Denis, Laurent

Dorovskyy, V. V.

Konovalenko, Alexander A.

Lecacheux, Alain

Rashkovsky, S. L.

Rucker, Helmut O.

Sidorchuk, K. M.

Sidorchuk, M. A.

Zakharenko, V. V.

Kinematika i fizika nebesnyh tel (Online)

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New methods and equipment of decametric radio astronomy for continuum observation at the UTR-2 radio telescope

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At present time the modernization of the giant decametric radio telescope UTR-2 is under way. New back-end facilities and methods which open up new possibilities for radio astronomical observations are developed. Some equipment was made in cooperation with Austrian and French radio astronomers. Current back-end facilities and methods used at the UTR-2 radio telescope are described and compared with former traditional methods, equipment and their characteristics. Some prospects regarding current progress in developing new generation of back-end facilities are also discussed. The main focus of the presentation is the observation methods and equipment applied at the UTR-2 radio telescope for the investigation of continuum radio sources: Galactic background, discrete sources (preparation for the catalogue of sources in the whole Northern Sky), SNR, HII regions. Some results of using the new back-end facilities (such as Digital Spectral Polarimeter) and processing methods are presented

Sidorchuk, K.M.

Konovalenko, A.A.

Rucker, H.O.

Lecacheux, A.

Denis, L.

Sidorchuk, M.A.

Rashkovsky, S.L.

Dorovsky, V.V.

Zakharenko, V.V.

Наукова електронна бібліотека періодичних видань НАН України (Vernadsky National Library of Ukraine)

