Status of the Small-Sized Telescopesof the Cherenkov Telescope Array
Akira OKUMURA
for the CTA Consortium (Thanks to the SST teams)
Institute for Space-Earth Environmental Research, Nagoya University

The extreme Universe viewed in very-high-energy gamma rays 2018
Oct 12, 2018cherenkov
telescopearray

oxon@mac.com

cherenkov
telescope
array

Small-Sized Telescopes (SSTs)

SST-1M

SST-2M GCT
SST-2M ASTRI
Image Credit: G. Pérez, IAC, SMM
2

roduction to CTA Science

1.1 Key Characteristics & Capabi

High-energy Frontier by CTA SSTs

(1)=l,b,(0y5–300 TeV(18HAssPaWLATCHA1year

WC5yearMAGIC 50h−1210SAITh05RVEthro

H.E.S.S. 50 h

ATCMST

LST

10−13ATCDifferential flux sensitivity

10−210−1

1Energy E (TeV)

1050Nht
u

50hh

So

www.cta-observatory.org/science/cta-performance/ (prod3b-v1)20)=l,b,(0y(18ssPaTLA

E2 x Flux Sensitivity (erg cm-2 s-1)

10−11

SST Core Energy Band,4(05)),0))CTA Consortium arXiV:1709.07997

102R3

Baseline Layout of CTA with 70 SSTs
CTA South
CTA North

Image Credit: G. Pérez, IAC, SMM

LSTMSTSST

2.5 kmAchieves a vast eﬀective area of ~5 km2 with 70 SSTs
Extends the highest-energy frontier to 300 TeV and beyond
4Southern Site – Paranal, Chile

Very Large Telescope
E-ELT

Proposed Site(~2150 m a.s.l)

📍Image © CNES/Airbus/DigitalGlobe/Google Earth5

PeVatron: Galactic PeV Cosmic-ray Accelerator

1010141510 1510E (eV)m-2 s-1 sr -1]
1.6Telescope Arrayπ0Auger
10Ankle1021.6HiRes 1
HiRes 2Telescope ArrayAuger

E [eV]

1112.6

Kascade Grande
IceTop-73

103

E F(E) [GeV

KascadeAnkle

m-2 s-1 sr -1]

m-2 s-1 sr -1]

1.6

1.6

2nd Knee

11315
161417
Kascade1618
1920
1010101010171710 16Kascade
10101010Grande
1018 101019 101020

E [eV]
IceTop-73

102

TA

18 18
1920
1010 10~300
10 TeV gamma
101910

2.6

14

2.6

113101013

JACEE
MGU
Tien-Shan
10Tibet07
Akeno
CASA-MIA
HEGRA
Fly’s EyeISM p/HeE F(E) [GeVHiRes 1
HiRes 2Telescope Array
Auger102
1.610

2nd Knee

2.6

IceTop-73

2.6ingammas
KascadeKascadeby
SSTsGrande103

Grigorov
2nd Knee
JACEE
MGU
310Tien-Shan
Tibet07
Ankle
Akeno
Knee
CASA-MIA
HEGRA
102
Fly’s Eye
Grigorov

E F(E) [GeV

102~3 PeV

m-2 s-1 sr -1]

103

JACEE
MGU
Tien-Shan
Tibet07
Akeno
4
CASA-MIA10
HEGRA
5–300
TeV
Fly’s Eye

13 PeV proton

PDG 2016

E F(E) [GeV

Grigorov

E F(E) [GeV

2.6
1.6-2 -2-1s-1-1sr-1)
E2.6 E
dN/dE
F(E) (GeV[GeV 1.6mms sr ]

104

29.
Knee Cosmic rays
104
Kneem-2 s-1 sr -1]

426

103

KneeEFiFigure 29.9: Expanded view ofHiRes 1
Figure29.8:
Theall-particle
spectrumasa function of EthFigure 29.8: The all-particleHiRes
spectrumas a function of E2
the cosmic-ray spectrum from da(energy-per-nucleus)
air shower measurements [90–105].
anTelescopeArray(energy-per-nucleus) from air shower
measurements from[90–105].
and the Auger Observatory [105]Auger
10Where do PeV cosmic rays come from? Do PeVatrons accelerate CRs up to Knee?
1Measurements
offluxgamma
with insmall
airthroughshower experimentsin the 18HiRe
0 decays
1 withMeasurementsof fluxsmall
airshower
experiments
the~3 PeV cosmic-ray
protons→~300
TeVrays
π
HiRes
data
[112]
isconsistentwith
13
15
16
18
19
20
14
17
10
10
10
10 diﬀer
10 by as10much as
10 a factor
10 of two,
10 indicative ofkneeregion(UHEknee region diﬀer by as much as a factor of two, indicative of(UHECR)
composition
gettinglighE[eV]systematic uncertainties
in interpretationof the data. (For a review
andA
systematic uncertainties in interpretationof the data. (Fora review
and helium above 1019eV,
while6
Figure 29.9:

