[PPT] - Atmospheric calibration of the Cherenkov Telescope Array Jan Ebr PowerPoint Presentation

SLIDE 1

Atmospheric calibration

f the Cherenkov

Telescope Array

Jan Ebr for the CTA Consortium

Institute of Physics Czech Academy of Sciences, Prague

www.cta-observatory.org

SLIDE 2

2/14

CTA: a precision VHE gamma-ray observatory

goal: systematic uncertainty in gamma photon energy <10 %
strict „budget“ for uncertainties due to the atmosphere:
Cherenkov light creation: 2 %
molecular extinction: 1 %

▶ molecular profile: GDAS, ECMWF + radio-sonde validation

slower evolution, long-time characterization
cirrus layers extinction: 1-2 %
boundary layer extinction: 1-2 %

▶ aerosols: Raman Lidar, FRAM (F/Photometric Robotic

Atmospheric Monitor), CTC (Cherenkov Transparency Coefficient)

time-scales of minutes, continuous monitoring
ozone absorption

SLIDE 3

3/14

CTA: an efficient and safe observatory

dynamic scheduling for better effective duty cycle
cloud position and movement prediction: All-Sky Cameras (ASC)
cloud height: Ceilometer
high-altitude clouds affect mainly low-energy showers
weather monitoring and forecast system to ensure safety of instruments

and humans

3 central 10-meter towers with anemometers and weather stations
additional towers to measure wind across the array
maximum reliability, array GUI integration
advance site characterization
weather stations + 30-meter tower for wind profile
ASC, Sun/Moon Photometer
FRAM, ARCADE Lidar

SLIDE 4

4/14

Raman Lidars

atmospheric extinction profile
355 and 532 nm lasers, Raman to 18 km, elastic to 25 km
1 in North, 2 in South
French (LUPM), Spanish (IFAE/UAB) and Argetinian (CEILAP) being unified
long-term tests on La Palma from later this year to finalize the design

poster: GA040

G. Vasileiadis

LUPM CEILAP IFAE/UAB

SLIDE 5

5/14

FRAM robotic telescopes

integral extinction across the CTA field of view using stellar photometry
15×15 degree extinction map every 60 s with ~0.03 OD precision
define time slots with constant atmospheric conditions
135 mm lens, 36×36 mm CCD camera, robotic equatorial mount
1 in North, 2 in South

SLIDE 6

6/14

FRAM: site characterization

scans in altitude: self-calibration and

precision aerosol data (VAOD)

advance deployment on sites to

study aerosol variability

shipment on the way to Chile
finalizing papers in La Palma
setups tested in Prague

฀0.05 ฀0.1 ฀0.15 ฀0.2 ฀0.25 ฀0.3 ฀0.35 ฀0.4 ฀0 ฀20 ฀40 ฀60 ฀80 ฀100 ฀120 ฀140 ฀160 ฀180 ฀200 VAOD scan฀no. 24–27 Feb 20 Apr–9 May

SLIDE 7

7/14

All-Sky Cameras

detection of clouds across the entire

sky using star visibility

also limited photometric

determination of extinction

all-sky cloud map every 60 s
dynamic online scheduling
4.5 mm lens, 15×15 mm CCD

camera

SLIDE 8

8/14

ASC: site characterisation

learn about cloud types and typical movement
La Palma since 10/2015, Chile since 11/2015
small, autonomous, reliable outdoor device: easy and fast to deploy

SLIDE 9

9/14

Ceilometer

Vaisala CL51 commercial IR Lidar - height for clouds detected by ASC
up to 3 layers, up to 13 km distance
widely used at airports etc.
sold fixed vertical; for CTA an alt-az mount designed

SLIDE 10

10/14

ARCADE Raman Lidar

355 nm laser, detection at 387.5 nm (nitrogen) and 407 nm (water vapor)
characterize aerosol profiles on sites in preparation for the full CTA operation
data used as input for MC simulations of different atmospheres
cross-calibration with CTA Raman Lidars and other instruments
ship to La Palma before end of 2017, after 1 year move to Chile

poster: GA284

L. Valore

SLIDE 11

11/14

Sun/Moon Photometer

Cimel CE318-T, worldwide standard, part of AERONET
precision integral AOD: daytime or Moon illum. >40 % and altitude >10 deg.
not directly applicable to most CTA observations
great for site characterization, AOD time development, cross-calibration
in Chile 06-09/2016, now returning from calibration

poster: GA024

J. Juryšek

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SLIDE 12

12/14

Chile: Site Characterisation Station

ASC, Sun/Moon Photometer, web-cams, seismometer
10-m tower: Reinhardt weather station
30-m tower: 3D Doppler anemometers (at 10, 20, 30 m)

Weathermonitoring

Goals

Strategy

6Doppler anemometers

Pluscomplementaryinstrumentssuchas: Dustcounters Electricfieldmills Accelerometers(onlyforCTA-S)

3weatherstations

Severalrain sensors

0.2 0.4 0.6 0.8 1

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5 10 15 20 25 30 35 N(>T) Temperature [C] All Night 0.2 0.4 0.6 0.8 1 10 20 30 40 50 60 70 80 90 100 N(>H) Humidity [%] All Night 0.2 0.4 0.6 0.8 1 10 20 30 40 50 60 70 80 N(>V) Wind speed [km/h] All Night

< < <

SLIDE 13

13/14

Cherenkov Transparency Coefficient

stereo trigger rates of IACT corrected for other effects depend on atmospheric

transparency

correct for: geometry, geomagnetic field, hardware efficiency
inversely, when transparency constant across array, get individual

telescope relative detection efficiencies

cross-check of other atmospheric monitoring results
performance in simulations: systematic uncertainty 4 % in transparency

poster: GA021

S. Štefánik

SLIDE 14

14/14

Summary

atmospheric monitoring is a key part of CTA design
data selection and correction, dynamic online scheduling
for each goal, appropriate devices and methods have been designed
LIDARs, FRAMs, ASCs, Ceilometers, CTC ...
additional weather monitoring for safety
extensive advance site characterisation