NEW METHODS AND EQUIPMENT OF DECAMETRICRADIO ASTRONOMY FOR CONTINUUM OBSERVATIONAT THE UTR-2 RADIO TELESCOPEK. M. Sidorchuk1, A. A. Konovalenko1, H. O. Rucker2, A. Lecacheux3, L. Denis3,M. A. Sidorchuk1, S. L. Rashkovsky1, V. V. Dorovsky1, V. V. Zakharenko11Institute of Radio Astronomy, NAS of Ukraine4 Chervonopraporna Str., Kharkiv, Ukrainee-mail: rai@ira.kharkov.ua2Space Research Institute6 Schmiedlstrasse, Graz, Austria3Observatoire de Paris – CNRS URA 1757F-92195 Meudon Cedex, FranceAt present time the modernization of the giant decametric radio telescope UTR-2 is under way.New back-end facilities and methods which open up new possibilities for radio astronomical obser-vations are developed. Some equipment was made in cooperation with Austrian and French radioastronomers. Current back-end facilities and methods used at the UTR-2 radio telescope are de-scribed and compared with former traditional methods, equipment and their characteristics. Someprospects regarding current progress in developing new generation of back-end facilities are alsodiscussed. The main focus of the presentation is the observation methods and equipment applied atthe UTR-2 radio telescope for the investigation of continuum radio sources: Galactic background,discrete sources (preparation for the catalogue of sources in the whole Northern Sky), SNR, H IIregions. Some results of using the new back-end facilities (such as Digital Spectral Polarimeter)and processing methods are presented.INTRODUCTIONAt present time the modernization of giant decameter UTR-2 radio telescope, located near Kharkiv is underway. A lot of work has been done by us and also together with French colleagues (Paris–Meudon Observatory)and Austrian colleagues (Space Research Institute, Graz). The main subject of our joint eﬀorts is the develop-ment and implementation of modern back-end facilities and data processing methods for the UTR-2 radiotelescope. In the frame of INTAS joint projects during last ﬁve years of cooperation we had conducted fourjoint observational campaigns [4] with use of new Digital Spectral Polarimeter [3], which allowed us to greatlyimprove sensitivity, time and frequency resolution of the radio telescope. Specially designed data processingmethods and observational algorithms allowed us to decrease duration of observations in several times. The mainfocus of the presentation is the observational methods and equipment applied at the UTR-2 radio telescope forthe investigation of continuum radio sources: Galactic background, discrete sources (preparation for the discretesources catalogue in the whole Northern Sky), supernova remnants, H II regions.MODERNIZATION OF THE UTR-2 RADIO TELESCOPET-shaped UTR-2 radio telescope [1] can be represented as shown in Fig. 1a: North–South antenna and East–West antenna are phased with electrical phase system based on switched delay lines. Phase system producesknife-like antenna patterns for both antennas. The phase system has six outputs: ﬁrst is the output from East–West antenna, i.e., knife-like beam, which is 0.5◦ wide in East–West direction and 15◦ wide in North–Southdirection. Outputs from second to sixth are ﬁve knife-like beams formed from North–South antenna. Thesebeams are 15◦ wide in East–West direction and 0.5◦ wide in North–South direction and are separated by 0.5◦in each declination. All these outputs are then put to back-end facilities. Before modernization of back-endfacilities the receivers were set, i.e., a ﬁlter bank with six frequency channels and ﬁve independent beams, thusforming 30-channel receiver. On output from the receiver we obtained ﬁve pencil-like beams, which were formedin a process of multiplication of North–South and East–West antenna signals. Later on thirty channels thesesignals were digitized and stored with eight-bit A/D converter.c© K. M. Sidorchuk, A. A. Konovalenko, H. O. Rucker, A. Lecacheux, L. Denis, M. A. Sidorchuk, S. L. Rashkovsky,V. V. Dorovsky, V. V. Zakharenko, 200457Figure 1. Scheme of UTR-2 radio telescope with classic and new back-end facilitiesA new scheme was developed in the process of modernization. As can be seen in Fig. 1b, antenna and phasesystem remained the same, and we only changed the old 30-channel ﬁlter bank with a new digital receiver.This receiver (Digital Spectral Polarimeter) had been created in France, at the Paris–Meudon Observatory incooperation with the Space Research Institute in Graz. The block diagram of the DSP is as follows: radiofrequency block for two independent inputs, which were fed with North–South antenna central beam and East–West antenna beam. The radio frequency block performs ﬁltration and frequency transformation to base band.The block is controlled through a serial communication port allowing to choose central frequency. The baseband signal is then digitized and passed to real-time by Fast Fourier Transform module which produces powerspectra of input channels and calculates cross spectrum–multiplication of both channels spectrum, thus creatingpencil-like beam.Each of 30 receivers used in the ﬁlter bank have their own tunable frequency and ﬁxed bandwidth of 40 kHz.Receivers are grouped in sets of six units per one antenna beam. In total, there are ﬁve such groups. Typically,we use following frequencies for these sets of receivers: 10, 12.6, 14.7, 16.7, 20, and 25 MHz. Dynamic rangeof this system is determined by A/D converter module used, and is 48 dB. During the observation we mustmanually slightly tune the frequencies of ﬁlter bank receivers in order to choose frequency bands free frominterferences.The ﬁlter bank covered frequency range from 10 to 25 MHz, but we could only use six diﬀerent frequencieswith 40 kHz bandwidth. In such conditions we could not avoid interferences, which are very common and strongat decameter wavelength band. We had implemented several methods of detecting interferences, but they werenot always successful. Still, a lot of data were damaged and could not be processed.Main characteristics of