BDT Radio – 1a – CMV 2009/09/01 Basic Detection Techniques Radio Detection Techniques Marco de Vos, ASTRON / Literature: Selected chapters from Krauss, Radio Astronomy, 2 nd edition, 1986, Cygnus- Quasar Books, Ohio, ISBN Perley et al., Synthesis Imaging in Radio Astronomy, 1994, BookCrafters, ISBN Selected LOFAR and APERTIF documents Lecture slides

BDT Radio – 1a – CMV 2009/09/01 Overview 1a (2011/09/20): Introduction and basic properties Historical overview, detection of 21cm line, major telescopes, SKA Basis properties: coherent detection, sensitivity, resolution 1b (2011/09/22 TBC): Single dish systems Theory: basic properties, sky noise, system noise, Aeff/Tsys, receiver systems, mixing, filtering, A/D conversion Case study: pulsar detection with the Dwingeloo Radio Telescope 2a (2011/09/26): Aperture synthesis arrays Theory: correlation, aperture synthesis, van Cittert-Zernike relation, propagation of instrumental effects Case study: imaging with the WSRT 2b (2011/09/27): Phase array systems Theory: aperture arrays and phased arrays, feed properties, sensitivity, calibration. Case study: the LOFAR system Experiment (2011/09/29 TBC): Phased Array Feed flux measurement Measurements with DIGESTIF (in Dwingeloo)

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

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 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 “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 1932 Discovery of cosmic radio waves (Karl Jansky) Galactic centre v=25MHz; dv=26kHz

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 ). 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)

1956

ESERO Docentendag - CMV 2008/11/

BDT Radio – 1a – CMV 2009/09/01

Connecting Europe …

BDT Radio – 1a – CMV 2009/09/01 Giant radio telescopes of the world m Jodrell Bank, UK ~ m Parkes, Australia ~ m Effelsberg, Germany ~ m Arecibo, Puerto Rico ~ m GreenBank Telescope (GBT), USA

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

ASTRON/LOFAR/SKA - CMV 2008/10/06 `

18 Dense Aperture Arrays 2500 Dishes Wide Band Single Pixel Feeds Phased Array Feeds 250 Sparse Aperture Arrays 3-Core Central Region Square Kilometre Array

21 Sparse Aperture Array stations (5 x LOFAR) Artist renditions from Swinburne Astronomy Productions SKA 1 baseline design Single pixel feed Central Region Baseline technologies are mature and demonstrated in the SKA Precursors and Pathfinders 250 x 15-m dishes

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)

BDT Radio – 1a – CMV 2009/09/01

BDT Radio – 1b – CMV 2009/09/04 Sensitivity Key question: What’s the weakest source we can observe Key issues: Define brightness of the source Define measurement process Define limiting factors in that process

BDT Radio – 1b – CMV 2009/09/04 Brightness function Surface brightness: Power received /area /solid angle /bandwidth Unit: W m -2 Hz -1 rad -2 Received power: Power per unit bandwidth: Power spectrum: w(v) Total power: Integral over visible sky and band Visible sky: limited by aperture Band: limited by receiver

BDT Radio – 1b – CMV 2009/09/04 Point sources, extended sources Point source: size < resolution of telescope Extended source: size > resolution of telescope Continuous emission: size > field of view Flux density: Unit: 1 Jansky (Jy) = W m -2 Hz -1

BDT Radio – 1b – CMV 2009/09/04 Antenna temperature, system temperature Express noise power received by antenna in terms of temperature of resistor needed to make it generate the same noise power. Spectral power: w = kT/λ 2 A eff Ω a = kT Observed power: W = kT Δv Observed flux density: S = 2kT / A eff Tsys = Tsky + Trec Tsky and Tant: what’s in a name After integration:

BDT Radio – 1b – CMV 2009/09/04 System Equivalent Flux Density What’s in Tsys? 3K background and Galactic radio emissionTbg Atmospheric emissionTsky Spill-over from the ground and other directionsTspill Losses in feed and input waveguideTloss Receiver electronicsTrx At times: calibration sourceTcal

BDT Radio – 1a – CMV 2009/09/01

Sampling

BDT Radio – 1a – CMV 2009/09/01 Subband separation Wideband input signal 80 or 100 MHz Separate into 512 small sub-bands kHz or kHz bandwidths Out-of-band rejection of a sub-band filter > 80 dB. Polyphase pre-filter, followed by FFT Optional near-perfect reconstruction of time-series

BDT Radio – 1b – CMV 2009/09/04 Reception pattern of an antenna Beam solid angle ( A = A/A 0 ) Measure of Field of View Antenna theory: A 0 Ω a = λ 2

BDT Radio – 2a – CMV 2009/10/06 Grating lobes

vdWaals symposium CMV 2007/12/18

BDT Radio – 1a – 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.