CTA Key Science Projects
for PeVatron Search7

Galactic Center
10–10

HESS 2016

E2 × flux (TeV cm–2 s–1)

10–11

10–1200

y.

ce

hysis
rest≈

HESS

10–13

CTA (Sim)
Diffuse emission (×10)
Model (best fit): diffuse emission
Model: diffuse emission E 68%
cut,p = 2.9 PeV
Model: diffuse emission E 90%
cut,p = 0.6 PeV
Model: diffuse emission E 95%
cut,p = 0.4 PeVHESS J1745–290
110Energy, E (TeV)

Figure 3 | VHE γ-ray spectra of the diffuse emission and HESSJ1745−290. The y axis shows fluxes multiplied by a factor E2, where E is theenergy on the x axis, in units of TeV cm− 2 s− 1. The vertical and horizontal
error bars show the 1σ statistical error and the bin size, respectively. Arrows
represent 2σ flux upper limits. The 1σ confidence bands of the best-fit
spectra of the diffuse and HESS J1745− 290 are shown in red and blue
shaded areas, respectively. Spectral parameters are given in Methods. The
red lines show the numerical computations assuming that γ-rays result fromthe decay of neutral pions produced by proton–proton interactions. The
fluxes of the diffuse emission spectrum and models are multiplied by 10 to

Galactic center is the best PeVatron candidate so far (possible CR cutoﬀ energyof a few PeV)
Deep survey of ~800 hours is planned (inc. dark matter search)8

Galactic Plane Survey (GPS)(Data)

Galactic Center

RX J1713

https://twitter.com/CTA_Observatory/status/996680795631816704
9

Galactic Plane Survey
https://www.hawc-observatory.org/news/HAWC(Sim.
1635Data)
hrs)(>20 TeV

Galactic CenterRX J1713

Any other PeVatron candidates by CTA?
https://twitter.com/CTA_Observatory/status/996680795631816704
10

Young Supernova Remnants (SNRs)

See Takeshi’s Talk

H.E.S.S. Collaboration: Observations of RX J1713.7 3946

The Astrophysical Journal, 840:74 (14pp), 2017 May 10. image with overlaid XMM contours

HESS ~160 hrs

HESS Col. 2018

CTA ~50 hrs (sim)Acero et al. 2017y excess image of RX J1713.7 3946 with overlaid XMM-Newton contours (1–10 keV).

Figure 1. forSimulatedgamma-ray images andof (a) otherAp /Ae=0.01(lepton-dominatedcase) and(b) Ap /Ae=100 (ha50-hrs exposure
RX J1713.7-3946
candidates
each afterGPSp

Ec = 300 TeV . The green contours show (a) XMM-Newton X-ray intensity (e.g., Acero et al. 2009) and (b) total interstel
smoothedinto>
match
the PSFof CTA.to
Thestudy
subtracted
image of (b)− (a) isalso hadronic
shown in (c). component
The black contours coMore photons
10 TeVregionspectralcutoﬀand−1(Aharonian et al. 2007a). The unit of color axis is counts pixel for all panels.

Higher angular resolution and sensitivity may be able to reveal escaping cosmicrays
3.2. Results and Analysis

Differences in the Maximum11L

SST Design Proposals

SST-2M GCT

Image Credit: G. Pérez, IAC, SMM

SST-2M ASTRI

SST-1M

Need 70 SSTs with less expensive technologies (4-m optics & compact camera)
Large FOV (8–10°) to detect photons with large core distances
SiPM cameras with 1296–2368 pixels and compact front-end electronics
12

Optical Systems for >8° FOV
Schwarzschild–Couder(ASTRI and GCT)

ASTRI
GCTCamera
SecondaryPrimary

Davies–Cotton (SST-1M)

Rodeghiero+ 2016

Credit: SST-1M/CTA
13

SiPM Cameras
LST Camera

GCT Camera

Eﬀ. φ30 mm~2 m~40 cm

https://twitter.com/masahiroteshima/status/931590497394745344

Use of SiPMs enables us to build
compact cameras with high pixel density

3 or 6 mm

Dedicated compact and modular readout
Very NSB tolerant and long SST-only observations (> 5 TeV) are possible
14

SST-2M GCT (Gamma Cherenkov Telescope)
GCT Prototype Telescope @ Paris Observatory

CCDCamera

4.0 m
2.0 m(mm)White 2017

Star ImageNarrow
4.00 m

Focal Plane
2.00 m

20M1M2

Wide 2D Gaus.
Narrow 2D Gaus.

