30TH INTERNATIONAL COSMIC RAY CONFERENCE

New method for atmospheric calibration at the Pierre Auger Observatory usingFRAM, a robotic astronomical telescope

SEGEV BENZVI, MARTINA BOHACOVA, BRIAN CONNOLLY, JIRI GRYGAR, MIROSLAV HRABOVSKY,TATIANA KAROVA, DUSAN MANDAT, PETR NECESAL, DALIBOR NOSEK, LIBOR NOZKA, MIROSLAVPALATKA, MIROSLAV PECH, MICHAEL PROUZA, JAN R IDKY, PETR SCHOVANEK, RADOMIR SMIDA,PETR TRAVNICEK, PRIMO VITALE, AND STEFAN WESTERHOFFFOR THE PIERRE AUGER COLLABORATION1

1 Pierre Auger Observatory, Av. San Martın Norte 304, 5613 Malargue, Argentinatravnick@fzu.cz

Abstract: FRAM - F/(Ph)otometric Robotic Atmospheric Monitor is the latest addition to the atmo-spheric monitoring instruments of the Pierre Auger Observatory. An optical telescope equipped withCCD camera and photometer, it automatically observes a set of selected standard stars and a calibratedterrestrial source. Primarily, the wavelength dependence of the attenuation is derived and the comparisonbetween its vertical values (for stars) and horizontal values (for the terrestrial source) is made. Further,the integral vertical aerosol optical depth can be obtained. A secondary program of the instrument, the de-tection of optical counterparts of gamma-ray bursts, has already proven successful. The hardware setup,software system, data taking procedures, and first analysis results are described in this paper.

Introduction

FRAM is part of the Pierre Auger Observatory, andits main purpose is to monitor continuously the at-mospheric transmission. FRAM works as an in-dependent, RTS2-driven [1], fully robotic system,and it performs a photometric calibration of thesky on various UV-to-optical wavelengths using a0.2 m telescope and a photometer. As a primaryobjective, FRAM observes a set of chosen stan-dard stars and a terrestrial light source at a dis-tance of 50 km. From these observations it ob-tains instant extinction coefficients and the extinc-tion wavelength dependence. The instrument wasinstalled during 2005 and after some optimizationsit is routinely taking data since June 2006.The main advantage of the system is fast mea-surement – data for one star in all filters are usu-ally taken in less than five minutes. In compari-son to Central Laser Facility (CLF) [2] or lidars[3] the other advantage of FRAM is that its mea-surements are completely non-invasive, i.e. pro-ducing no light at all and not anyhow affecting thedata acquisition of the fluorescence detector (FD).

Furthermore, some of the stars are well-calibratedand non-variable standard light sources with pre-cisely known and tabulated intensities in variousfilters, thus allowing straightforward comparisonwith measurements done using the same set of fil-ters. On the other hand, the main disadvantage ofthis instrument is that it is capable only of integralmeasurements through the whole atmosphere (orfor the whole distance between telescope and ter-restrial light source).

Instrument setup

The telescope has its own laminate enclosure thatis located about 30 meters from the building of flu-orescence detector at Los Leones, on the southernedge of Auger observatory array and about 13 kilo-meters from the town of Malargue in Argentina.As the primary instrument we use a 20 cmCassegrain-type telescope with an Optec SSP-5photometer and with integrated 10-position filterslider. Effective telescope focal length is 2970 mmand focal ratio ∼1:15. The system is further

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equipped with an electronic focuser Optec TCF-S. This Crayford-style motorized focuser is in-stalled in secondary Cassegrain focus and bears thephotometer on its moving end. A beam-splittingdichroic mirror is installed behind the focuser. Thered and infrared light is reflected into narrow-fieldpointing CCD camera (Starlight XPress MX716)and ultraviolet and visible light passes through themirror into the photometer.

Figure 1: FRAM telescope and its enclosure duringsunset at Los Leones site.

