12 - Lezione OA AA1920 - Telescopi gamma - Modalità...

Ottica Astronomica AA1920

Telescopi γ-rays

LE-HE (10s MeV/100 Gev) Space-based Observations

Coded-mask telescopes for the low-energy range Compton telescopes for the medium-energy range Pair creation telescopes for the high-energy range

VHE-HE (>10s GeV) Ground-based Observations

Imaging Air Cherenkov Telescopes (IACTs) or Water Cherenkov Detectors Extensive Air Shower (EAS) detectors.

https://commons.wikimedia.org/wiki/File:TheElectromagneticSpectrum.jpg


Cherenkov Telescopes

n ∼ 1.00029 @ STP

1.5°

Instruments to detect photons derived from the interactions of the primary γ-ray with the upper layers of the troposphere. The atmosphere is a key part of the detection chain.

Above 50 GeV the production of atmospheric electromagnetic showers by primary γ-rays is dominated by the pair production and bremsstrahlung mechanisms 0.2°



ultra-relativistic electron through a layer of 10 km

UV G. R

odeghiero 2015N photons emitted by a charged

particle of charge ze per unit path

length x and unit wavelength λ

Observations mainly at UV-VIS waves in a photon starved regime.

What about the atmospheric diffuse background during a night of the new moon?

Bckg (350 nm - 450 nm) =1012 photons*sr/m2/s

Cherenkov flash flux 50 photons/m2 in 5 ns(*) in a radius of 100 m (light pool), angle subtended almost 1 deg (3*10-4 sr) with these numbers the residual bckg is of the order of 1-2 photons/m2

(*) differently from conventional optical astronomy, the image can’t be improved increasing the exposure….

Ottica Astronomica AA1920 de Naurois & Mazin, 2015

Direction reconstruction: from single telescope towards array of telescopes to allow geometric reconstruction of shower direction and impact in stereoscopic mode

What about the atmospheric diffuse background during a night of the new moon?Bckg (350 nm - 450 nm) =1012 photons*sr/m2/sCherenkov flash flux 50 photons/m2 in 5 ns in a radius of 100 m, angle subtended almost 1 deg (3*10-4 sr) with these numbers the residual bckg is of the order of 1-2 photons/m2



Ground-based γ-ray astronomy with Imaging Atmospheric Cherenkov Technique was pioneered with the Whipple, CAT and HEGRA telescopes.In the field of very high energy gamma-ray astronomy (VHE, energies >100 GeV) recent IACTs are:

H.E.S.S.MAGIC VERITASCANGAROO

and the future is

CAT Cherenkov Telescope Array



H.E.S.S.High Energy Stereoscopic Systemfour 12 m diameter IACTs + one 28 m IACT

Phase 1Four 12m in diameter telescopes on a square with 120m side length

Phase 2One single huge dish with about 600 m2 mirror area(28m in diameter)

H.E.S.S. is located in Namibia, near the Gamsberg mountain, an area well known for its excellent optical quality.



MAGICMajor Atmospheric Gamma Imaging Cherenkovtwo 17 m diameter IACTs F/1.03 separated by 85 mVHE (30 GeV - 100 TeV)

2200 m a.s.l. on the Roque de los Muchachos European Northern Observatory on the Canary Island of La Palma (28°N, 18°W)



VERITASVery Energetic Radiation Imaging Telescope Array Systemfour 12 m optical reflectors VHE (50 GeV - 50 TeV)

Fred Lawrence Whipple Observatory (FLWO) in southern Arizona, USA



CANGAROOCollaboration of Australia and Nippon (Japan) for a GAmma Ray Observatory in the Outbackfour

four 10 m optical reflectorsVHE (100 GeV)

Woomera, Australia



Northern Hemisphere Site rendering; credit Gabriel Pérez Diaz, IAC SMM

CATCherenkov Telescope ArrayTwo array sites = CTAObservatoryEnergy range (20 GeV - 300 TeV)

Large-Size Telescope (LST) 23 m diameter – parabolic shapeMedium-Size Telescope (MST) 12 m diameterSmall-Size Telescope (SST) 4 m diameter

La Palma – Canarie (see MAGIC)