0.35 m10

3.05 mWide3.56 m

0Built at Meudon site of the Paris Observatory’s (very bright sky)
6 segmented aspherical primary mirrors and semi-monolithic secondary
Star images by a CCD camera show 6–7 mm PSF size (narrow component) while wide
component (scattering by micro roughness) needs to be improved with new Al mirrors
15SST-2M GCT Camera
First Prototype with Multi-anode PMTs (2015)

Second Prototype with SiPMs (2018)

Pre-amps

1 p.e.

New ASICs

Sampling andTigger ASICs

2 p.e.
64-ch MAPMT

64-ch SiPM

Updated the first prototype camera with SiPM arrays

Charge (arbitrary units)‣Better photon detection eﬃciency, uniform pixel gain, and better charge resolution‣

New sampling and trigger ASICs for better dynamic range, lower noise, improved trigger eﬃciency, etc

Lab tests and on-telescope observations
16

GCT First Lighthttps://www.cta-observatory.org

CTA’s first ever Cherenkov images taken on Nov 26, 2015
CR hadron observations with the first camera and telescope prototype at Paris Observatory
The second camera to be tested on the ASTRI telescope prototype in 201817

SST-2M ASTRI

mirror replication technology(Astrofisica con Specchi a Tecnologia replicante Italiana)
On-axis Simulation

A&A 608, A86 (2017)

7 mm (pixel size)

1.8 m

E. Giro et al.: Optical validation of the ASTRI SST-2M Cherenkov telescope

4.3 m
CCD Images

Credit: CTA/ASTRI

Fig. 2. On-axis PSF of the ASTRI SST-2M telescope. The colourcoding of the contributions of the di↵erent segment types is as in Fig. 1.
The red square represents the dimension of the ASTRI camera pixel.

Fig. 3. Compa
tween its idealcurve) values.
design requirem
The pixel size o
2 arcsec on theto the actuators allowing tip-tilt adjustment in the range of 1.5
with a resolution of a few arcseconds. Three actuators control theposition of M2 permitting 15 mm motion along the optical axis
with a resolution of 0.1 mm and a maximum range of tip-tilt correction of 0.25 . To align the OS, a large CCD (37 mm ⇥ 37 mm,corresponding to 1 ⇥ 1 in the sky)3 was placed at the nominal
position of the telescope focal plane pointing at the Polaris star.
The image was then analysed to deduce alignment correctionsfor M2 and each individual M1 segment. As described in Sect. 2,
the on-axis image of a point source for the ideal realizationofGiro+2017
ASTRI SST-2M OS has an extended image whose structure depends on the alignment of the OS components. Figure 2 shows
Fig. 5. PSF of the ASTRI SST-2M telescope across the focal plane.
optical performanceat 3image. The Cherenkov
theAlignment
on-axisandsimulatedstructurehaveof beensuchoptimized
an ideal
in whichcamera pixel size is overplotted for each PSF.
di↵erent colours denote the contributions from the rings illustrated in Fig. 1. The alignment procedure consists of moving all
observations
at the highertheenergies(>1 ofTeV),
a goodsharper than the others. The di↵erence between imagesthetaken
OS elements
to reproduce
structuretheshowing
ideal PSF.
As a Fig. 4. EE parawith the expected
at the same angles is no more than 6%.
first stepagreementin the alignment
processperformances.
the distance between M2 and ferent angular
To calculate the D80 parameter, a background image wastheacfocal surface has been adjusted to match the dimension of the the SiPM pixel
quired on the nearby sky patch without bright stars in the CCD Acknowledgements. The authors acknowledge the support of the local sta↵ ofimages
segmentsof ideal
PSF. Once
this distance
the M1
Serra La
Nave observing
station,on-axis
the INAF-Osservatorio
Astrofisico
di Catafield and it was directly subtracted from the corresponding
PSF ofandtip-tilt
the wholecorrections
ASTRI team. They
also applied
acknowledgetocompanies
involved in
is fixed,nia,
thewereeachindividualimage. The D80 values for each o↵-axis position were calculated
the realization of the telescope, in particular the GEC consortium (composedsegment
its contribution
to the idealPSF.
Non- 3.2. PSF cha
using the total photon flux contained in the subtracted PSF M1
imageof EIE toandreproduce
the Galbiati group),MediaLario Technologies,Flabeg,and Zaot.
supported by ofthe figures
Italian Ministryof Education,M1
University,andof the entire CCD field and then plotted in Fig. 3. The obtained
idealitiesThis
ofworkthe isfabricationof individualsegments
Research (MIUR) with funds specifically assigned to the Italian National Instivalues (filled circles on dashed curve) follow a pattern similar
and toM2 tuteas wellastheM1-to-M2
distance
misalignment
residualof Astrophysics (INAF) for the Cherenkov Telescope Array (CTA), and by
To characteriz
the ideal PSF design (continuous curve) with an additional aber- the Italian Ministry of Economic Development (MISE) within the “Astronomia
18
were not taken into account at this stage. A detailed description

