Blažek, Jiří

Cunniffe, Ronan

Ebr, Jan

Ebrová, Ivana

Janeček, Petr

Jelínek, Martin

Juryšek, Jakub

Karpov, Sergey

Mandát, Dušan

Mašek, Martin

Pech, Miroslav

Prouza, Michael

Trávníček, Petr

English

Pierre Auger Collaboration

KITopen

FRAM telescopes and their measurements of aerosol content at the PierreAuger Observatory and at future sites of the Cherenkov Telescope ArrayPetr Janeček1, for the CTA Consortium, Jan Ebr1, Jakub Juryšek1, Michael Prouza1, Jiří Blažek1, PetrTrávníček1, Dušan Mandát1, Miroslav Pech1 for the Pierre Auger Collaboration and the CTA Consortium,Sergey Karpov1, Ronan Cunniffe1, Martin Mašek1, Martin Jelínek2, and Ivana Ebrová1,31Institute of Physics of the Czech Academy of Sciences, Prague2Astronomical Institute of the Czech Academy of Sciences, Ondřejov3Nicolaus Copernicus Astronomical Center, Polish Academy of Sciences, WarsawAbstract. A FRAM (F/(Ph)otometric Robotic Atmospheric Monitor) telescope is a system of a robotic mount,a large-format CCD camera and a fast telephoto lens that can be used for atmospheric monitoring at any sitewhen information about the atmospheric transparency is required with high spatial or temporal resolution andwhere continuous use of laser-based methods for this purpose would interfere with other observations. Theoriginal FRAM has been operated at the Pierre Auger Observatory in Argentina for more than a decade, whilethree more FRAMs are foreseen to be used by the Cherenkov Telescope Array (CTA). The CTA FRAMs arebeing deployed ahead of time to characterize the properties of the sites prior to the operation of the CTAtelescopes; one FRAM has been running on the planned future CTA site in Chile for a year while two othersare expected to become operational before the end of 2018. We report on the hardware and current status ofoperation and/or deployment of all the FRAM instruments in question as well as on some of the preliminaryresults of integral aerosol measurements by the FRAMs in Argentina and Chile.1 Hardware and operational status1.1 Auger FRAMThe first FRAM (F/(Ph)otometric Robotic AtmosphericMonitor) telescope was installed in 2005 at the PierreAuger Observatory in Malargüe, Argentina [1] and hasundergone several hardware iterations and explored sev-eral methods of atmospheric monitoring using stellar pho-tometry [2]. Since 2009 it is routinely used for “rapidmonitoring”, that is to determine the possible presence ofclouds in the direction of a recently recorded ultra-high-energy cosmic-ray shower [3, 4]. The dataset thus gener-ated has inspired the idea of the determination of the ver-tical aerosol optical depth (VAOD) using series of wide-field images at different elevations (“scans”) [5, 6]. Tothis end, the Auger FRAM routinely takes such scans evenin the absence of a cosmic-ray trigger since the begin-ning of 2016; however only a fraction of the scans takencan be used for aerosol determination so far because evenscattered distant clouds (otherwise irrelevant for the PierreAuger Observatory itself) in the distance interfere with thefitting procedure and those are a relatively common occur-rence at the Auger site.The current Auger FRAM setup [7] has been installedin February 2013. The key part for the VAOD analysise-mail: janecekp@fzu.czhttp://www.cta-observatory.org/Full author list: http://www.auger.org/archive/authors_2018_09.htmlis the “wide-field camera” which consists of a MoravianInstruments large-format G4-16000 CCD and a Nikkor300/2.8 lens, together yielding a 7 ◦ × 7 ◦ field of viewover 16 Mpix; the installation of this instrument marksthe beginning of high-quality VAOD data. A major up-grade of the “narrow-field camera” is being carried outin September 2018: the replacement of the current 12-inch Meade SCT with a small G2-1600 CCD with theOrion UK 12-inch “Optimised Dall-Kirkham” telescopewith ASA 3-inch focuser and another G4-16000 CCD willincrease the field of view tenfold to roughly 1 sq. de-gree, possibly allowing VAOD studies with the large tele-scope (which has so far been used mainly for supple-mentary astronomical observations). During the upgrade,the Paramount ME mount will be retired after almost 10years of service under heavy load and replaced with a new10Microns GM2000 mount. While the Paramount workedreliably after a proper maintenance schedule was estab-lished, the 10Micron mount promises at least 10 years ofmaintenance-free operation, easier handling of the rela-tively heavy setup as well as absolute encoders on bothaxes allowing for additional level of safety in remote op-eration.1.2 CTA FRAMsThe FRAM concept has been proposed to be inte-grated within the atmospheric monitoring for the futureEPJ Web of Conferences 197, 02008 (2019) https://doi.org/10.1051/epjconf/201919702008AtmoHEAD 2018© 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/).