Lucarelli, F

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29th International Cosmic Ray Conference Pune (2005) 5, 367370Development and first results of the MAGIC central pixel system foroptical observations.F. Lucarelli  , P. Antoranz , M. Asensio  , J.A. Barrio , M. Camara , J.L. Contreras ,R. de los Reyes , M.V. Fonseca , M. Lopez , J.M. Mirandaand I. Oya for the MAGICcollaboration(a) Dpto. F·sica Atomica, Facultad de Ciencias F·sicas, Universidad Complutense, 28040 Madrid, Spain.(b) Dpto. F·sica Aplicada III, Facultad de Ciencias F·sicas, Universidad Complutense, 28040 Madrid, Spain.(c) Dept. Infra., I. Sistemas Aeroespaciales y Aerop., Universidad Politcnica, 28040 Madrid, Spain.(d) Now at the Dip. di Fisica, Universita degli Studi di Roma La Sapienza, Ple. Aldo Moro 5, 00185 Roma, Italy.Presenter: F. Lucarelli (lucarel@gae.ucm.es), spa-lucarelli-F-abs1-og27-posterThe MAGIC telescope has been designed for the observation of the Cˇerenkov light generated in Extensive AirShowers. However, its 17 m. diameter and optical design makes it suitable for optical observations as well. Inthis work, we report on the development of a system based on the use of a dedicated photo-multiplier (PMT)for optical observations installed at the center of the MAGIC camera (the central pixel). An electro-opticalsystem has been developed in order to transmit through optical fiber the PMT output signal to the countingroom, where it is digitized and stored for off-line analysis. First tests of this system using the Crab nebula ascalibration source show its optical pulsation.1. IntroductionThe MAGIC telescope [1] is an innovative detector aimed to detect very high- energy  -rays from astrophysicalsources using the Imaging Atmospheric Cˇerenkov technique. The telescope collects the very short flashesof atmospheric Cˇerenkov radiation (5-20 nsec in duration) emitted during the development of the ExtendedAtmospheric Showers (EAS) produced in the interaction of the cosmic  -rays with the atmospheric nuclei.The main characteristics of the telescope are its 17m. tessellated mirror, a system of analog signal transmissionbased on optical fiber and signal digital sampling with 300 MHz Flash ADCs. The telescope is especiallydesigned to reach the extremely low energy threshold for primary  -rays of 30 GeV. The main detector, orcamera, is located at the focus of the telescope and consists of a matrix of 576 fast-response PMTs.Besides the main  -ray observations, the large collection area of the MAGIC telescope can also be used toperform optical observations of varying astronomical objects. That can be done by integrating the slow DCcurrent output of a PMT. This technique was already applied by other Cˇerenkov telescopes (HEGRA CT1 [2],Whipple [3], Celeste [4], HESS [5]) which detected the optical pulsed emission from the Crab pulsar andestimated the photon content of the nebula surrounding the pulsar itself [2].At commissioning time, the center of the MAGIC camera was deliberately left empty in order to host a dedi-cated PMT for optical observations. Such modified PMT, the so-called central pixel, was installed at the endof March ’05 and tested successfully with the detection of the optical pulsed emission from the Crab pulsar.In what follows, we will describe the main technical aspects of the installation of the central pixel and the testsperformed with the observation of the Crab pulsar.368 F. Lucarelli et al.Figure 1. (a) Sketch of the transmission system for the central pixel. (b) Transceiver bandwidth. (c) On-site measurementsat La Palma: pulsed signals as received at the end of the transceiver chain in two cases: for a 100 mV input signal at thetransmitter (upper plot) and a 10mV input signal (lower plot).2. The central pixelThe PMT installed at the center of the MAGIC camera is a standard 1” MAGIC PMT, ET9116 [6], especiallydesigned for fast pulsed-light detection. The DC branch of the pre-amplifier placed at the PMT base hasbeen modified in order to have an integration constant 	 msec. The overall tension was set to 1.08kV,corresponding to a gain of around 20k.The transmission of the DC output signal from the PMT base to the counting house for its digitalization andstoring is made through optical fiber. The electro-optical transceiver (see Figure.1.a) was designed in orderto transmit Crab-like signals, that is, analog pulsed signals with 10-100 Hz periods and widths of the orderof milliseconds. Thus, instead of the high-speed VCSELs of the MAGIC DAQ chain (which detect the fastCˇerenkov pulses), a wide dynamic range LED was used (Honeywell HFE 4050-014 [8]). The optical fiber(a graded index multimode fiber 50/125mm core/cladding, (  =850nm.)) and the connectors (Diamond MATE-2000) were already installed in the camera. The length of the optical fiber from the camera to the countinghouse is of about 170m.At the emitter, the LED is set to an operation point of 40mA, thus providing linear operation over a widedynamic range (up to 200 mV). The transceiver built allows to measure DC variations at the level of 0.2%.At the receiver (inside the counting house), a pin diode with an analog pre-amplifier (Honeywell HFD3038-002 [9]) implements the optical-electric conversion. The bandwidth of the transceiver (Figure 1.b) is set from1Hz to 4kHz, thus allowing transmission of Crab-like signals and rejecting high and low frequency noise (andDC background).Figure 1.c shows the on-site tests of the whole transceiver system, where a series of pulsed signals (P=32Hz)Development and rst results of the MAGIC central pixel ... 