Narrow-field pointing CCD camera has resolutionof 752×580 pixels and field of view of 7′×5′. It isprimarily used for the fine centering of the targetedstar into a field of view of the photometer, whichhas only 1′ in diameter.Photometer Optec SSP-5A is a high-precision stel-lar photometer. A Fabry lens projects an imageof the primary mirror onto the cathode of photo-multiplier (PMT). The Hamamatsu R6358 PMTwas selected for our setup, because of extendedspectral response from 185 nm to 830 nm. AFabry lens is of B270-type glass that has enhancedUV-transmission. This still somewhat cuts downthe transmission below 350 nm, but does not ad-versely affect the transmission of any of the usedfilters. For star measurements we use the set offour Stromgren uvby filters and Johnson U fil-ter, for terrestrial source observation we use alsofour narrowband filters having central wavelengths340 nm, 365 nm, 394 nm, and 412 nm.Atop the telescope is installed wide-field CCDcamera – Finger Lake Instruments MaxCam CM8with Carl Zeiss Sonnar 200mm f/2.8 telephotolens. This CCD camera uses Kodak KAF 1603

ME chip with 1536 × 1024 pixels, thus assuring240′×160′ field of view. The effective diameter ofthe lens is 57 mm and the limiting magnitude underoptimum conditions reaches R ∼ 15.0 for a 30 sexposure. This CCD camera is further equippedwith Finger Lake Instruments filter wheel CFW 2with set of Johnson-Coussins UBVRI filters andwith 380-nm and 391-nm narrowband filters. Thiswide-field CCD camera is primarily dedicated forastrometry, i.e. for geometrical comparisons of starpositions in the images with catalogue, what thenenables precise justification of the telescope point-ing on the sky. Mount is a commercially availableLosmandy G-11, which uses the standard GEM-INI GOTO system equipped with two servomotorswith relative optical encoders.

Software

The system is driven by RTS2, or Remote Tele-scope System, 2nd Version, software package [1].RTS2 is an integrated package for remote telescopecontrol under the Linux operating system. It is de-signed to run in fully autonomous mode, selectingtargets from a database table, storing CCD imageand photometer metadata to the database, process-ing images and storing their identified coordinatesin the database. RTS2 was developed and is main-tained under open-source license in collaborationwith robotic telescope projects of BART, BOOTESand WATCHER [4].

Observed targets and observationschedule

FRAM is primarily designed to provide the atmo-spheric extinction model. The data for this modelare collected by the photometer with the help ofboth CCD cameras. The observation targets are se-lected bright (brighter than 6.5 mag) standard starsfrom photometric catalogue of Perry & Olsen [5]that features star measurements in Strømgren uvbyphotometric system.The target cycle begins with a slew to the positionfollowed by a short WF camera exposure to checkthe pointing accuracy. The position of the pho-tometer aperture within WF camera’s image is wellknown, so if the initial pointing is not satisfactory,

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a correction could be made. This image also servesas a test of atmospheric conditions: target may becanceled, if the necessary conditions are not met(clouds or fog resulting in no image astrometry).After the star of interest was successfully centeredwithin the WF aperture, a control exposure withthe NF CCD camera is done. The star is identifiedas the brightest source in the field of view and, ifneeded, the mount position is corrected again andthe star is moved to the center of photometer aper-ture. The photometer then does two sequences ofmeasurements per filter of interest. Each sequencetypically consists of five 1s integrations to obtainthe signal value and its variance in each filter. Si-multaneously both CCD cameras take exposures,so that pointing may be improved in real-time. TheWF camera provides also a measurement in set ofJohnson-Coussins UBVRI filters. The completeset of measurements is then stored in the structureof PSQL database.

Terrestrial light source observations

The Horizontal Attenuation Monitor (HAM) [6]uses continuously radiating Mg-Xe HID lamp sit-uated about 50 km away from our telescope. Wecan observe HAM lamp also with FRAM, usingboth our CCD cameras and photometer.The hourly automatic observations of the HAMlight source started during October 2005. Thebrightest light source is identified within the im-age and then centered to the desired position onthe WF camera, using iterative procedure. Whensuch center position is achieved, the photometerflux is checked and observation script started. Theultimate goal of these observations is to obtainthe independent values for the coefficient of theaerosol attenuation wavelength dependence andthen check, whether the characteristic of horizon-tal aerosol distribution, obtained from observationsof this terrestrial source, agrees with the results of’vertical’ measurements of standard stars.

