Sky Calibration January 2011

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1 Sky Calibration January 2011
Workshop AERONET Sky Calibration January 2011

2 The spheres used by PHOTONS
The calibrations are made in a dark room. This room is not rigorously “clean”, but with a specific area for the calibration and without material storage space. Currently, at the PHOTONS Lab, we use 3 small spheres LOA Spheres sisters Manufacture name Sphere diameter Aperture Power (W) System controller Sphere H Labsphere LPS-45-H HLS SL 6” (15.25cm) 2” (5cm) 20W (3.3A) Yes Sphere VIII Labsphere LPS-200-H HLS SF 2”75 (7 cm) 75W (6.25A) Sphere Beta150W Gamma 100W SpherOptics LRS-8z ITHS 150 8" (20cm) 2.8" (7cm) (3% of the sphere surface) 100W1 and 150W Yes 2 GSFC Labsphere USS-2000S (Eris) 20" (50cm) (4% of the sphere surface) 4 lights 35W ??? Table Watt external halogen lamp with attenuator. The position of the attenuator can be selected to achieve different radiance configuration. 2 System controller: keithley 6485 (photopic detector D8-P-100) Names (letters of different alphabets) are changed after every calibration by Labsphere or Sphereoptics. For example: Sphere L is the old Sphere K of old Sphere J….

3 And the new big sphere We use it in 2 configurations:
-6 lampes (3000W) -4 lamps (2400W) homogeneity Better < 0.5% in a circle of 10cm at the center

4 Why we chose these spheres?
The dynamic of the photometer measurement is (2^15) digital numbers (DN). The better evaluation of the coefficients calibration is when the DN signal is close to the maximum. In practical, it must be in the last third part (20000 to 32678DN).

5 Impact of distance between the photometer and the sphere's aperture.
F= Rad329VIII(03/2010)-Rad329VIII(07/2010) It is the differences (%) with 2 successive calibrations March 2010 and July 2010. In march 2010 we adjusted calibration with a mask in front of the sphere. We always calibrated the photometers in the same conditions of sphere photodiode. Since july, we put the photometers at a distance of 15cm of the sphere aperture . The power current is checked to always stay at the same value.

6 Variation of the #329 Calibration (2005 to 2010)
On average (for all wavelengths) the coefficients vary between 0 and 2.25% over the period

7 Calibration results July/2010 4 Spheres calibrated by J. Cooper
The differences between the Cooper calibration and the « classic » calibration with the master #329: Big Sph 6lamps : <0.85% Big Sph 4lamps : <1% Sph vIII : 0.8%<X< 2.75% Sph Gamma : 2.3%<X< 3.6%

8 Unexpected variations of the Master #329
Before and after the travel at Izana! May be the same problem in May 2010 at GFSC (cal 7H and 6H)

9

10 Vicarious calibration of reference spheres
… E0 History V0 & Ca records filter Ω of photometer T corr Vicarious calibration Simultaneously use of multiple instrument, can reduce uncertainties. Taking filter profile and temperature corrections as well as use appropriate E0 can assume the high precision. Historical Ω can improve precisions. Vicarious Ca New coeff. of sphere

11 New LOA Developpement / Big Sphere / Simultaneous calibrations
Sasha L.

12 Multi-photometers Sky calibration Device-using the Big Sphere
-All the photometers see the same point (in a circle of 10 cm) -Sun master Photometers in the central position of the Box. -Calibration SKY for 8 photometers in the same time. Monitoring of the sphere with the #396 (reference). This instrument allows to make a dynamic calibration of the sphere (at each calibration photometers).

13 How the sun photometer position can change the measurements?
Whatever the position (For the #396), the maximum variation is 0.5% For the 4 lamps case the number of photometers change the values <0.2%

14 For the 4 lamps case the number of photometers change the values <0

15 No good for the master #329 Cause  stray light

16 Conclusion/perspectives for the SKY PHOTONS calibrations
About travelling master: -Need a more stable photometer (new). Why not 2 Master? -New collimator -We would need an EPROM to calibrate UV channels for the Dual Polar photometers. Concerning the spheres: - Use of the Big sphere with dynamic calibration with the #396 photometer. For small spheres, a check will be done through the #396. Automatic checking with Vicarious method. - We appreciate that J.Cooper will come from time to time to verify the spheres.

17 For information New Wet sensor to stop several photometers (Rain/Snow/remote stop)

18 The control box and photometers connector for 16 photometers
We can too to stop photometers by Internet The detector (IBR) Capacitive sensor with heater resistor behind.

19 Capteur de pluie "IBR273" Le capteur "IBR273" est une platine sur substrat céramique destinée à la réalisation de détecteur de pluie. Utilisés principalement jusqu'alors en agriculture, de tels détecteurs trouvent désormais de nombreuses autres applications.     Conçu sur une plaque en substrat céramique (Alumina), le capteur "IBR273" est doté d'excellentes caractéristiques thermo/électrique associé à une bonne résistance aux agressions "mécaniques".     Une mini résistance chauffante associé à un capteur de température (au dos du module) permettront de s'affranchir des possibilités de détections intempestives dues à la "rosée du matin".      Le recours à des matériaux de haute technologie assure une haute stabilité à l'ensemble et une grande résistance aux produits de nettoyage les plus divers.   Son principe de fonctionnement repose sur une détection de type capacitive. Ainsi l'accumulation de pluie sur la surface de la plaque aura pour conséquence de modifier la valeur de sa capacité. Le capteur devra être associé à une électronique externe (non livrée) de conversion "fréquence / tension" dont le niveau de sortie sera lié à la valeur de la capacité de la platine "IBR273". Features - Mode de détection: capacitif - Technologie: Substrat céramique (AL2O3) - Capacité nominale: 100 pF (10 %) - Résistance chauffante: 42 ohms 10 % (Pr = 3,5 Watt à 12 V / I=292 mA / Tf = 106 °C) - NTC: 1 Kohm / 25 °C