Southern Hemisphere Site rendering; credit Gabriel Pérez Diaz, IAC SMM

Atacama desert - Chile

CATCherenkov Telescope ArrayTwo array sites = CTAObservatoryEnergy range (20 GeV - 300 TeV)

Large-Size Telescope (LST) 23 m diameter – parabolic shapeMedium-Size Telescope (MST) 12 m diameterSmall-Size Telescope (SST) 4 m diameter



Main telescope optics parameters:

Reflective area to define the amount of collected light (large aperture and only a few

optical elements) PSF/large FoV

to be compared with pixel diameter (see photomultiplier tube), i.e. low angular resolution (arcmin)

Time dispersion different light paths through the telescope smaller than c*3 ns (average

Cherenkov light pulse from a gamma-ray shower) Large throughput

Large FoV (≈15 deg) -> 103 m2 deg2 > 102 m2 deg2 optical telescopes

Optics configurations:

Parabolic reflector Davies-Cotton design

Schwarzschild–Couder telescopes Aplanatic Telescope

single-mirror

two-mirrors



Davies-Cotton telescope(originally proposed as a solar concentrator)

Each facet has a spherical shape with a curvature radius = 2f and is distributed along a sphere of radius f

VERITAS telescope (Davies-Cotton design) 350 identical hexagonal spherical mirrors (of area 0.322 m2 and radius-of-curvature of approximately 24m) giving a total reflector area of 110 m2



Parabolic mirroradvantage: minimum time dispersion (isochronous)

Davies–Cotton design advantages: better off-axis imaging (large FoV)

many small, identical and spherical facetstrivial optical alignmenteasy mirror replacement and mirror re-coating

Both single-mirror solutions are affected by off-axis aberrations, limiting the maximum aperture. Increasing the aperture, decreases the focal ratio (f) while primary aberrations are amplified with the field of view (δ):

only two-mirrors could also correct coma.



Parabolic mirroradvantage: minimum time dispersion (isochronous)

Davies–Cotton design advantages: better off-axis imaging (large FoV)

many small, identical and spherical facetstrivial optical alignmenteasy mirror replacement and mirror re-coating

Bernlohr et al. 2003

PSF

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H.E.S.S.

Davies–Cotton layout Davies–Cotton layout f/1.2 (bottom)Parabolic layout (top)i.e. spherical facets on parabola

Davies–Cotton layout f/1.2with mirror facets with different PSF

Increased cost, lower resonant freqs + mechanical complications (see spider for camera)



H.E.S.S.

Mirror facet support unit

Mirror mountings

The 499 PMT pixel camera. The focus box is 1.8 m square. A remotely operated shutter usually covers the camera during daylight hours.

VERITAS

An image of Polaris in the focal plane of the telescope recorded with a CCD camera. The point spread function is 0.06° FWHM. The circle indicates the size of a VERITAS PMT (0.15° diameter).


Ottica Astronomica AA1920 W. Hofmann



H.E.S.S.Mirror facets alignment

Cornils et al. 2003

Images of a star on the camera lid before alignment. Each spot corresponds to the star image of a mirror facet. Spots spread radius ~1 deg -> 0.5° facet tilt (doubled due to reflection)


Ottica Astronomica AA1920 W. Hofmann



Schwarzschild–Couder designadvantage: large FoV

The ASTRI (*) prototype, the first Schwarzschild-Couder telescope to be built and tested, was inaugurated in September 2014 and has been undergoing testing at the Serra La Nave observing station on Mount Etna in Sicily. The ASTRI is one of three proposed Small-Size Telescope designs for CTA. Credit: Cherenkov Telescope Observatory

(*) Astrofisica con Specchi a Tecnologia Replicante Italiana

Vercellone S. et al. 2012

Cherenkov Telescopeshttps://www.media.inaf.it/2019/05/07/cielo-gamma-due-specchi-astri/




Schwarzschild aplanatic telescope (1905)f/3 with FoV = ±1.4°large obscuration ratio (0.5) and focus between M1 and M2M1=hyperboloid and M2=oblate spheroid difficult to manufacture and test.

