An outdoor test facility for the Cherenkov Telescope Array mirrors
M. C. Medina, B. Garcia, J. Maya, A. Mancilla, J. J. Larrarte, E. Rasztocky, M. Benitez, J. Dipold, M. Platino
aa r X i v : . [ a s t r o - ph . I M ] J u l ND I NTERNATIONAL C OSMIC R AY C ONFERENCE , R
IO DE J ANEIRO T HE A STROPARTICLE P HYSICS C ONFERENCE
An outdoor test facility for the Cherenkov Telescopes Array mirrors M EDINA , M. C. , G ARC ´ IA , B. , M AYA , J. , M ANCILLA , A. , L ARRARTE , J. J. , R ASZTOCKY , E. , B ENITEZ , M. ,D IPOLD , J. AND P LATINO , M. FOR THE
CTA C
ONSORTIUM . Instituto Argentino de Radioastronomia, CCT La Plata-CONICET, Argentina ITeDA, Mendoza, Argentina IFSC, USP, Brazil ITeDA, Buenos Aires, Argentina [email protected] Abstract:
The Cherenkov Telescopes Array (CTA) is planned to be an Observatory for very high energy gammaray astronomy and will consist of several tens of telescopes which account for a reflective surface of more than10000 m . The mirrors of these telescopes will be formed by a set of facets. Different technological solutions,for a fast and cost efficient production of light-weight mirror facets are under test inside the CTA Consortium.Most of them involve composite structures whose behavior under real observing conditions is not yet fully tested.An outdoor test facility has been built in one of the candidate sites for CTA, in Argentina (San Antonio de losCobres [SAC], 3600m a.s.l) in order to monitor the optical and mechanical properties of these facets exposed tothe local atmospheric conditions for a given period of time. In this work we present the preliminary results of thefirst Middle Size Telescope (MST) mirror-monitoring campaign, started in 2013. Keywords:
Cherenkov Telescopes, mirrors, environmental tests.
The next generation of Very High Energy (VHE) g -ray tele-scope array is CTA (Cherenkov Telescope Array), whichis currently in the development phase[1]. Two sites, onein the Northern and one in the Southern Hemisphere, areplanned to provide full-sky coverage. In each of these sitesan array of telescopes of multiple sizes will be installed;there will be small (5 m), medium (12 m) and large (23 m)diameter telescopes (called, from now on, Small Size Tele-scope or SST, Medium Size Telescope or MST and LargeSize Telescope or LST, respectively), each optimized fordifferent energy ranges.The final configurations of these arrays are not yet com-pletely defined but the southern site of CTA will be com-posed of at least 50 telescopes of 3 different sizes and atotal of over 5,000 m of mirrors will be necessary. Thenorthern site, which is intended to be smaller, will requireof the order of 3,500 m of mirrors. Because of its largesize, the reflector of a Cherenkov telescope is composed ofmany individual mirror facets. In particular, for the MST,hexagonal mirrors of 1.2 m (flat to flat) diameter will beused, with a spherical shape of about 32 m of radius of cur-vature. In order to fulfill the specifications on optical prop-erties, mechanical behavior and costs, different technolog-ical solutions are under study [1][2]. Most of them involvea composite structure, supporting a slim reflective surface,which is assembled and glued using the cold slump tech-nique [3][4].In this work we present the first outdoor test facility forthe CTA composite mirrors. This facility is placed in oneof the Argentinean candidate sites for the southern obser-vatory and it will allow the characterization of the behaviorof different mirror designs under real environmental con-ditions. The paper is organized as follows: in section 2 themirrors under test are described. In section 3 a small de-scription of the site is given. Section 4 is dedicated to the description of the facility and tests, while in section 5 somevery preliminary results are presented. Finally, section 6contains the conclusions and the perspectives of this work. The Irfu-CEA CTA team, together with the Kerdry com-pany provided two MST mirrors manufactured as de-scribed in [4] to be tested with the facility installed at SanAntonio de los Cobres (SAC, from now on). The most im-portant characteristics of these particular mirrors are de-scribed in Table 1. Schematics of the inner structure of themirrors is shown in Fig.1. They are part of the first seriesof prototypes intended to be used for testing the behaviorof the mirrors against different typical parameters on sev-eral astronomical sites, i. e. extreme temperature gradients,humidity levels, atmospheric dust, etc.
