Transcript Sep 1
Basic Detection Techniques Radio Detection Techniques Marco de Vos, ASTRON [email protected] / 0521 595247 Literature: Selected chapters from Krauss, Radio Astronomy, 2nd edition, 1986, CygnusQuasar Books, Ohio, ISBN 1-882484-00-2 Perley et al., Synthesis Imaging in Radio Astronomy, 1994, BookCrafters, ISBN 0-937707-23-6 Selected LOFAR and APERTIF documents Lecture slides BDT Radio – 1a – CMV 2009/09/01 Overview 1a (2009/09/01): Introduction Measurement properties, EM radiation, wavelength regimes, coherent & incoherent detection, caveats in interpretation. Historical example: detection of 21cm line Tour d’horizon, system perspective 1b (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 2a (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) 2b (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 – 1a – CMV 2009/09/01 BDT Radio – 1a – CMV 2009/09/01 BDT Radio – 1a – CMV 2009/09/01 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 – 1a – 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 – 1a – 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 – 1a – 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 – 1a – CMV 2009/09/01 EM waves Directionality (RA, dec, spatial resolution) Time (timing accuracy, time resolution) Frequency (spectral resolution) Flux (total intensity, polarization properties) I Q f (t , , l , m, ) U V BDT Radio – 1a – CMV 2009/09/01 Energy levels BDT Radio – 1a – CMV 2009/09/01 Different wavelengths, different properties BDT Radio – 1a – CMV 2009/09/01 Windows of opportunity BDT Radio – 1a – 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 – 1a – CMV 2009/09/01 ESO VLT Hawk I CCD BDT Radio – 1a – 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 – 1a – CMV 2009/09/01 SCUBA bolometer BDT Radio – 1a – 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 @ 300K) 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 – 1a – CMV 2009/09/01 Horn antennas BDT Radio – 1a – CMV 2009/09/01 Wire antennas, vivaldi BDT Radio – 1a – CMV 2009/09/01 BDT Radio – 1a – CMV 2009/09/01 “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 – 1a – CMV 2009/09/01 Early days of radio astronomy v=25MHz; dv=26kHz Galactic centre 1932 Discovery of cosmic radio waves (Karl Jansky) BDT Radio – 1a – 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 – 1a – 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 – 1a – CMV 2009/09/01 BDT Radio – 1a – CMV 2009/09/01 1956 BDT Radio – 1a – CMV 2009/09/01 1971 BDT Radio – 1a – CMV 2009/09/01 BDT Radio – 1a – CMV 2009/09/01 Connecting Europe … BDT Radio – 1a – CMV 2009/09/01 Giant radio telescopes of the world 1957 ~1970 ~1970 ~1970 ~2000 BDT Radio – 1a – CMV 2009/09/01 76m Jodrell Bank, UK 64-70m Parkes, Australia 100m Effelsberg, Germany 300m Arecibo, Puerto Rico 100m GreenBank Telescope (GBT), USA EVLA 27 x 25m dish BDT Radio – 1a – CMV 2009/09/01 BDT Radio – 1a – CMV 2009/09/01 Grote vragen Voor de antwoorden is een grote telescoop nodig De Square Kilometre Array BDT Radio – 1a – CMV 2009/09/01 BDT Radio – 1a – CMV 2009/09/01 A systems perspective Simulations, analyses, ... Science Applications Document Science User Requirements Document ify ce ec n Sp lia mp Co Analysis Reports System Architectural Design Document System Requirement Specification Reference documents Background documents Science consortium System Design & Engineering Operations Plan Subsystem Requirement Specification Subsystem Architectural Design Document Interface Control Document SDE/workpackages Prototype designs Workpackages Engineering reports BDT Radio – 1a – CMV 2009/09/01 LOFAR – the science Epoch of Reionisation Wide-area Surveys Transients Cosmic Rays Magnetism Solar System Science BDT Radio – 1a – CMV 2009/09/01 RCU Board A/D Filter Filter RSP Board Beamformer Central Processing Facilities WAN Filter Backplane & RF Shield Output control Distributed Beamforming GbE switch fabric (231 outputs) Filter Station GbE switch (24 ports) GbE RSP board 24 calibration Sync. Correlator / Beamformers (Blue Gene /L) Storage Buffering Image creation Delay Ionosphere Calibration RFI Mitigation User applications WAN fibre connections Station 77 BDT Radio – 1a – CMV 2009/09/01 Archive Export and GRID on/off delaystep on/off delaystep 1..16 BDT Radio – 1a – CMV 2009/09/01 to receiver BDT Radio – 1a – CMV 2009/09/01 Sampling observation mode I 10 - 90 observation mode II 110 - 190 optional observation mode III 170 - 230 30 10 90 I : 0-80 BDT Radio – 1a – CMV 2009/09/01 Filters 160 MHz clock Nyquist Zones 110 II: 80 - 160 I: 0 - 100 0 observation mode IV 210 - 250 II: 100 - 200 100 200 MHz clock Nyquist Zones III: 160 - 240 III: 200- 300 200 300 frequency [MHz] 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 – 1a – CMV 2009/09/01 BDT Radio – 1a – CMV 2009/09/01 CEntral Processing Facility 10 Tbyte/day 25000 Tbyte/day BDT Radio – 1a – CMV 2009/09/01 250 Tbyte/ day BDT Radio – 1a – CMV 2009/09/01 BDT Radio – 1a – CMV 2009/09/01