-0.05 0 0.05 0.1 0.15 0.2 0  100  200  300  400  500  600VAOD-0.05 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 700  800  900  1000  1100  1200  1300VAOD-0.05 0 0.05 0.1 0.15 0.2 1300  1400  1500  1600  1700  1800  1900VAOD-0.05 0 0.05 0.1 0.15 0.2 2000  2100  2200  2300  2400  2500  2600VAOD-0.05 0 0.05 0.1 0.15 0.2 2600  2700  2800  2900  3000  3100  3200VAODobservation no.September                                     October                          Nov              DecJanuary                                                            FebruaryMarch                                                                  AprilApril                                                                 MayMay                                                                 JuneFigure 1. Time series of VAOD measurements taken by the CTA FRAM telescope located near Cerro Paranal in Chile from 09/2017to 06/2018. Each point represents one scan in altitude from the zenith to the horizon in seven 30s exposures. Consecutive points of thesame color were taken during the same night, but there can be several nights skipped due to weather or hardware issues for each colorchange.2EPJ Web of Conferences 197, 02008 (2019) https://doi.org/10.1051/epjconf/201919702008AtmoHEAD 2018-0.05 0 0.05 0.1 0.15 0.2 0  100  200  300  400  500  600VAOD-0.05 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 700  800  900  1000  1100  1200  1300VAOD-0.05 0 0.05 0.1 0.15 0.2 1300  1400  1500  1600  1700  1800  1900VAOD-0.05 0 0.05 0.1 0.15 0.2 2000  2100  2200  2300  2400  2500  2600VAOD-0.05 0 0.05 0.1 0.15 0.2 2600  2700  2800  2900  3000  3100  3200VAODobservation no.September                                     October                          Nov              DecJanuary                                                            FebruaryMarch                                                                  AprilApril                                                                 MayMay                                                                 JuneFigure 1. Time series of VAOD measurements taken by the CTA FRAM telescope located near Cerro Paranal in Chile from 09/2017to 06/2018. Each point represents one scan in altitude from the zenith to the horizon in seven 30s exposures. Consecutive points of thesame color were taken during the same night, but there can be several nights skipped due to weather or hardware issues for each colorchange.-0.05 0 0.05 0.1 0.15 0.2 0.25 0  100  200  300  400  500  600  700VAOD-0.05 0 0.05 0.1 0.15 0.2 0.25 800  900  1000  1100  1200  1300  1400  1500VAODobservation no.               2014                                      2015                             summer 2016fall 2016                                                        winter 2016                     springFigure 2. Time series of VAOD measurements taken by the Auger FRAM telescope located near Malargue, Argentina from 03/2014 to11/2016. Each points represents one scan in altitude from 30 degrees elevation to the horizon in at least five 30s exposures. Consecutivepoints of the same color were taken during the same night, but there can be many nights skipped due to weather or hardware issues foreach color change.Cherenkov Telescope Array gamma-ray observatory [8].During the CTA operation, the FRAM would continuouslymonitor the field(s) of view of the Cherenkov telescopesfor any sign of changes in atmospheric transmission. How-ever, a number of altitude scans is also foreseen for nightlyoperation as those provide direct self-calibration for theFRAM [9]. To get a complex view of the atmosphericproperties of the future CTA sites, several devices are be-ing deployed ahead of time, including the FRAMs: one isalready deployed on the future CTA-South site near CerroParanal in Chile, the second one is almost completely as-sembled in Prague and will be probably installed on the fu-ture CTA-North site on La Palma, Canary Islands in Octo-ber/November 2018; a third FRAM, again for the Chileansite, should follow within several months. When deployedahead of the CTA itself, the FRAMs, in the absence ofother duties, take altitude scans continuously, resulting inlarge amounts of data; as the Chilean site has exception-ally clear weather, the majority of those can be used forVAOD determination.The wide-field detectors for all three CTA FRAMs areidentical: again a G4-16000 CCD but with a Zeiss 135/2lens, extending the field of view to 15 ◦ × 15 ◦. The twoFRAMs in Chile use Paramount MYT mounts, which areprobably the lightest robotic mounts on the market thatsatisfy requirements for remote use. The FRAM for LaPalma uses 10Microns GM2000 as it will be accompa-nied by a narrow-field telescope previously used in theBART project of the Institute of Astronomy of the CzechAcademy of Sciences; this telescope will be removed be-fore the CTA operation commences. All three setups havebeen or will be extensively tested on the provisional plat-forms in the courtyard of the Institute of Physics in Pragueand their components measured for spectral properties inthe optical laboratory in Olomouc and for non-linearity ofthe CCDs in Prague.2 VAOD DataAll the FRAM setups are equiped with BVRI photometricfilters, but most of the measurements (and all of those pre-sented in this section) are taken through the B filter. TheB filter is the closest easily accessible standard photomet-ric filter to the relevant wavelength range for both Augerand CTA as the fluorescence lines observed