Mirror Focal length (m) Reflectivity (%) CoatingSACK010-0213 ( A ) 16.51 77.7 Al+SiO +HfO +SiO SACPNG01-0213 ( B ) 16.37 65.3 Al+SiO +HfO +SiO Table 1 : Irfu-CEA/Kerdry MST mirror facets characteris-tics.The two MST mirrors are not identical but they weightapproximately 25 kg, being both completely watertight.Facets for the MST, built using other technologies, areexpected to be provided by other groups to perform thesame test at SAC.
The important optical properties that should be stable inreal observation conditions are: the mirror focal length, the utdoor test facility33 ND I NTERNATIONAL C OSMIC R AY C ONFERENCE , R
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Figure 1 : Exploded view of the Irfu-CEA and Kerdry mir-rors.ability to focus determined by the size of the Point SpreadFunction (PSF) and the reflectivity (global and local). Thefocal length and the PSF can be evaluated using a simpli-fied “2f” system (see section 4). In the case of the reflec-tivity, only the local reflectivity measurement could be eas-ily implemented at the site, using an adapted commercialspectrometer. The optical specifications should be fulfilledwithin the operational temperature range measured at theSAC site and the mirrors shall not suffer any damage orirreversible change of optical properties from temperaturevariations within -25 ◦ C to +60 ◦ C. The global reflectivity,defined as the percentage of reflected light that is focusedby the mirror inside a circular area with an integration di-ameter of 1 mrad, shall not change more than 3%. If themirror shape is affected by the atmospheric variations, thiswill impact on the PSF and finally on the global reflectivity.From the mechanical point of view, the mirrors should re-sist winds loads of ∼
120 km/h, dust abrasion and eventualimpacts, without suffering permanent damage or change.This would be detected as variations in the focal length orthe reflectivity of the mirrors.
The San Antonio de los Cobres site is located at Lat. 24 ◦
02’ 42.7” S and Lon. 66 ◦
14’ 05.8” W, in the Province ofSalta, Argentina. Altitude of the site is 3600 meters a. s. l.It is a flat area of approximately 4 km × Figure 2 : Panoramic view of the San Antonio de los Co-bres site.
Inside the fenced area at the SAC site, it was possibleto place only four MST mirrors in a suitable position forthe tests. The system consists of four dedicated structuresto hold the mirrors in three different positions: parking (mirrors facing down), observation (mirrors facing up, at -45 ◦ from the zenith) and test (mirrors in vertical positionfacing the horizon). During daytime the mirrors remain at parking position. After sunset, they move to observation ,coming back to parking at sunrise. The test position is usedwhen the optical properties are tested using a portable “2f”system.Each mirror is viewed by an IP camera (Ubiquiti Air-Cam ) and images of the mirrors are taken every 10 min-utes. In order to distinguish the fog or ice formation on themirror surfaces when it happens, they are illuminated byhigh intensity white LED (SMD cold white 5060 ) dur-ing the taking of photos (see Fig. 3). The cameras and themovement of the mirrors are controlled by a SBC TS-7260 . The images and sensors data (temperature and humid-ity) are stored in a 32G pen-drive. A power control systemhas been developed for controlling the high intensity LEDwith the SBC.Two pairs of sensors for temperature and relative humid-ity (RTD and HIH-400 , respectively) are placed on thesurface of both mirrors, and one additional pair measuresthe air temperature and humidity. The data is sent via theRS-232 port when the SBC requests it. Figure 3 : Mirrors test facility at San Antonio de los Co-bres, Province of Salta, Argentina.In order to evaluate the evolution of the optical prop-erties we have developed a portable ( f = mirror focallength) system following the specifications described in[4]. With this system the mirror is uniformly illuminatedby a light source placed at twice the mirror focal length(2f) and close to its optical axis. The light source should bepoint-like (much smaller than the mirror PSF). The light re-flected by the mirror will ideally produce at an inverted1:1 scale image of the source. The spread of this image is Resistance Temperature Detectors ND I NTERNATIONAL C OSMIC R AY C ONFERENCE , R
IO DE J ANEIRO twice the mirror PSF. In our case, we use a 3 Watts blueLED to illuminate the mirror and the image formed at thescreen placed next to the light source is captured with acommercial camera (see right panel of Fig. 4). The screenand the light source are attached to a tripod which allowsto adjust the distance of the system to the mirrors (see leftpanel of Fig. 4).
