Transcript Astronomy makes things happen

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 (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
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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
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Wire antennas, vivaldi
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“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)
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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)
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1956
1956
ESERO Docentendag - CMV
2008/11/05
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Connecting Europe …
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Giant radio telescopes of the world
1957
~1970
~1970
~1970
~2000
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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
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`
ASTRON/LOFAR/SKA - CMV
2008/10/06
Square Kilometre Array
2500 Dishes
Dense Aperture Arrays
3-Core Central
Region
250 Sparse Aperture Arrays
18
SKA1 baseline design
250 x 15-m dishes
Baseline technologies
are mature and
demonstrated in the SKA
Central Region
Precursors and
Pathfinders
Sparse Aperture Array
stations (5 x LOFAR)
Single pixel feed
21
Artist renditions from Swinburne Astronomy Productions
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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
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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
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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 = kT
Observed power:
W = kT Δv
Observed flux density: S = 2kT / Aeff
Tsys = Tsky + Trec
Tsky and Tant: what’s in a name
After integration:
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Tsky  Trec
T 
B
System Equivalent Flux Density
What’s in Tsys?
3K background and Galactic radio emission
Atmospheric emission
Spill-over from the ground and other directions
Losses in feed and input waveguide
Receiver electronics
At times: calibration source
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Tbg
Tsky
Tspill
Tloss
Trx
Tcal
on/off
delaystep
on/off
delaystep
1..16
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to receiver
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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]
Reception pattern of an antenna
Beam solid angle (A = A/A0)
Measure of Field of View
Antenna theory: A0 Ωa = λ2
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Grating lobes
BDT Radio – 2a – CMV 2009/10/06
vdWaals symposium CMV 2007/12/18
vdWaals symposium CMV 2007/12/18
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