Couder aplanatic anastigmatic telescope (1926)M1-to-M2 distance exactly twice the EFL (f’), i.e. a less compact design but a lower obscuration ratio.

modified by

Wilson R.N. 2007




Wilson R.N. 2007




SC aplanatic telescopes for application in γ-ray astronomy with IACTs was developed by V. Vassiliev et al. (see Refs).

Significant reduction of the plate scale with respect to single-mirror solutions making it compatible with cameras of PMTs and Winston cones that have dimensions in the order of centimeters.

ASTRI focal length value (f’ = 2141.1 mm), the derived plate scale is PS = 96"/mm.Angular resolution in sky of ∆J=0.166°.


Telescopi γ-rays







Water Cherenkov Telescopes

HAWC is located on Sierra Negra volcano near Puebla, Mexico at an altitude of 4100 meters. The detector has an instantaneous FoV ≈ 15% of the sky and during each 24 hour period HAWC observes two-thirds of the sky.The detector contains 300 tanks with 1200 PMTs in total

HAWCHigh-Altitude Water Cherenkov observatory (from MILAGRO’s heritage)VHE (100 GeV and 100 TeV)


Water Cherenkov Telescopes

The water is dense (relative to air), and so a γ-ray produces an e+e- pair once it enters the tank. These charged particles then emit Cherenkov radiation as they speed through the water.

G. R

odeghiero 2015

41°

HAWC corrugated steel tanks 4 m high and 7.3 m in dia with 4 PMTs sensitive at UV wavelengths

nwater = 1.33


Telescopi γ-rays







Extensive Air Shower detectors

Detectors that measure particles of the shower tail reaching the ground. This method provides a snapshot of the shower at the moment it reaches the ground and constitutes the so-called “particle sampler" technique.Those detectors have a very large duty cycle (potentially 100%), but rather high energy threshold (as high energy showers are more penetrating and produce charged particles at lower altitude than lower energy showers). Moreover, as they only have access to shower tails, they usually have a rather poor capability to discriminate the showers induced by γ-rays from the much more numerous showers induced by protons and charged nuclei. Such detectors are usually installed at high altitude to collect more charged particles.


Extensive Air Shower detectors

The field array (200m x 200m) consists of 252 detector stations arranged on a rectangular grid with a distance of 13 meters to each other.

KASKADE

In each station there are up to four electron/gamma-detectors and one muon-detector under a iron-lead-absorber

A calorimeter is one of the main components of KASCADE: the hadrons and their interactions

are important for the understanding of the shower development within the atmosphere


Telescopi γ-rays







Coded-mask telescopes

The telescope is a coded mask: each direction in the sky results in a distinctive shadow pattern (shadowgram, see PSF). The image is recovered by deconvolution.

IBIS imager aboard INTEGRAL, an exampleThe coded mask (a) partially covers the operture of the telescope. It is made of opaque plates and holes optimally distributed. The detector (b) records the shadow of the mask projected by the gamma-ray sources located within the field of view (c). In the case of two sources, the superimposition of the shadows of the mask onto the detector is more complex.

Credits: ISDC/M. TürlerESA – IBIS-INTEGRAL coded mask

a square of size 1064 × 1064 × 16 mm3 made up of 95 ×95 individual square cells of size 11.2 × 11.2 mm2


Compton telescopes

Coded mask patterns can be apparently random (Beppo-SAX/WFC) or highly structured (INTEGRAL/IBIS) but always satisfying established principles.

Different mask patterns exhibit different image resolutions, sensitivities and background-noise rejection, and computational simplicities and ambiguities, aside from their relative ease of construction.

FZP = Fresnel Zone PlateORA = Optimized RAndom patternURA = Uniformly Redundant Array

HURA = Hexagonal Uniformly Redundant ArrayMURA = Modified Uniformly Redundant Array

LevinVeeraraghavan


Coded-mask telescopesSchematic view of the SPectrometer of INTEGRAL (SPI)energy range 20 keV - 8 MeV

A 3-D plot of the shadowgram of SPI showing the counts received in each of the 19 hexagonal Ge detectors during one pointed observation of the INTEGRAL spacecraft.

