Building a lidar for cta
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Building a LIDAR for CTA. What is the new “thing” at IFAE?. Imaging Atmospheric Cherenkov Technique. PHYSICS OF SHOWERS Cosmic rays and gammas impinge the atmosphere Electromagnetic cascades e-e+ pairs bremsstrahlung Cherenkov radiation and Hadronic cascades pions and muons

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Imaging atmospheric cherenkov technique
Imaging Atmospheric Cherenkov Technique


  • Cosmic rays and gammas impinge the atmosphere

  • Electromagnetic cascades

    • e-e+ pairs

    • bremsstrahlung

    • Cherenkov radiationand Hadronic cascades

    • pions and muons

  • Typical Cherenkov signal is bluish light with few ns duration



~ 10-20 km

~ 1o

Cherenkov light cone

~ 120 m

Atmosphere in iact
Atmosphere in IACT

  • Atmosphere is actually a part of the detector

  • Need to characterize it for accurate measurements:

  • Atmospheric Profile:

    • Can change seasonally

    • Affects first interaction point and Cherenkov yield for a given shower.

    • Can be measured with Radiosondes.

  • Aerosols

    • High level (e.g. clouds) can occur around shower-max and so affect Cherenkov yield & image shape etc.

    • Low Level (near to ground level) which act as a filter, lowering the Cherenkov yield.

    • Can be measured with LIDARs


  • Light Detection and Ranging

  • Same name, many different applications:

    • Industry: Automation, vehicle cruise control, video clips, traffic monitoring

    • Geology: Elevation models, terrain surveys

    • Military: Long range 3D imaging, missile guiding

    • Nuclear physics: Density profile of fusion reactors plasma

    • Astronomy: Distance to moon, relativity measurements

    • Meteorology

Basic lidars
Basic LIDARs

  • Mainly used to measure distances

  • Pretty common use

  • A short pulse is emitted and backscattered

  • Distance is proportional to time between emission and reception

  • Low energy laser, high rate

  • Single / dual axis mirror systems

Extended lidars
Extended LIDARs

  • LIDAR technique is continuously evolving:

    • Coherent detection

    • Optical heterodyne techniques

    • Inelastic scattering

  • LIDARs can measure many things:

    • Distance

    • Speed

    • Rotation

    • Chemical composition and concentration

Lidar for atmospheric measurements

A short light pulse is emitted to the atmosphere

A portion of the light is scattered back toward the lidar system

The light is collected by a telescope and focused upon a photo detector.

LIDAR for atmospheric measurements

Laser source


We measure the amount of backscattered light as a function of distance to the LIDAR

The lidar equation
The LIDAR equation

  • Some assumptions have to be made to solve the equation

  • Klett inversion has associated systematic uncertainty of around 30%

Backscattering coefficient



Extinction Coefficient




Typical response
Typical response

What’s this?

Cloud, aerosol,…?

Clean atmosphere

Attenuation, when, why?

Inelastic scattering raman
Inelastic scattering: Raman

  • Not all scattering is elastic

  • In some cases molecules change their vibrational and/or rotational state (Raman process), adding or absorbing part of photon’s energy

  • Shift on the wavelength of scattered light, depending on molecule states

  • Raman nitrogen/oxygen signals can be used to retrieve aerosol extinction coefficients with low uncertainty

  • Cross section for Raman is orders of magnitude smaller than elastic

  • Powerful lasers, large telescopes, efficient detectors and photon counting are required

Clue experiment
CLUE experiment

  • Old experiment in La Palma, sharing space with HEGRA

  • Aim to measure matter/antimatter ratio in cosmic radiation observing the Cherenkov light produced by air showers

  • Not a big success…

  • But can be recycled for a Raman LIDAR!

Open clue container
Open CLUE container

  • Fully robotized lids, “petals” and telescope frame

  • Easy to transport

  • One still in La Palma

Clue telescope
CLUE Telescope

Multiwire proportional chamber filled with C4H11NO


d=1.8 m


High FOV

Excellent luminosity

Big hole in the center

Electronics behind mirror

Clue good bad things
CLUE good / bad things

    • Robotized housing for the LIDAR

    • Motorized telescope frame with big mirror

    • Space for electronics on the same frame

    • Mirror may be even too big and in not so good shape

    • Obsolete control electronics

    • Almost no written documentation

    • Tons of things to do, few experience

Telescope frame
Telescope frame

  • Mechanical model redone from scratch

    • Finite elements simulation


  • Raman LIDAR usually use Nd:YAG lasers 355 nm (tripled)

  • Plan to buy one with adjustable power and firing rate for development.

  • Two possible locations:

    • Installed on the center of the mirror, on the other side of the hole

    • On the focal plane, behind photodetector / fiber

  • Photosensor near laser to read the actual power and length of each pulse

  • Powerful lasers and airports do not mix well

    • Authorization required?

Optical setup

1st stage

2nd stage

What else?

Optical setup

Can get very complicated!!!

(from UPC)

Optical setup ii
Optical setup II

  • Build custom mechanical pieces for compact and precise optical setup.

  • Fiber and setup are attached to the telescope frame, no relative movements.

  • Easily extendable to receive extra wavelengths.

  • Use narrow-band filters or diffraction grating?


  • Raman signal is much smaller than Rayleigh:

    • Dual DAQ systems: standard digitization for low altitudes (big signals) and photon counting for extended range.

  • DAQ with high dynamic range and fast data transfer, but not a lot of BW needed

    • 40 MHz sampling rate -> 3.75 m per sample

    • 30 Km -> 4000 samples memory

    • Dynamic range >16 bits (20 bits)

    • For rates of 1Khz, many channels ~50MB/s.

The future
The future

  • Motor control for telescope movement and container aperture

    • Ethernet based motor driver already in development

    • Waiting for the container to know specific motor requirements

  • Decide on a Laser, create control SW/HW.

  • Decide on sensors, order components and build optical setup.

  • Clean the telescope mirror, verify optical characteristics and modify mechanical structure to adapt to laser, optical setup and DAQ.

  • Design/Order Acquisition HW and SW.