BDT Radio – 1a – CMV 2009/09/01

Basic Detection Techniques

Radio Detection TechniquesMarco de Vos, ASTRONdevos@astron.nl / 0521 595247

Literature: Selected chapters from

Krauss, Radio Astronomy, 2nd edition, 1986, Cygnus-Quasar Books, Ohio, ISBN 1-882484-00-2Perley et al., Synthesis Imaging in Radio Astronomy, 1994, BookCrafters, ISBN 0-937707-23-6

Selected LOFAR and APERTIF documentsLecture slides

BDT Radio – 1a – CMV 2009/09/01

Overview

1a (2011/09/20): Introduction and basic propertiesHistorical overview, detection of 21cm line, major telescopes, SKABasis properties: coherent detection, sensitivity, resolution

1b (2011/09/22 TBC): Single dish systemsTheory: basic properties, sky noise, system noise, Aeff/Tsys, receiver systems, mixing, filtering, A/D conversionCase study: pulsar detection with the Dwingeloo Radio Telescope

2a (2011/09/26): Aperture synthesis arraysTheory: correlation, aperture synthesis, van Cittert-Zernike relation, propagation of instrumental effectsCase study: imaging with the WSRT

2b (2011/09/27): Phase array systemsTheory: aperture arrays and phased arrays, feed properties, sensitivity, calibration.Case study: the LOFAR system

Experiment (2011/09/29 TBC): Phased Array Feed flux measurementMeasurements 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 wavesActive dipole antennaDish + feed horn + LNARequires full receiver chain, up to A/D conversionRadiomm (turnoverpoint @ 300K)IR (downconversion by mixing with laser LOs)

Phase is preserved

Separation of polarizations

Typically narrow bandBut tunable, and with high spectral resolutionFor 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 techniquesVery 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 UniverseInformation 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. 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)

1956

ESERO Docentendag - CMV 2008/11/05

1956

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 76m Jodrell Bank, UK

~1970 64-70m Parkes, Australia

~1970 100m Effelsberg, Germany

~1970 300m Arecibo, Puerto Rico

~2000 100m 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 B

and Single

Pixel F

eeds

Phased A

rray

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

SKA1 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

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)

),,,,( mltf

V

U

Q

I

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) = 10-26 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/λ2 Aeff Ωa = kTObserved power: W = kT Δv

Observed flux density: S = 2kT / Aeff

Tsys = Tsky + TrecTsky and Tant: what’s in a name

After integration: B

TTT

recsky

BDT Radio – 1b – CMV 2009/09/04

System Equivalent Flux Density

What’s in Tsys?3K background and Galactic radio emission Tbg

Atmospheric emission Tsky

Spill-over from the ground and other directions Tspill

Losses in feed and input waveguide Tloss

Receiver electronics Trx

At times: calibration source Tcal

BDT Radio – 1a – CMV 2009/09/01

to receiver

1..16on/off delaystep

on/off delaystep

BDT Radio – 1a – CMV 2009/09/01

BDT Radio – 1a – CMV 2009/09/01

BDT Radio – 1a – CMV 2009/09/01

Sampling

I: 0 - 100 II: 100 - 200 III: 200- 300

200 M Hz clockNyquist Zones

0 100 200 300

160 M Hz clockNyquist Zones

frequency [M Hz]

o bse rv ationmo de I10 - 90

Filte rs30

10 90 110

optiona l

ob se rv ationmo de II

110 - 190

ob se rv ationmo de IV210 - 250

ob se rv ationmo de III170 - 230

I : 0-80 II: 80 - 160 III: 160 - 240

BDT Radio – 1b – CMV 2009/09/04

Reception pattern of an antenna

Beam solid angle (A = A/A0)Measure of Field of View

Antenna theory: A0 Ωa = λ2

BDT Radio – 2a – CMV 2009/10/06

Grating lobes

vdWaals symposium CMV 2007/12/18

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.