Basic Detection Techniques Radio Detection Techniques Marco de

Basic Detection Techniques Radio Detection Techniques Marco de Vos, ASTRON devos@astron. nl / 0521 595247 Literature: Selected chapters from Krauss, Radio Astronomy, 2 nd edition, 1986, Cygnus. Quasar Books, Ohio, ISBN 1 -882484 -00 -2 Perley et al. , Synthesis Imaging in Radio Astronomy, 1994, Book. Crafters, ISBN 0 -937707 -23 -6 Selected LOFAR and APERTIF documents Lecture slides BDT Radio – 1 a – CMV 2009/09/01

Overview 1 a (2009/09/01): Introduction Measurement properties, EM radiation, wavelength regimes, coherent & incoherent detection, caveats in interpretation. Historical example: detection of 21 cm line Tour d’horizon, system perspective 1 b (2009/09/04): Single pixel feeds Theory: basic properties, sky noise, system noise, Aeff/Tsys, receiver systems, mixing, filtering Case study: the LOFAR Low Band Antenna 2 a (2009/10/06): Array antennas Theory: aperture arrays & phased array feeds, beamforming, tile calibration, … Case study: the DIGESTIF Phased Array Feed Experiment (2009/10/08 TBC) Measurements with DIGESTIF (in Dwingeloo) 2 b (2009/10/09): Synthesis arrays Theory: aperture synthesis, van Cittert-Zernike relation, propagation of instrumental effects, … Concluding case studies: WSRT MFFE, EVLA, LOFAR HBA BDT Radio – 1 a – CMV 2009/09/01

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Measurement process Atmospheric effects Imaging system Instrumentation Conditioning of radiation before detection Spectroscopes, photometers, phase modulators, … Detectors From photon/free space wave to … Digital signal processing Real-time conditioning of detected data Calibration & Modelling Determining and removing instrumental signatures Deriving physical quantities from measurements Assessing significance by comparison with predictions BDT Radio – 1 a – CMV 2009/09/01

Observables Neutrinos Matter (cosmic rays, meteorites, moon rocks) Gravitational waves (<=c) EM waves Directionality (RA, dec, spatial resolution) Time (timing accuracy, time resolution) Frequency (spectral resolution) Flux (total intensity, polarization properties) BDT Radio – 1 a – CMV 2009/09/01

Neutrino’s Super-Kamiokande Neutrino Detector water tank showing the thousands of photon detectors each about the size of a beach ball Sudbury Neutrino Observatory BDT Radio – 1 a – CMV 2009/09/01

Gravitational waves Indirect measurement through pulsar observations? Gravitational wave causes optical path differences. A Michelson interferometer is used to detect the phase differences thus induced. BDT Radio – 1 a – CMV 2009/09/01

EM waves Directionality (RA, dec, spatial resolution) Time (timing accuracy, time resolution) Frequency (spectral resolution) Flux (total intensity, polarization properties) BDT Radio – 1 a – CMV 2009/09/01

Energy levels BDT Radio – 1 a – CMV 2009/09/01

Different wavelengths, different properties BDT Radio – 1 a – CMV 2009/09/01

Windows of opportunity BDT Radio – 1 a – CMV 2009/09/01

Photon detectors Respond to individual photons: Bio/chemical: eye, photographic plate Electrical: CCD (photo excitation), photomultipliers (photo emission) X-ray/gamma-ray detectors: scintillators, … Phase not preserved!!! Incoherent detection Often integrating (e. g. CCD) Inherently broadband Need instrumentation to get spectral resolution/accuracy Sensitive above threshold energy BDT Radio – 1 a – CMV 2009/09/01

ESO VLT Hawk I CCD BDT Radio – 1 a – CMV 2009/09/01

Energy detectors Absorb energy Bolometer: temperature rises with total EM energy deposited “Read-out” by measuring electrical properties change with temperature Used in FIR en sub-mm Phase not preserved!!! Incoherent detection Inherently broadband with slow response Need instrumentation to get spectral resolution/accuracy No threshold energy BDT Radio – 1 a – CMV 2009/09/01

