Calibration for LHAASO_WFCTA Yong Zhang, LL Ma on behalf of the LHAASO collaboration 32 nd 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: 5m 2 spherical mirrors with reflectivity 82% Camera: 16 × 1 6 PMTs Pixel size: 1 °× 1 ° FOV: 14 °× 1 6° Electronics: DC coupling, FADC 10bits 50M Hz Physics Goal: to study the energy spectrum & compositions of cosmic rays(10 13 —10 15 eV). Prototype of Cherenkov telescope YBJ ARGO-YBJ HALL

Light source(1) calibration: using calibrated Hybrid Photo Diode (HPD) to measure light flux from UVLED(355nm): I Hires = #photons/mm 2 WFCT_Probe : two PMTs (XP3062), measuring the flux from the same source. C Hires =k*I Hires (k=QE*G*A PMT ) Light source(2) calibration: I YBJ =C YBJ /C hires *I Hires absolute gain: G= C FADC /(I YBJ *A PMT ) (FADC count/pe) CR measurement: in observations, #photons=C CR /G Number of photons is then measured. Photometric calibration(1) ——Using HPD PC Inverse- polarity Amplifier Pulse Generator UV LED trigger HPD WFCTA _Probe UV light 355nm This work is done at Hires lab WFCTA _Probe UV LED Mirror WFCTA cluster trigger This work is done at YBJ

Calibration result  Resolution: -- HPD: CRTNT Probe: 5% => I LED : 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 arc 3 、 Pyroelectric energy meter+radiomter ： Calibration Accuracy is ± 3% 4 、 Sky windows: 1m×1m 5 、 Up/down flat ： controlled by motor This laser calibration system is built in a container and is able to controlled remotely by login a local PC104. Up/down flat N 2 laser Figure 1 ： The mechanical structure of laser calibration system Theodolite Sky window Container Photometric calibration(2) ——Using Nitrogen laser featureparameters Wavelength Spectral bandwidth Pulse width (FWHM) Pulse energy Energy stability Peak power Average power Beam size Beam divergence (full angle) Repetition rate 337.1nm 0.1nm <3.5 ns 170 μJ 3% std. dev. (at 10 Hz) 45kW 3mW (at 20 Hz) 3.7mm 5. 8 mrad 1 to 20 Hz Table 1: Parameters of nitrogen laser

2.52kmLaserDetector θ1θ1 θ2θ2 T M1 T M2 SMSM Figure 2: Geometry of laser calibration system ● This system had been installed at ARGO-YBJ site from March ● 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, Figure 3 shows the example image of laser track. Photometric calibration(2) ——Using Nitrogen laser

LaserDetector θ1θ1 θ2θ2 T M1 T A1 T M2 T A2 SMSASMSA 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 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 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!