by Auger liebetween 300 to 400 nm and this is also where the at-mospheric Cherenkov spectrum peaks.The effective wave-length for VAOD measurements in B filter is between 430and 440 nm depending on the type of aerosols and the spe-cific hardware setup. Reaching even slightly shorter wave-3EPJ Web of Conferences 197, 02008 (2019) https://doi.org/10.1051/epjconf/201919702008AtmoHEAD 2018lenghts is difficult because it requires custom optics anddetectors instead of off-the-shelf components and there isno directly applicable all-sky stellar catalog for that pur-pose.Fig. 1 shows all currently available VAOD measure-ments from the CTA FRAM. Apart from a period inOctober/November 2017 when the roof of the telescopehousing was not working due to improper reaction ofthe firmware to low oil levels and some small downtimedue to internet connection issues, the telescope was oper-ated continuously without issues until the cooling of thecamera stopped working in June 2018, thus producing alarge amount of measurements, from which roughly a halfhas been processed so far due to limitations on computerpower on site and bandwidth to transfer images. As theroof is a prototype of the unified housing system for allthe CTA FRAMs, it is acceptable to show issues and thefailure was used to develop better handling of unexpectedconditions in the firmware; the camera was replaced inJuly 2018, but new data taken since then will have to beanalyzed separately, because the necessary corrections aredifferent for each individual camera. The cooling issue hasbeen worked out with the vendor and all cameras liable tothis problem are being upgraded.The data series show an obvious population of outliersof unknown origin. So far we are not able to reject thosebased purely on a single observation, but the comparisonwith the Sun/Moon Photometer measurements (see [6])show that these are likely of instrumental origin. Barringthe outliers, the conditions on the future CTA site are oftenstable for many days with very small aerosol content.Fig. 2 shows selected VAOD measurements fromthe Auger FRAM. The Auger FRAM underwent severalhardware changes, including several changes of thecamera due to vendor improvements in the environmentalprotection of the camera electronics and reliability ofthe filter wheel. For each camera, the corrections fornon-linearity and bias signal described in [6] have to bedeveloped separately and for now, only the data from theperiod 03/2014 to 11/2016 have been fully processed. Inthe first two years, the observations were only carried outfor rapid monitoring purposes and thus they depend onthe triggers from the Observatory; due to problems withthe trigger, the 2015 data are limited. Moreover, most ofthe triggers are designed to look for anomalous showers,which can be often caused by clouds – this means thatthe rapid monitoring triggers specifically find clouds onthe sky, further limiting the amount of data available foraerosol analysis. Since the implementation of dedicatedaerosol observations in early 2016, the amount of dataavailable increased significantly and yet even moredata from 2017 and 2018 are awaiting processing withthe non-linearity corrections. The aerosol content ofthe Auger site is more variable, particularly in australSummer, when thunderstorms and other severe weathermay occur, however significant stable periods also existson the site. Interestingly, the outlier problem found on theCTA FRAM does not seem to occur at the Auger FRAM.AcknowledgmentsThe authors are grateful for the support by the grants ofthe Ministry of Education, Youth and Sports of the CzechRepublic, projects LTT17006, LM2015046, LTT18004,LM2015038, CZ.02.1.01/0.0/0.0/16_013/0001402, CZ.02.1.01/0.0/0.0/16_013/0001403 and CZ.02.1.01/0.0/0.0/15_003/0000437. This research was supported in partby the Polish National Science Centre under grant2017/26/D/ST9/00449. The work is partially performedaccording to the Russian Government Program of Com-petitive Growth of Kazan Federal University.References[1] The Pierre Auger Collaboration, Nucl. Instrum. Meth.A 798, 172 (2015)[2] Prouza, M. et al., Advances in Astronomy 2010,849382 (2010)[3] The Pierre Auger Collaboration, Journal of Instrumen-tation 7, P09001 (2012)[4] Ebr, J. et al. (The Pierre Auger Collaboration), Rev.Mex. AA (SC) 45, 53 (2014)[5] Ebr, J. et al. (The Pierre Auger Collaboration), EPJWeb Conf. 144 01011 (2017)[6] Ebr, J. et al. (The Pierre Auger Collaboration and CTAConsortium), these proceedings[7] Ebr, J. et al., Rev. Mex. AA (SC) 45, 114 (2014)[8] CTA Consortium, Astroparticle Physics 43, pp. 3–18(2013)[9] Janecek, P. et al. (CTA Consortium), EPJ Web Conf.144 01012 (2017)4EPJ Web of Conferences 197, 02008 (2019) https://doi.org/10.1051/epjconf/201919702008AtmoHEAD 2018

FRAM telescopes and their measurements of aerosol content at the Pierre Auger Observatory and at future sites of the Cherenkov Telescope Array

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FRAM telescopes and their measurements of aerosol content at the Pierre Auger Observatory and at future sites of the Cherenkov Telescope Array

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