Mathes, Hermann-Josef

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Pierre Auger Collaboration

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A novel method for the absolute end-to-end calibration of the Auger fluo-rescence telescopesHermann-Josef Mathes1,∗ for the Pierre Auger Collaboration1Karlsruhe Institute for Technology, Institute for Nuclear Physics, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, GermanyAbstract. The fluorescence detector technique is using the atmosphere as a calorimeter. Besides theprecise monitoring of the parameters of the atmosphere a proper knowledge of the optical propertiesin the UV range of all optical components involved in the measurements of the fluorescence light isvital.Until now, the end-to-end calibration was performed with a 4.5 m2 large, uniformly lit light sourceattached to the aperture of the telescopes. To improve the maintainability we propose an alterna-tive setup where a small and lightweight light source of known optical properties re-samples themeasurement of the big light source piece by piece. This will be achieved by moving the lightsource based on an integrating sphere in two dimensions in front of the aperture. A prototypesetup has been installed and we are currently optimizing the parameters of the system and theprocedures. The aim is to reduce the effort for the procedure without diminishing the quality ofthe measurement.First measurements with this setup have already been performed and the measurements of thegeometrical and optical properties of the light source are an ongoing activity. We present ourcalibration scheme and the first, preliminary results.1 IntroductionThe fluorescence detector (FD) of the Pierre AugerObservatory [1] consists of 27 fluorescence telescopesat 4 different sites [2], [3] overlooking an area of3000 km2 equipped with over 1600 water Cherenkovdetectors. The FD measures the fluorescence light ofextensive air showers on clear, moonless nights. Theamount of measured fluorescence light is an indicatorfor the energy of the primary particle. However thevariable optical features of the atmosphere, mainlyattenuation and scattering, as well as the propertiesof the FD telescope are crucial for a precise measure-ment.For the FD of the Pierre Auger Observatory vari-ous systems are used to monitor the variability of theatmosphere and the relative night-to-night changesof the FD. The measurement of the night-to-nightchanges is included as a standard procedure withinthe data taking of the FD. The main reasons for thesenightly changes are dust accumulation on mirrors andfilters and the slow aging of the photo multiplierstubes (PMTs) caused by the exposure to the nightsky background.In order to determine the calibration constants ofthe PMTs of the FD telescope after a certain timeperiod an absolute calibration with an absolutely cal-∗e-mail: hermann-josef.mathes@kit.eduibrated light source is done. This procedure uses alarge, uniformly lit area as a light source (see fig. 1)covering the whole aperture. This so called "drum" isdirectly mounted to the UV filter of the FD telescopewhich acts also as aperture. With a diameter of over2.40 m this device requires several persons to moveand operate it. From this it is understandable thatthis absolute calibration procedure is only applied ona biennial time scale or even less frequently.In addition and for obvious reasons it is desirablethough not very practical that after any change ofcomponents either of an FD telescope or one of itsrelative calibration system a new absolute calibrationshould be performed rather close in time in order toget a new reference calibration. The resulting gapafter a change in components was bridged in the pastby relying on the stability of the relative calibrationsystem and adjusting the calibration constants thenin a suitable way.2 The ConstructionThus we are proposing a new type of absolute cali-bration system which consists of a portable and timestable light source (so that it can be carried to any lab-oratory for detailed measurements or FD site easily)which can be utilized for a fast absolute calibrationmeasurement. As a result of its reduced physical size,© The Authors, published by EDP Sciences. This is an open access article distributed under the terms of the Creative Commons Attribution License 4.0(http://creativecommons.org/licenses/by/4.0/). EPJ Web of Conferences 210, 05013 (2019) https://doi.org/10.1051/epjconf/201921005013UHECR 2018Figure 1. Scheme of the drum calibration inside theFD aperture (upper image); the drum is lifted into theaperture and several people are required for this action(lower photo).its light output window is much smaller than the sizeof the aperture of the FD. Therefore the idea is tomove this light source around in the aperture of theFD and by summing over all positions of the smalllight source the big drum should be approximated asmuch as possible and as necessary.Another advantage of being small is that measure-ments with a closed shutter (if necessary) are possi-ble. From earlier built light sources for the FD ithas become obvious that a sufficient stabilization ofthe properties of the light source must be guaranteed.This includes mainly a temperature stabilization anda sufficient mechanical and long term stability of thelight source.2.1 Mechanics and OpticsThe aperture boxes of the fluorescence telescopes arecustom made with support structures constructedfrom Bosch Rexroth type profiles. For instance theUV filters of the FD are mounted within these pro-files. The grooves of these profiles offer the possibilityto mount further, lightweight equipment permanentlyto the aperture boxes.Thus the new light source must be low in weight aswell as the system moving this light source. We havechosen to implement the moving part by two verticallymounted profile guides (the y-axis) on which anotherprofile guide (the x-axis) is