPT conditions

Maximum at 1 TeV shower: Height: 8 km, 200 photons/m2 in the Visible

Angular spread: 0.50

The CTA Observatory

  • The Cherenkov Telescope Array (CTA) is aproposed advanced facility for ground based highenergygamma ray astronomy.

  • This approach hasproven to be extremely successful for gamma raysof energies above 100 GeV.

  • The facility will consistof an array of telescopes enhancing the all skymonitoring capability and using low, medium and high energy sections.

Atmospheric monitoring for CTA

  • The monitoring direction has to follow the event direction.

  • The contribution of the NTUA Team to the CTA experiment deals with the appropriate instrumentation for high-accuracy atmospheric monitoring.

Developing a multi-wavelength HSRL

higher night sky background

The spectral region of the detector’s sensitivity practically, lies in the range about 325 - 525 nm.

The spectral region of interest for multi-wavelength atmospheric monitoring can be narrower, in the range 350 to 450 nm.

  • The HSRL allows higher sensitivity in comparison with Raman LIDAR because of the greater cross section of the interaction.

  • The effort now is to assemble a pulsed SLM coherent laser system at 355 nm. Via Raman cells it can provide additional UV and near UV wavelengths (multi-wavelength feature).

  • Fabry-Perot etalon pair is used to distinguish the signal contributions of aerosol and molecular scattering.

Spectral structure of the signal

Backscattering coefficient

Extinction coefficient

The Lidar equation describing the backscattered signal by molecules (m) and aerosols (particulates) (p) in the atmosphere:

Signal composition

Height determination:

Spectral discrimination:

  • The height of the measurement is pre-selected by the timing of the detection.

  • An accurate determination of the height can be achieved using the correlation function between the transmitted and the backscattered pulse.





Design configuration of HSRL

Sub-systems and specification of a multi-wavelength HSRL:

(1) Light source (transmitter):pulsed, narrow line SLM laser at neat UV 355 nm + gas Raman cells (being developed)

(2) Spectral discriminators (receivers):two, high overall finesse F-P etalons with different FSRs (being developed)

(3) Receiver telescope:mounting and rotation mechanism (prototype) + parabolic mirror, polishing quality ~λ/8(available)

(4) Signal detectors:high-sensitivity LN-cooled CCD (available and recently tested) + UV band-pass optical filter (available)

An indicative design of HSRL in backscattering mode

HSRL in synergy with RAMAN

RAMAN:the signal depends on αalone

the molecular number density of the reference gas (nitrogen)

: extinction coefficient for aerosols at l0

: extinction coefficient for molecules at l0

: reference Raman signal

HSRL: the signaldepends on both β and α

The methodology illustrating the how to determine the optical parameters of the atmospheric constituents

[A. ANSMANN, et al, Appl. Phys. B 55, 18-28 (1992)

M. IMAKI, Y. TAKEGOSHI and T. KOBAYASHI, Japanese Journal of Applied Physics Vol. 44, No. 5A, 2005, pp. 3063–3067]

: the molecular volume-backscatter coefficient

: the molecular extinction coefficient

: the volume backscatter coefficient of aerosol and clouds

: the Rayleigh backscatter power

: the Mie backscatter power

RAMAN Lidar setup in NTUA

  • Recently acquired Quanta Ray laser 1.2 J per pulse (not SLM) operating at 1064, 532 and 355 nm.

  • Option of injection seeding for frequency stabilization.

  • The NTUA Raman lidar with a multi-wavelength detection box at: 355-387 (nitrogen 1st Stokes)-407 (water vapor 1st stokes)-532-607 (water vapor 1st Stokes)-1064 nm.

RAMAN LIDAR setup of NTUA Atmospheric Environment group with 300 mm telescope

If the seeder is funded, then it is feasible to operate in HSRL mode

Studying the laser mode competition

1.05 A

2.00 A

1.55 A

Τest of Nd:YVO4 DPSS CW SLM Laser at 532 nm using a spectrum analyzer (scanning confocal F-P etalon 2 GHz FSR).

Capture of the spectrum analyzer signal observed at the oscilloscope.

A recently purchased Nd:YVO4 DPSS CW SLM Laser at 1064 nmhas to be converted to pulsed and amplified. In a subsequent stage a Second Harmonic Generationand a Sum Frequency Modewill used for providing the 355 nm beam.

Studying the frequency drift

  • (1) SLM CW DPSS Laser at 532 nm

  • (2) Beam reflector ~4 % (glass)

  • (3) Scanning confocal Fabry-Perot

  • (4) Laser Power Supply

  • (5) Digital Oscilloscope

  • (6) Sawtooth voltage generator

  • (7) Voltage Amplifier

  • After about 7 h (temperature stabilization) :

    • The frequency drift rate, following an exponential-shape curve by time, tends to zero.

    • The SLM feature is achieved.

    Molecular channel test at UV region





    Low pressure mercury lamp


    Triplet at 365 nm

    Experimental setup for testing molecular F-P etalon (1) with spacer d=13.015 mm: Use of mercury low pressure spectral lamp (2) with a narrow filter (3) at 365 nm.

    A system of three fringe patterns corresponding to the triplet at 365 nm

    (3 transition lines spaced by 0.5 and 1 nm).

    2-D plot of intensity showing the three fringe system.

    Evaluation of the etalon parallelism



    The excess fraction (ε) is determined

    in a grid of x,y points. The variation of ε reflects the effective spacing variation (parallelism defect) .

    Wavelength used: λ=435 nm)

    The relative spacing variation for a commercial as a reference (5 mm).

    Parallelism=λ/50 (P-V)

    The relative spacing variation of the etalon under test (13 mm).

    Parallelism=λ/4 (P-V)

    D=70 mm

    Modular Equation system

    Optimum solution for e´1 and e´2

    Not feasible

    Optimum-ideal solution for d

    Optimum solution for d

    Determination of the effective spacing

    A precision of few nm is feasible !

    • 1st stage of analysis: determination of the “excess fractions” ei(i=1,2) of the 2-D fringe pattern for both wavelengths.

    • 2nd stage of analysis: optimum solution investigation algorithm in the “Excess Fraction Space” by a novel method developed.

    Modular equation system to be solved:


    At least two well-known lines (wavelengths l1 and l2 with an accuracy of the order of 2×10-8).


    Mounting type of receivers

    Etalon for the aerosol channel:

    Mounting system proposed of this etalon (100 mm spacer):

    a cylinder made of “zerodur”.

    Etalon for the molecular channel:

    Mounting type proposed for this etalon (13 mm spacer):

    “Hansen mount” type applied at Dynamic Explorer Fabry-Perot made of “invar”.

    [ T. Killean et al, Appl. Opt. 21, 3903-3912 (1982) ]

    Conclusions and Prospects

    • A multi-wavelength in near UV HSRL design for atmospheric monitoring in CTA observatory is on the way at NTUA.

    • The synergy of HSRL/RAMAN lidar promises higher accuracy of atmospheric monitoring, thus we are studying this possibility.

    • The frequency stability and SLM feature of the tested CW laser is established after certain time of operation.

    • Our methods for characterization of the F-P etalon receivers were verified and can help us for serious improvements.

    • A near future prospect of our team is to accomplish funding for injection seeding to the Quanta Ray laser of 1.2 J per pulse for operating in HSRL mode.

    Backup transparencies

    Determination of the effective spacing - 2

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