Calibration for LHAASO_WFCTA

Yong Zhang, LL Ma on behalf of the LHAASO collaboration

32nd International Cosmic Ray Conference, Beijing 2011

Large High Altitude Air Shower Observatory Wide Field of view Cherenkov Telescope Array —LHAASO_WFCTA

outline• Introduction• Calibration

• Photometric calibration– Using Hybrid Photo Diode (HPD)

– Using Nitrogen Laser

• Weather calibration– Using Nitrogen Laser

– Using Infrared detector

– Using Star light

• summary

Introduction

Light collector: 5m2 spherical mirrors with reflectivity 82%Camera: 16 × 1 6 PMTsPixel size: 1 °× 1 °FOV: 14 °× 1 6°Electronics: DC coupling, FADC 10bits 50M HzPhysics Goal: to study the energy spectrum & compositions of cosmic rays(1013—1015eV) .

Prototype of Cherenkov telescope

Prototypes @ YBJ

ARGO-YBJ HALL

• Light source(1) calibration: using calibrated Hybrid Photo Diode (HPD) to measure light flux from UVLED(355nm): IHires = #photons/mm2

• WFCT_Probe : two PMTs (XP3062), measuring the flux from the same source. CHires=k*IHires (k=QE*G*APMT) • Light source(2) calibration: IYBJ =CYBJ/Chires*IHires

• absolute gain: G= CFADC/(IYBJ*APMT) (FADC count/pe)• CR measurement: in observations , #photons=CCR/G

Number of photons is then measured.

Photometric calibration(1) ——Using HPD

PC

Inverse-polarity

Amplifier

PulseGenerator

UV LEDtrigger

HPD

WFCTA_Probe

UV light355nm

This work is done at Hires lab

WFCTA_Probe

UV LED

Mirror

WFCTA clustertrigger

This work is done at YBJ

Calibration result

Resolution: -- HPD: 4.8 -- CRTNT Probe: 5% => ILED : 6.9%

Photometric calibration(1) ——Using HPD

Calibration results of the two prototypes

The laser calibration system (shown in figure 1) includes: 1 、 Nitrogen laser ： parameters are shown in Table 1.2 、 theodolite ： Resolution is 0.26 second of arc3 、 Pyroelectric energy meter+radiomter ： Calibration Accuracy is ± 3%4 、 Sky windows: 1m×1m5 、 Up/down flat ： controlled by motorThis laser calibration system is built in a container and is able to controlled remotely by login a local PC104.

Up/down flat

N2 laser

Figure 1 ： The mechanical structure of laser calibration system

Theodolite

Sky window

Container

Photometric calibration(2) ——Using Nitrogen laser

feature parameters

WavelengthSpectral bandwidthPulse width (FWHM)Pulse energyEnergy stability Peak powerAverage powerBeam size Beam divergence (full angle)Repetition rate

337.1nm0.1nm<3.5 ns170 μJ3% std. dev. (at 10 Hz)45kW3mW (at 20 Hz)3 .7mm5 . 8 mrad1 to 20 Hz

Table 1: Parameters of nitrogen laser

2.52km Laser Detector

θ1 θ2

TM1 TM2

SM

Figure 2: Geometry of laser calibration system

● This system had been installed at ARGO-YBJ site from March 2011.

● This system is located 2.52km apart from two telescopes station.

● The light received by the telescope is proportional to the energy of the laser pulse

● The absolute laser energy can be measured accurately by Pyroelectric energy

meter.

Photometric calibration(2) ——Using Nitrogen laser

Figure 3: Image of laser track with 65◦ in elevation

We tested this laser calibration system on April 2 and8, 2011. Figure 3 shows the example image of laser track.

Photometric calibration(2) ——Using Nitrogen laser

LaserDetector

θ1θ2

TM1TA1

TM2TA2

SMSA

Figure 4: Geometry of laser calibration system

● This system is located 176m and 71m apart from the two prototypes

of Cherenkov telescope respectively.

● Backscattering light by molecules and by aerosols is received.

●We will measure the daily variation of atmosphere using this system

from next observation season.

Weather calibration(1) ——Using Nitrogen laser

• Monitor clouds. • scan the whole sky once/15min

Figure 5: The infrared temperature of the whole sky

Figure 6: The distributions of the infrared temperature

Weather calibration(2) ——Using Infrared detector

Cloudy condition

Good weather condition

• The telescope can observe the night

sky background(NSB).

• A clear correlation between the star

light and the FADC counts recorded

by the telescope can be seen clearly

• The correlation is disappeared under

the bad weather condition.

Advantages:• The flux of star is very stable • Almost have the same path

with Cherenkov photons

Weather calibration(3) ——Using Star light

NSB measured by one PMT in one night

NSB measured by all PMTs of one cluster in one night

steps of the weather selection

• 1: on hourly scale: – A linear fit between the flues of

the star light and the FADC counts is done. If the differences between the FADC counts and the fitted value are larger than 4RMS, the points are subtracted as bad weather conditions

• 2: on the whole night scale:– The selection is based on the

correlation coefficient between the FADC counts and the fluxes of the star light.

The distribution of the correlation coefficient of the 39 days of Nov. and Dec. 2009.

Good weather condition

Summary

• photometric calibration using HPD had been done,

Resolution is 7%.

• The laser calibration system had been installed at

ARGO-YBJ site from March 2011. This system will be

operated from next observation season

• 133 nights are calibrated using stars light. 99 nights is

good weather, the value of correlation coefficient are

larger than 0.8

Thank You!