Optical follow-ups of gamma-ray bursts

The RTS2 software system was originally devel-oped especially for the search of optical transientsof gamma-ray bursts. This software system was

significantly modified to achieve FRAM main aimsin atmospheric monitoring, however it is still veryeasy to activate special observation mode for op-tical transients. The main computer of the sys-tem receives in such case the alerts about detectedgamma-ray bursts via network, slews there andmakes images of the given sky region.This alert system was activated on FRAM in late2005 and already during January 2006 a very suc-cessful observation was made. An extraordinarilybright prompt optical emission of the GRB 060117was discovered and observed with a wide-fieldCCD camera atop the telescope FRAM from 2 to10 minutes after GRB. Optical counterpart identi-fied in our images was characterized by rapid tem-poral flux decay with slope exponent α ∝ 1.7±0.1and with a peak brightness of 10.1 mag in BesselR filter. Later observations by other instrumentsset a strong limit on the optical and radio transientfluxes, unveiling an unexpectedly rapid further de-cay. We presented more details in [7].

Calibration and results

Our main goal is to provide the so-called Angstromexponent γ, which is often used for parametriza-tion of wavelength (λ) dependence of aerosol op-tical depth τA: τA(λ) = τA0 · (λ0/λ)γ , where λ0

is the reference wavelength and τA0 is the aerosoloptical depth measured for this wavelength. More-over, the Johnson U filter has almost the same cen-tral wavelength as have the lasers used for mea-surement of vertical aerosol optical depth (VAOD)at CLF [2] and at lidar stations [3]. The integralvalue of VAOD (h =∞) in U filter thus can be usedfor direct cross-checks with these instruments.We analyzed our database of photometer countssince June 2006, when telescope entered era of sta-ble operation, until March 2007. We initially usedpart of data for calibration, where we fitted thedependence of difference between observed andtabulated magnitude on airmass for several stableand high-quality nights. The resulting dependenceshould be linear and the fit parameters characterizethe extinction (slope), but importantly also the in-strument zeropoint (intercept). The knowledge ofthe zeropoint then allowed us to directly computethe extinctions for our whole database. After that,

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Figure 2: Histograms of measured distribution of Angstrom coefficient γ (left) and of integral aerosol opticaldepth τA (right) for all data obtained from June 2006 until March 2007. Only the hardware quality cutswere applied, consequently some reconstruction artifacts are still apparent. For the aerosol optical depthdistribution the left (negative) tail is due to mis-identified stars and the more prominent right tail (τA > 1)is due to observations through clouds.

we converted extinction expressed in magnitudesinto total optical depth and then subtracted molec-ular Rayleigh part, using model from [8].For the standard star measurements (see Fig-ure 2) we obtained a preliminary mean value ofγ =−0.1 ± 0.9 that is lower than the results fromHAM (γ = 0.7 ± 0.5) [9], however still within 1-σ limits. Moreover, FRAM γ =−0.1 is in goodagreement with theoretical expectations for atmo-sphere in desert-like environment (γ∼ 0) [10].In the right panel of Figure 2, a resulting peakvalue of VAOD (h =∞) is ∼ 0.15 for measure-ments in Johnson U filter. It should be notedthat the mean VAOD value is not relevant, be-cause the distribution is strongly biased with itsprominent tail of high extinction values that wereobtained for observation through clouds. How-ever, even the position of the peak indicates highervalues of integral VAOD compared to the resultsfrom CLF and lidars (VAOD (h =∞) ∼ VAOD(h = 10 km)+0.02 = 0.08) [9]. The reliable com-parison will be possible only after the applicationof the quality cuts that will follow the ongoing pro-cessing of CCD camera control images.Acknowledgments: The telescope FRAM was built andis operated under the support of the Czech Ministry of

Education, Youth, and Sports through grant programsLA134 and LC527.

References

[1] P. Kubanek et al., Proc. of SPIE Vol. 6274(2006) 62741V1.

[2] B. Fick et al., JINST 1 (2006) P11003.[3] S. Y. BenZvi et al., Nucl. Instrum. Meth. A

574 (2007) 171.[4] P. Kubanek et al., Nuovo Cimento B Vol. 121

(2007) in press.[5] C. L. Perry, E. H. Olsen, D. L. Crawford, As-

tron. Soc. Pacific 99 (1987) 1184.[6] R. Cester et al. [Pierre Auger Collaboration],

Proc. 29th ICRC, Pune (2005), 8, 347.[7] M. Jelınek et al., Astron. Astrophys. 454

(2006) L119.[8] A. Bucholtz, Appl. Optics 34 (1995) 2765.[9] S. Y. BenZvi [Pierre Auger Collaboration],

these proceedings, #0399.[10] T. F. Eck et al., Journal of Geophysical Re-

search, Vol. 104 No. D24 (1999) 31333.

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