SCUBA bolometer BDT Radio – 1 a – CMV 2009/09/01

Coherent detectors Responds to electric field ampl. of incident EM waves Active dipole antenna Dish + feed horn + LNA Requires full receiver chain, up to A/D conversion Radio mm (turnoverpoint @ 300 K) IR (downconversion by mixing with laser LOs) Phase is preserved Separation of polarizations Typically narrow band But tunable, and with high spectral resolution For higher frequencies: needs frequency conversion schemes BDT Radio – 1 a – CMV 2009/09/01

Horn antennas BDT Radio – 1 a – CMV 2009/09/01

Wire antennas, vivaldi BDT Radio – 1 a – CMV 2009/09/01

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$“Unique selling points” of radio astronomy Technical: Radio astronomy works at the diffraction limit$

“Unique selling points” of radio astronomy Technical: Radio astronomy works at the diffraction limit ( /D) It usually works at ‘thermal noise’ limit (after ‘selfcalibration’ in interferometry) Imaging on very wide angular resolution scales (degrees to ~100 arcsec) Extremely energy sensitive (due to large collecting area and low photon energy) Very wide frequency range (~5 decades; protected windows ! RFI important) Very high spectral resolution (<< 1 km/s) achievable due to digital techniques Very high time resolution (< 1 nanoseconds) achievable Good dynamic range for spatial, temporal and spectral emission Astrophysical: Most important source of information on cosmic magnetic fields No absorption by dust => unobscured view of Universe Information on very hot (relativistic component, synchrotron radiation) Diagnostics on very cold - atomic and molecular - gas BDT Radio – 1 a – CMV 2009/09/01

Early days of radio astronomy v=25 MHz; dv=26 k. Hz Galactic centre 1932 Discovery of cosmic radio waves (Karl Jansky) BDT Radio – 1 a – CMV 2009/09/01

The first radio astronomer (Grote Reber, USA) Built the first radio telescope "Good" angular resolution Good visibility of the sky Detected Milky Way, Sun, other radio sources (ca. 1939 -1947). Published his results in astronomy journals. Multi-frequency observations 160 & 480 MHz BDT Radio – 1 a – CMV 2009/09/01

Radio Spectral-lines Predicted by van der Hulst (1944): discrete 1420 MHz (21 cm) emission from neutral Hydrogen (HI). Detected by Ewen & Purcell (1951) BDT Radio – 1 a – CMV 2009/09/01

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1956 BDT Radio – 1 a – CMV 2009/09/01 1971

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Connecting Europe … BDT Radio – 1 a – CMV 2009/09/01

Giant radio telescopes of the world 1957 ~1970 ~2000 BDT Radio – 1 a – CMV 2009/09/01 76 m Jodrell Bank, UK 64 -70 m Parkes, Australia 100 m Effelsberg, Germany 300 m Arecibo, Puerto Rico 100 m Green. Bank Telescope (GBT), USA

EVLA 27 x 25 m dish BDT Radio – 1 a – CMV 2009/09/01

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Grote vragen Voor de antwoorden is een grote telescoop nodig De Square Kilometre Array BDT Radio – 1 a – CMV 2009/09/01

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A systems perspective BDT Radio – 1 a – CMV 2009/09/01

LOFAR – the science Epoch of Reionisation Wide-area Surveys Transients Cosmic Rays Magnetism Solar System Science BDT Radio – 1 a – CMV 2009/09/01

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Sampling BDT Radio – 1 a – CMV 2009/09/01

Timing Rubidium (Rb) laser reduces variance in the GPS-PPS to < 4 ns rms over 105 sec. The output of the Rb reference is distributed to the Time Distribution Sub-rack (TDS). Reference frequency is converted to the sampling frequency: using 10 MHz reference and Phase Locked Loops (PLL) in combination with a Voltage Controlled Crystal Oscillator (VCXO), the jitter of the output clock signals are minimized. Within a sub-rack all clock distribution is done differentially to reduce noise picked up by the clock traces and to reduce Electro Magnetic Interference (EMI) by the clock. BDT Radio – 1 a – CMV 2009/09/01

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CEntral Processing Facility 10 Tbyte/day 25000 Tbyte/day BDT Radio – 1 a – CMV 2009/09/01 250 Tbyte/ day

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