369were transmitted from the MAGIC camera to the counting house by feeding them directly into the transmitterby means of a pulse generator.3. Crab observationsThe whole system has been installed at the end of March ’05 and tested by observing the Crab pulsar. The DCcurrent from the central pixel, converted to voltage, was digitized at a sampling frequency rate of 2 kHz bymeans of a 16-bits National Instruments PCI-6034E ADC card [10] and stored for off-line analysis. The ADCcard was set to a resolution of 0.16  -volts.A time stamp reflecting this frequency was associated at each event (the absolute event times were not availableat that time) and transformed to an inertial reference frame, which is assumed as the solar barycenter system.For this transformation, the TEMPO software [12] has been used.As the Crab pulses are completely embedded in noise (generated by the night sky background and the electron-ics), in order to observe its optical pulsation at the expected frequency, the corrected times must be folded withthe radio ephemerides corresponding to the observation time provided by the Jodrell Bank observatory [13]. Aphaseogram was produced around the expected Crab rotational frequency for each independent test frequencydefined by the Independent Fourier Spacing IFS=1/T  . The folded intensities were tested against a uniformdistribution by performing the Z  statistical test. Figure 2 (lower plot) shows the Z  ﬀ test scanning differentfrequencies around the expected Crab frequency. A clear peak appeared at the frequency expected for theobservation epoch. Figure 2 (upper plot) shows the well-known double-peaked lightcurve calculated at themaximum Z  ﬁ value and after 20 min. of observation.4. ConclusionsIn this work, we have reported about the installation of the central pixel system of the MAGIC telescope,dedicated to optical observations. The central pixel has been tested successfully by the detection of the Craboptical pulsations. The minimum time requested for a 5 ﬂ detection was lower than 1 minute. The expecteddetection time was 30 sec [2]. However, due to extreme weather conditions preceding the weeks immediatelybefore the observations, the optical conditions of the telescope were not the optimal. The pointing error of thetelescope was estimated offline in about 0.1 ﬃ , while the Point Spread Function (PSF) was around 0.1 ﬃ (FWHM).This limited the amount of collected light over the Central Pixel to about 7-9% of the total light emitted bythe pulsar. Besides that, the observation was done at very high zenith angle, with a strong background due tozodiacal light. Recent alignments of the mirror have reduced the PSF to less than 0.05 ﬃ . Thus, we expect toimprove the detection time by a factor at least 10.The applications of the central pixel will be focused mainly in the simultaneous observations of the Crab pulsarin the optical and  regimes, in order to have real-time ephemerides for periodicity search in  -rays. Beforethat, the central pixel will be also used to test the whole timing system of the MAGIC telescope [11]. Opticalobservations of flaring AGNs (Mrk 421, 501, ..) and X-ray binary systems (AE Aquarii) are also contemplated.5. AcknowledgementsThe authors wish to thank the financial support given by the CICYT (project FPA2003-9543-C02-01) to makethis work and the IAC for providing excellent working conditions in La Palma. The MAGIC telescope ismainly supported by BMBF (Germany), CICYT (Spain), INFN and MURST (Italy).370 F. Lucarelli et al.Figure 2. Upper plot: Crab light-curve. Lower plot: Z  statistical test. In the box on the left, a zoom of the region aroundthe expected frequency.References[1] Fonseca, M.V., Acta Physica Polonica B, vol. 30, 2331 (1999).[2] On˜a-Wilhelmi, E., et al., Astropart. Phys. 22, 95 (2004).[3] Srinivasan, R., et al., Proc. Towards a Major Atmospheric Cˇerenkov Detector V, Durban (1997) p.51.[4] De Naurois, M., et al., ApJ 566, 343 (2002).[5] Franzen, A., et al., Proc. 28th ICRC, Tsukuba (2003).[6] Ostankov, A., et al., NIM A, vol. 442, Issue 1-3, 117 (2000).[7] http://www.magic.iac.es/subsystems/centralpixel/[8] http://content.honeywell.com/sensing/prodinfo/infrared/catalog/0331eng.pdf[9] http://content.honeywell.com/sensing/prodinfo/fiberoptic/application/on7eng.pdf[10] http://sine.ni.com/nips/cds/view/p/lang/en/nid/11916[11] Lucarelli, F., et al., ”The timing system of the MAGIC telescope”, Proc. of the 19th ECRS, Florence(Italy), 2004 (in preparation) and http://www.magic.iac.es/subsystems/timing.html.[12] Manchester, R.N., et al., MNRAS 328, 17 (2001).[13] http://www.jb.man.ac.uk/ pulsar/crab.html

Development and first results of the MAGIC central pixel system for optical observations.

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