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
Prototypes @ YBJ
ARGO-YBJ HALL
Prototype of Cherenkov telescope
Light collector: 5m2 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(1013—1015eV) .
Photometric calibration(1)
——Using HPD
• 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.
Pulse
Generator
PC
WFCTA
_Probe
Mirror
WFCTA
_Probe
UV light
355nm
UV LED
UV LED
trigger
HPD
Inversepolarity
Amplifier
trigger
This work is done at Hires lab
WFCTA cluster
This work is done at YBJ
Photometric calibration(1)
——Using HPD
Calibration results of the two prototypes
Calibration result
Resolution:
-- HPD: 4.8
-- CRTNT Probe: 5%
=> ILED : 6.9%
Photometric calibration(2)
——Using Nitrogen laser
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.
Table 1: Parameters of nitrogen laser
feature
parameters
Sky window
N2 laser
Theodolite
Up/down flat
Container
Figure 1：The mechanical structure of
laser calibration system
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
Photometric calibration(2)
——Using Nitrogen laser
SM
TM1
TM2
θ2
θ1
Laser
2.52km
Detector
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
We tested this laser calibration system on April 2 and8, 2011. Figure 3
shows the example image of laser track.
Figure 3: Image of laser track with 65◦ in elevation
Weather calibration(1)
——Using Nitrogen laser
SMSA
TM1TA1
TM2TA2
θ2
Detector
θ1
Laser
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(2)
——Using Infrared detector
• Monitor clouds.
• scan the whole sky once/15min
Good weather condition
Cloudy condition
Figure 6: The distributions of the infrared temperature
Figure 5: The infrared temperature
of the whole sky
Weather calibration(3)
——Using Star light
• 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
NSB measured by one PMT in one night
• 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
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.
Good weather condition
The distribution of the correlation coefficient of
the 39 days of Nov. and Dec. 2009.
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!