mounted. The x-axis ismounted at both ends to a sliding carriage which ismoved by the tooth belt of the y-axis. In the samemanner, a sliding carriage at the x-axis holds the lightsource. We have chosen the drylin rW profile guidesfrom the company igus [6]. Because of their specialcoating and the plastic bushings of the sledges theyare maintenance and lubrication free and advertisedas being resistant to dirt, dust and humidity. Withthe chosen dimensions of the mechanical parts, themaximum weight of the light source should be below2.5 kg. The carriages on both axes are moved byindependent motors of sufficient strength via toothbelts. During firing of light pulses a brake is used tokeep everything in the selected position. This meansthat there is no current flowing through the motorsto achieve low electromagnetic interference. Basicallythe carriages can be moved with sub-millimeter preci-sion and the system applies auto-corrective measuresif some steps are lost during movement.The light source uses a 5.3′′ diameter general pur-pose integrating sphere of the type 4P-GPS-053-SLmanufactured by labsphere [7] to create as much aspossible a Lambertian distribution of the light. Thesphere used has a large exit port of up to 2.5′′ diame-ter which can be reduced by port reducers to 2′′ and1′′ respectively. Most of the measurements describedhere are performed with the 2′′ port reducer with awindowless exit port.This sphere is oriented with the large exit port to-wards the optical filter of the fluorescence telescope.The other two ports hosting the LEDs and the moni-toring photodiode are in a plane parallel to the filterand oriented at 90 ◦ of each other (see fig. 2, rightphoto).2.2 ElectronicsOne of these ports is equipped with the so called LEDhead, which is a temperature stabilized copper diskholding up to 3 UV LEDs (type UVLED365-10E fromRoithner LaserTechnik). The LEDs are operated withshort pulses (1 to 8 µs) and with currents in the range0.1 to 20 mA. The second port also holds a photo-diode (of type S1336-44BQ from Hamamatsu) in acopper disk to measure the light output of the LEDs.This is mainly meant to monitor the shot-to-shot sta-bility of the generated light pulses but not for an ab-solute calibration. The temperature stabilized copperdisks of both heads are kept at a temperature of 30◦ Cwhich is well above the expected temperatures duringthe nights when the light source is used. The heatingpower applied to the disk, though low, is sufficient2EPJ Web of Conferences 210, 05013 (2019) https://doi.org/10.1051/epjconf/201921005013UHECR 2018Figure 2. Full view of the XY scanner mounted to theaperture box of the Coihueco telescope 4 (upper photo).Detailed view of the integrating sphere with mountedflasher electronics (lower photo, grey case on top of thesphere), photodiode and LED head (upper right).to heat the system to the set value of 30◦ C startingfrom temperatures around 0◦ C.The temperature control, the steering of the LEDsand the read back from the photodiode is handledby a micro controller of the MSP430 family whichis placed together with all required components on aprinted circuit board mounted outside the integratingsphere but close to the heads. The power supply ofthis board and the trigger pulse input are done via asingle coaxial line. The control of the board is doneusing a wireless connection. The device is able togenerate light pulses with a rate up to 10 Hz if in self-trigger mode and up to 250 Hz if externally triggered.3 MeasurementsA full measurement consists of a large number of scan-ning positions distributed uniformly over the area ofthe aperture window. For example for the case of a2′′ wide exit window, there are roughly 2300 scan po-sitions. First measurements were conducted with theaim to• adjust the necessary current for the LED(s) in orderto result in a certain signal height in the ADCs ofthe FD camera,• determine how sparse the scan grid could be,• and to determine the number of light flashes perscan position.Before and after each scanning measurement, aso-called Calibration A calibration reference measure-ment was performed in order to track the stabilityof the PMT camera over the duration of the scan-ning measurement. This gives us also the possibilityof rejecting the scanning measurement if it turns outafterwards that the drift of the PMT camera was toolarge. This calibration A reference consists of an LEDlight source whose light is sent directly to the PMTcamera from the center of the mirror of the telescope.After the first measurements the LED current wasset to 3 mA or 2.7 mA for an improved version of theflasher electronics which resulted in a relative magni-tude of PMT amplitude of about 80 − 85 % of theADC scale for the brightest camera pixel.To illustrate the second point one should keep inmind that putting circles of 5 cm diameter onto theaperture circle of 1300 cm diameter in the optimalhexagonal pattern, one would need 2301 scan points.Going to a scan distance of 6 cm, only 1613 scanpoints are needed and this number reduces furtherto 569 if a distance of 10 cm is chosen.Simulations show that the achievable accuracywith one light flash per position is sufficient. As thesemeasurements are carried out in such a way that allPMT channels are read out after a flash, which allowsfor a trigger rate of roughly 1 Hz, this results in anoverall measurement time of about three quarters ofan hour. This is an acceptable value which lies wellwithin our expectations.As the chosen exit port size of the selected inte-grating sphere is rather large compared to the over-all size of the sphere, a certain inhomogeneity is ex-pected. In addition, to shield the exit port from di-rect light from the LED port, a so called light baffleis placed inside the integrating sphere by the manu-facturer. This baffle is a source of further inhomo-geneities. These inhomogeneities (spatial and angu-lar) as well