Basic Detection Techniques

1. Basic Detection Techniques • Radio Detection Techniques • Marco de Vos, ASTRON • devos@astron.nl / 0521 595247 • Literature: • Selected chapters from • Krauss, Radio Astronomy, 2nd edition, 1986, Cygnus-Quasar 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

2. 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

5. 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

6. 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)

7. 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

8. 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.

9. EM waves • Directionality (RA, dec, spatial resolution) • Time (timing accuracy, time resolution) • Frequency (spectral resolution) • Flux (total intensity, polarization properties)

10. Energy levels

11. Different wavelengths, different properties

12. Windows of opportunity

13. 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

14. ESO VLT Hawk I CCD

15. 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

16. SCUBA bolometer

17. 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

18. Horn antennas

19. Wire antennas, vivaldi

21. “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

22. Early days of radio astronomy 1932 Discovery of cosmic radio waves (Karl Jansky) v=25MHz; dv=26kHz Galactic centre

23. 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

24. Radio Spectral-lines • Predicted by van der Hulst (1944):discrete 1420 MHz (21 cm) emission from neutral Hydrogen (HI). • Detected by Ewen & Purcell (1951)

26. 1956 1971

29. Connecting Europe …

30. Giant radio telescopes of the world • 1957 76m Jodrell Bank, UK • ~1970 64-70m Parkes, Australia • ~1970 100m Effelsberg, Germany • ~1970 300m Arecibo, Puerto Rico • ~2000 100m GreenBank Telescope (GBT), USA

31. EVLA • 27 x 25m dish

33. Grote vragenVoor de antwoorden is een grote telescoop nodigDe Square Kilometre Array

35. A systems perspective

36. LOFAR – the science

40. Sampling

41. 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.

43. CEntral Processing Facility 10 Tbyte/day 25000 Tbyte/day 250 Tbyte/ day