Credits: SPI Team

Credits: IS

DC

coded mask

19 germanium detectors

complex AntiCoincidence System (ACS) to actively shield the tube and the bottom of the telescope

Photo courtesy IN

TA, S

pain

127 hexagonal elements: 63 = 3 cm thick tungsten (absorption efficiency >95% at 1 MeV) 64 = nearly free of matter


Telescopi γ-rays







Compton telescopes

Telescope design based on Compton scattering: a photon hits an electron and some of the photon energy is transferred to the charged particle.

the angle of the incoming gamma ray is inferred using the data from both sets of phototube detectors. This angle defines a ring (event circle) in the sky where the gamma ray could have come from.


Compton telescopes

COMPTEL Compton Telescope that flew on the Compton Gamma-Ray Observatory (CGRO)

Compton telescopes are usually constructed in two layers. A cosmic gamma ray enters and scatters on an atom in the first layer. The resulting electron is detected through scintillation in the top layer and is observed by phototubes. The photon that results from the interaction passes through to the second layer of the telescope where it is absorbed and detected by another set of phototubes. The phototubes viewing the two levels can approximately determine the interaction points at the two layers and the amount of energy deposited in each layer.

Credit: NASA's Goddard Space Flight Center


Telescopi γ-rays







Pair-conversion telescopes

Large Area Telescope (LAT) on the Fermi Gamma-ray Space Telescope(Fermi), formerly the Gamma-ray Large Area Space Telescope (GLAST).Observation in the energy range between 20 MeV and 300 GeV.

LAT is a pair-conversion telescope with:

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a precision converter-tracker , The converter-tracker has 16 planes of high-Z material in which γ-rays can convert to an e+e− pair. The converter planes are interleaved with position-sensitive detectors that record the passage of charged particles, thus measuring the tracks of the particles resulting from pair conversion to reconstruct the directions of the incident γ-rays.

a calorimeterto measure the energy deposition due to the electromagnetic particle shower that results from the e+e− pair produced by the incident photon and to image the shower development profile. Each calorimeter module has 96 CsI(Tl) crystals.

an anti-coincidence detectorto provide charged-particle background rejection; therefore its main requirement is to have high detection efficiency for charged particlesThe telescope’s dimensions

are 1.8 m × 1.8 m × 0.72 m. with a mass of 2.7 tons


A flight tracker trayCompleted tracker array

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layers of silicon strip (Si) detectors interleaved with layers of tungsten foil (W) to facilitate the pair-creation

The probability distribution for the reconstructed direction of incident γ-rays from a point source is referred to as the Point Spread Function (PSF). Multiple scattering of the e+e− limits the obtainable resolution. To get optimal results requires that the e+e− directions be measured immediately following the conversion.



LAT calorimeter module

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Anticoincidence Detector (ACD)



2.4 sr at 1 GeV, where Aeff is the effective area of the LAT (~1m2)

What about the field of view of such a telescope?

Effective area versus energy at normal incidence (solid curve) and at 60◦ off-axis (dashed curve) for Source analysis class

W.B. Atwood et al. 2009

Effective area versus energy at normal incidence for Diffuse (dashed curve), Source (solid curve), and Transient (dotted curve) analysis classes.



γ-ray observations comparison

G. Rodeghiero 2015

To summarise ….


References

• H.E.S.S. - http://www.mpi-hd.mpg.de/hfm/HESS• MAGIC - http://magic.mppmu.mpg.de• VERITAS - http://veritas.sao.arizona.edu• CANGAROO - http://icrhp9.icrr.u-tokyo.ac.jp/• CTA - http://www.cta-observatory.org• HAWC - http://www.hawc-observatory.org/• KASKADE - https://web.ikp.kit.edu/KASCADE/• COMPTEL - http://heasarc.gsfc.nasa.gov/docs/cgro/comptel/compteldesc.html• The CTA Consortium, Exp Astron (2011) 32:193–316• Vassiliev, V.V., et al., Astropart. Phys. 28, 10 (2007) [astro-ph/0612718]• Holder J. et al. Astropart. Phys. 25, 391 (2006)• Wilson R.N., Reflecting telescopes, Springer Ed• Longair M. S. High Energy Astrophysics, Cambridge University Press

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