Figure 4 : Left : Portable system schematics. Right : Sys-tem used at SAC for the outdoor measurements.In principle, the focal length of the mirrors can be de-termined by scanning the spot size with the distance tothe mirror. However, the lack of uniformity on the ground(presence of a slope and high density of bushes) made verydifficult a smooth movement of the system, preventing asuccessful scan of the image size. Some improvements onthe portable system will be implemented in the nextmonths in order to avoid these difficulties. These will con-sist basically on a more stable support, with a better lightcollimator and a fixation for the camera for keeping thesame relative position between the screen and the camerain each measurement. The system is working since May 10th, 2013. The first im-ages of the mirrors (see Fig. 5) show that no condensationis produced on the mirror surfaces during the first nightof operation. The values of humidity and temperature reg-istered by the sensors for the same night, can be seen inFig.6.
Figure 5 : Mirrors surface captured by the IP Camera dur-ing the night of May 12th, 2013. No features due to ic-ing or condensation are seen in the images.
Left : MirrorSACK010-0213 (A).
Right : Mirror SACPNG01-0213 (B).They are compared to a reference pair of sensors whichwas placed nearby the mirrors. We can see that the relativehumidity maximum value is less than 50% with a tempera-ture minimum greater than -3 ◦ C. Figure 6 : Humidity and temperature evolution during May12, 2013 night.
Top : Relative humidity registered by thesensors placed near the mirrors reflective surface (magentaand cyan lines) compared to the one taken by the reference sensor (blue line).
Bottom : Temperature registered by thesensors placed together with the humidity ones. The samecolor scheme is used in this plot.
Even though the system shall be improved soon, wewere able to capture the spot produced by the two mir-rors and estimate their focal length, finding the smallestspot size. This is shown in Fig. 7. These are very pre-liminary results but the minimal image size was found at ∼ ± ∼ ± Figure 7 : Left : Image produced at twice the focal lengthof the mirror A.
Right : Image produced at twice the focallength of the mirror B.
The first outdoor test facility for mirrors has been installedat the northern Argentinean candidate site for CTA (SAC, utdoor test facility33 ND I NTERNATIONAL C OSMIC R AY C ONFERENCE , R
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Province of Salta). This facility consist of a dedicated struc-ture to securely fix the mirrors together with a remote po-sitioning system which allows to move the mirrors froma parking position during daytime to an observation posi-tion at night. The first two MST mirrors provided by theIrfu-CEA group and the Kerdry company have been al-ready installed at the site. Temperature and humidity sen-sors have also been installed near the mirror surfaces. Thebehavior of the mirrors regarding fogging and icing is mon-itored by means of pictures taken by an IP camera everyten minutes during night time. Images of the mirrors are be-ing taken and the weather conditions are being monitored.The very first results related to fogging phenomena are en-couraging but more statistics and a longer time coverageis needed. The optical stability will also be tested for thenext 6 months using an improved version of the portable2f system. Other mirrors built with different technologiesare also intended to be installed at the SAC site in the fol-lowing months.
Acknowledgment:
We gratefully acknowledge support fromthe following agencies and organizations: Ministerio de Ciencia,Tecnolog´ıa e Innovaci´on Productiva (MinCyT), Comisi´on Na-cional de Energ´ıa At´omica (CNEA) and Consejo Nacional deInvestigaciones Cient´ıficas y T´ecnicas (CONICET) Argentina;State Committee of Science of Armenia; Ministry for Research,CNRS-INSU and CNRS-IN2P3, Irfu-CEA, ANR, France; MaxPlanck Society, BMBF, DESY, Helmholtz Association, Ger-many; MIUR, Italy; Netherlands Research School for Astron-omy (NOVA), Netherlands Organization for Scientific Research(NWO); Ministry of Science and Higher Education and the Na-tional Centre for Research and Development, Poland; MICINNsupport through the National R+D+I, CDTI funding plans andthe CPAN and MultiDark Consolider-Ingenio 2010 programme,Spain; Swedish Research Council, Royal Swedish Academy ofSciences financed, Sweden; Swiss National Science Founda-tion (SNSF), Switzerland; Leverhulme Trust, Royal Society, Sci-ence and Technologies Facilities Council, Durham University,UK; National Science Foundation, Department of Energy, Ar-gonne National Laboratory, University of California, Universityof Chicago, Iowa State University, Institute for Nuclear and Par-ticle Astrophysics (INPAC-MRPI program), Washington Univer-sity McDonnell Center for the Space Sciences, USA. The re-search leading to these results has received funding from theEuropean Union’s Seventh Framework Programme ([FP7/2007-2013] [FP7/2007-2011]) under grant agreement n 262053.