Built at INAF-Catania mountain station on Mt. Etna (very active volcano)

18 segmented aspherical primary mirrors + monolithic aspherical secondary

ASTRI prototype telescope is the first realization of the Schwarzschild–Couder optics with fullmirror configuration

SST-2M ASTRI Camera

Image credit: CTA/ASTRI
Prototype Camera

Camera Lids
Front Window

ASTRI camera performance
8 × 8 ch SiPM

Pulsed and continuous un

Backend Electronics• Trigger alignment within± 1/20 pe
equivalentPhotonDetection Module• Relative gain(SiPMcalibrationand frontend)• Photoelectron
equivalent• Dynamical range up to1350 pe

SiPM

Climatic Chamber

Pulse height distributionof a single SiPM array

Response linearity measurem

PDM gain calibration

8 × 8 ch MPPC × 37 = 2368 MPPC pixels at the focal plane
Dedicated SiPM-readout ASICs (CITIROC) used in front-end electronics
Compatible design with SST-2M GCT19

ASTRI First Lighthttps://www.cta-observatory.org

Orion’s Belt

Image credit: CTA/ASTRI

Achieved first light of air-shower images on May 25, 2017
Observed Crab and Mrk 501 but not significant yet
Also able to image stars by measuring pixel amplitude variance (proportional to star flux)
20

SST-1M
Simulation
0.0 deg

DataPixel size

4.0 m
Actis+ 2011

5.1 deg

Image credit: CTA/SST-1M

Conventional Davies–Cotton optics with 18 segmented spherical mirrors (lessexpensive than Schwarzschild–Couder)
Fully automated system installed at IFJ, Krakow, Poland
Optical performance has been verified with star images
21

SST-1M Camera
Image credit: CTA/SST-1M

DigiCamPhoton Detection Plane
Entrance
Window

Measured Charge Resolutioncharge resolution

Light Concentrators
on Hexagonal SiPMs

10-1
no NSBNSB 40 MHz
NSB 80 MHz
NSB 660 MHz
Poissongoal (NSB 125 MHz)requirement (NSB 125 MHz)

Photon Detection Plane Module

8.9° FOV with 1296-pixel SiPMs and light concentrators

10102

103

detected light intensity [pe]Dead-time free fully digital camera (DigiCam)
Not compatible with SST-2Ms but use similar technologies with MST FlashCam
22

SST-1M First Lighthttps://www.cta-observatory.org

Stacked showers

Significance Map

Crab 4.2σ (1.2 hrs)

Image credit: CTA/SST-1MAchieved first light on Aug 31, 2017
Prototype detected the Crab nebular with 4.2σ excess in test observations
New observation campaigns are ongoing in 201823

SST Harmonization Started
Three diﬀerent approaches are matured and have verified
the concept of SSTs
But it is time to consolidate the optics and camera designs
before SST pre-production phase“SST harmonization” process started in May 2018 to simplify
the southern array with easier maintainability and lessconstruction cost
Final SST design proposals to be submitted Oct 2018
Review and evaluation of “the” final SST design will follow in
2019
Only single acronym, the “SST”, will be used afterwards :-)24

Timeline

We are here!

Site hosting
agreement for CTA
south to be signed

Very Large Telescope
E-ELT
Proposed Site(~2150 m a.s.l)
Image © CNES/Airbus/DigitalGlobe/Google Earth25

ASTRI Mini Array

Image credit: CTA/ASTRI

9 ASTRI telescopes to be built as ASTRI Mini Array in parallel to the SST harmonization
Stereo imaging, array trigger, array control etc. will be thoroughly tested
ASTRI and GCT cameras can be mounted
26

Summary
CTA Small-Sized Telescopes will explore the highest-energy
gamma-ray frontier from the ground‣Core energy coverage of 5–300 TeV‣

70 telescopes in CTA South‣

Wide-FOV optical system and SiPM camera‣

PeVatrons and cosmic-ray origins

Three SST designs; GCT, ASTRI, and SST-1M‣

Verified their functionalities in labs and by first light‣

Harmonization process is ongoing‣

Bigger single SST group will be formed and quickly move toward
pre-production and completion27