as possible means to avoid them are cur-rently under investigation in the laboratory and bysimulation studies.For the current way of mounting the integratingsphere on the XY scanning system, the orientation ofthe axis of maximal inhomogeneity is 45◦ with respectto the horizontal axis. It is also expected that thisshould be seen during the measurements in the FDaperture. Therefore two full scans were done, one withthe standard orientation of the sphere and the otherone where the sphere was rotated by 180◦ around itsoptical axis (defined by the vertical to the exit portplane).4 First ResultsSo far the XY scanning calibration system was oper-ated during two measurement campaigns, in March2018 and in November 2018. The second campaigngave us the opportunity to repeat some of the earliermeasurements with the same, untouched light sourceto check the stability of the overall system. It turnedout that the differences between the two sets of mea-surements were of the order of 1 %, already correctedby the aging effect of the PMTs of the FD.3EPJ Web of Conferences 210, 05013 (2019) https://doi.org/10.1051/epjconf/201921005013UHECR 2018Fig. 3 shows the difference of the response of thecamera between a scan grid of 6 cm and of 10 cm.The difference between both scan grids is less than4 %. The rather regular structure comes from thearrangement of the PMTs in the camera in columns.Figure 3. Calibration constants for the 440 pixels of aFD camera in arbitrary units for a sample scan grid sizeof 6 cm (upper plot) and the difference between this and10 cm scan grid (lower plot).These findings are also confirmed by simulationstudies using the same two grid sizes, the expectedsystematic error having the same size, see fig. 4. Theshape of the distribution is however not yet compati-ble with the measured one.Figure 4. Difference of calibration constants of the FDcamera between a scan grid of 6 cm and of 10 cm obtainedfrom simulations.As shown in fig. 5 the effect of the baffle expressedby the difference of the two measurements is of theorder of 15 % when an exit port of 2′′ size is used. Ifthe size of the exit port is reduced to for instance 1′′,the effect gets smaller and is then of the order of 9 %.Figure 5. Calibration constants for the 440 pixels of a FDcamera in arbitrary units when the sphere is mounted asseen in fig. 2, lower picture (upper plot). A slight increasefrom the left to the right is visible. When the sphere isrotated by 180◦ a left-to-right decrease is expected andthe difference between both configurations is shown (lowerplot).5 Conclusion and OutlookOur series of measurements have shown that with anintegrating sphere used as a light source and withstable operating conditions the repeatability of themeasurement is rather good, i.e. the system provesalready to be a valuable tool. Limiting aspects of ournew technique, mainly due to some spatial inhomo-geneities of the light distribution of the source can bemeasured and put into the simulation model to reducethe expected systematic error. There is addionionallyongoing work (both simulations and measurements)to modify the internal components of the integratingsphere (namely the baffle and the LED positioning)to reduce these inhomogeneities further.In order to optimize the time needed for a fullscan of the aperture of a single telescope, a sparsergrid of scan points can be used without a significantreduction of the overall quality of the measurement.This means that it is not necessary to aim for a fullcoverage of the aperture.To achieve the initial goal of providing an absolutecalibration of the FD telescopes, the absolute calibra-tion of this light source is urgently required.So far the design of the light source used is beingoptimized to allow for imitating the drum measure-ment as much as possible. However these types ofmeasurements are mainly targeting the degradationof the telescope due to aging or change of the trans-mission or reflectivity. For the analysis of the data theso called point-spread function is also important. For4EPJ Web of Conferences 210, 05013 (2019) https://doi.org/10.1051/epjconf/201921005013UHECR 2018that a directional light source will be constructed andmounted to the carriage of the x-axis in the future.During the next campaigns further measurementswill be made with the two systems already installedwill be done to check the long term behavior of thesystem itself and to test if the system remains stableover longer periods in time. Further portable systemswill be prepared for additional calibration of multipletelescopes.Acknowledgments: We would like to acknowl-edge the support of this project by the BMBF grantsnumber 05A17VK1 and 05A17PX1. This work wasalso supported by the Ministry of Education, Youthand Sports of the Czech Republic - Mobility ProjectNo. 8J18DE008 and by the DAAD grant No.57393793.We acknowledge the contributions from S. Michal andJ. Šupík from the Palacký University, RCPTM, JointLaboratory of Optics of Palacký.References[1] A. Aab et al., Nuclear Instrum. Methods Phys.Res. Sect. A 798, 172-213 (2015)[2] J. Abraham et al., Nuclear Instrum. MethodsPhys. Res. Sect. A 620, 227-251 (2010)[3] H.-J. Mathes et al., Proc. of the 32nd ICRC, Bei-jing, China, 153-156 (2011)[4] A. Rovero, P. Bauleo, J. Brack, J. Harton, R.Knapik, Astropart. Phys. 31 (4), 305-311 (2009)[5] A. Aab et al., Astropart. Phys. 95, 44-56 (2017)[6] igus rGmbH, Spicher Str. 1a, D-51147 Köln,www.igus.de[7] Labsphere Inc., 231 Shaker St., North Sutton, NH03260, USA, www.labsphere.com5EPJ Web of Conferences 210, 05013 (2019) https://doi.org/10.1051/epjconf/201921005013UHECR 2018

A novel method for the absolute end-to-end calibration of the Auger fluorescence telescopes

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A novel method for the absolute end-to-end calibration of the Auger fluorescence telescopes

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