the old and new systems are listed in Table 1. The modernization lifted us to a newTable 1. Characteristics of old and new back-end facilitiesFrequency resolution, kHz Number of channels Dynamic range, dBFilter bank 40 5×6 48DSP 12 2×1024 7058level. With the DSP we cover 12 MHz continuous frequency band, divided into 1024 channels. Each channel is12 kHz wide, so typical interference signal ﬁts completely in one or two channels. We developed and implementeda method, which allowed us to automatically clean spectrum from interferences.Now we can perform radio astronomical observations in very diﬃcult interference conditions. For 80% ofobservational time on average 15% of spectrum is aﬀected by interferences. With the cleaning algorithm wecan use 85% of frequency band. Thus, frequency coverage of the full DSP band is 12×0.85 MHz, instead of40×6 kHz maximum for the ﬁlter bank. The spectrum of observed objects is a power-law, i.e., it does notcontain any ﬁne structures, thus allows us to increase sensitivity by 42 times in comparison with old back-endsystem.DISCRETE SOURCES SURVEYOwing to the modernization and the progress of computers and numerical techniques the data processingmethods for various radio astronomical tasks have been considerably improved. We had developed a newautomatic algorithm of discrete sources detection [2]. It has allowed us to make the data processing procedurenearly completely automatic and increase reliability of the whole process. Nowadays, we had observed andprocessed seventy percents of the Northern Sky. The latest part of the discrete sources survey was processedusing improved methods. In the latest observational campaign we covered approximately one steradian ofthe sky. As a result, 483 sources were detected. The coverage map is shown in Fig. 2.Figure 2. Coverage of the celestial sphere by the present part of the surveyGALACTIC BACKGROUND RADIATION OBSERVATIONSUsing the new back-end facilities we had developed improved algorithm for Galactic background radiationobservations. To speed up the process of observation we implemented fast scanning technique. Each 10 secondsthe diagram pattern of the radio telescope switches to target slightly diﬀerent sky region. We change the beamposition in declination. Right ascension scanning is performed due to rotation of the Earth. During ten-dayexperiment we covered thirty percents of the Northern Sky. In Fig. 3 you can see contour brightness map ofsky region, namely from 12 to 60 degrees in declination. Frequency range for the map is 17–25 MHz. To obtaina map we used our interference-cleaning algorithm. Some areas still contain interference. The map is onlyobtained from North–South antenna. Its beam has knife-like form. You can see traces from Cassiopeia A,Cygnus A, and Crab Nebula. In former conditions and with old equipment and methods it would have takenus several years to build such a big map.Using new equipment and methods of map building and interference cleaning we observed a lot of othercontinuum sources: supernova remnants, H II regions, and Galactic clusters.Figure 3. Experimental Galactic background map59PROSPECTSThe modernization of UTR-2 radio telescope is continuing now. We are developing new digital receiver togetherwith French and Austrian colleagues. It will have the possibilities similar to the Digital Spectral Polarimeterwith some new features.Figure 4. Diagram of perspective digital receiver for UTR-2Radio frequency block, as the DSP, has tunable central frequency. But it does not perform frequencyconversion to base band. The signal on output from the block is placed at 70 MHz starting frequency with14 MHz bandwidth. The next block is an A/D converter, which operates at 56 MHz sampling frequency,performing undersampling frequency of input signals. Then undersampled data is processed in Digital DownConversion (DDC) block, which produces output base band signal. Due to ﬂexibility of modern digital circuitswe can choose sub-bands of full bandwidth from 14 MHz down to 800 kHz. The next block is a programmabledigital processing unit. Typical task for it is FFT transformation. But it is also possible to get raw waveformdata or reprogram the block any time to perform some diﬀerent tasks. The characteristics of this receiver arelisted in Table 2.Table 2. Characteristics of perspective back-end facility for UTR-2Frequency range Frequency resolution Time resolution Continuous frequency band10–70 MHz 400 Hz – 14 kHz 1 ms /waveform 14 MHz– 800 kHzCONCLUSIONWe had signiﬁcantly improved performance of the world’s largest decameter UTR-2 radio telescope and deve-loped new methods and equipment, which allowed us to obtain new astrophysical results. Also, we got a lotof experience with operating giant decameter radio telescope and processing its data. In a scope of LOFARconcept our experience can be very useful. The modernization of the UTR-2 is not stopped. We are continuingto develop modern back-end facilities and data processing methods to use with UTR-2.Acknowledgements. The work was partially supported by INTAS with the grants INTAS 97–1964 and INTAS03–51–5727.[1] Braude S. Ya., Megn A. V., Ryabov B. P., et al. // Astrophys. and Space Sci.–1978.–54, N 3.[2] Braude S. Ya., Rashkovsky S. L., Sidorchuk K. M., et al. Decametric survey of discrete sources in the northernsky // Astrophys. and Space Sci.–2002.–280, N 3.–P. 235–300.[3] Lecacheux A., Rosolen C., Clerc V., et al. Digital techniques for ground based, low frequency radio astronomy //Proc. SPIE.–1998.–3357.–P. 533–542.[4] Rucker H. O., Lecacheux A., Konovalenko A. A., et al. New frontiers in decameter radio astronomy // Planetaryradio emissions: Proc. of the 5th Intern. Workshop, Graz, Austria, April 2–4, 2001 / Eds H. O. Rucker, M. L. Kaiser,Y. Lelanc.–P. 51–61.60

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New methods and equipment of decametric radio astronomy for continuum observation at the UTR-2 radio telescope

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