basic detection techniques
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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 (2009/09/01): IntroductionMeasurement properties, EM radiation, wavelength regimes, coherent & incoherent detection, caveats in interpretation. Historical example: detection of 21cm lineTour d’horizon, system perspective
1b (2009/09/04): Single pixel feedsTheory: basic properties, sky noise, system noise, Aeff/Tsys, receiver systems, mixing, filteringCase study: the LOFAR Low Band Antenna
2a (2009/10/06): Array antennasTheory: 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 arraysTheory: 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 detectionSpectroscopes, photometers, phase modulators, …
DetectorsFrom photon/free space wave to …
Digital signal processingReal-time conditioning of detected data
Calibration & ModellingDetermining and removing instrumental signaturesDeriving physical quantities from measurementsAssessing significance by comparison with predictions
BDT Radio – 1a – CMV 2009/09/01
Observables
Neutrinos
Matter (cosmic rays, meteorites, moon rocks)
Gravitational waves (<=c)
EM wavesDirectionality (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
Gravitational wave causes optical path differences. A Michelson interferometer is used to detect the phase differences thus induced.
Indirect measurement through pulsar observations?
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
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 plateElectrical: CCD (photo excitation), photomultipliers (photo emission)X-ray/gamma-ray detectors: scintillators, …
Phase not preserved!!!Incoherent detectionOften integrating (e.g. CCD)
Inherently broadbandNeed instrumentation to get spectral resolution/accuracySensitive 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 energyBolometer: 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 responseNeed 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 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
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)
BDT Radio – 1a – CMV 2009/09/01
BDT Radio – 1a – CMV 2009/09/01
1956 1971
BDT Radio – 1a – CMV 2009/09/01
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
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
S c ien c e Ap p lic a tio n sD o c u m en t
S c ien c e Us erR eq u ir em en ts
D o c u m en t
S y s tem R eq u ir em en tS p ec if ic a tio n
S y s tem Arc h itec tu r a lD es ig n D o c u m en t
S u b s y s temR eq u ir em en tS p ec if ic a tio n
S u b s y s temAr c h itec tu r a l
D es ig n D o c u m en t
Sp ec ify
C omp liance
O p er a tio n sP lan
I n ter f ac e C o n tr o lD o c u m en t
An aly s isR ep o r ts
P r o to ty p ed es ig n s
R ef er en c ed o c u m en ts
Bac k g r o u n dd o c u m en ts
S c ien c ec o n s o r tiu m
S y s tem D es ig n& E n g in eer in g
S D E /w o r k -p ac k ag es
W o r k p ac k ag es
E n g in eer in gr ep o r ts
S im u la tio n s ,an a ly s es , . . .
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 B oard
A /D
B ac k plane& RF S hie ld
D is tributedB eam form ing
GbE
Sta
tio
nG
bE
sw
itch
(2
4 p
ort
s)
Gb
E s
wit
ch f
ab
ric
(23
1 o
utp
uts
)
W A N fibrec onnec t ions
S y nc .
Delay
B uffering
Corre lator /B eam form ers
(B lue G ene /L)
Calibrat ionRF I M it igat ion
S torage
c alibrat ion
Im age c reat ion
Us er applic at ionsRSP board 24
Station 77
Centra l P roc es s ing F ac ilit ies
A rc hiveE x port and
G RID
RS P B oard
W A N
Ionos phere
Outputc ontro l
Filter
Filter
Filter
Filter
B eamformer
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
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 – 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.
BDT Radio – 1a – CMV 2009/09/01
BDT Radio – 1a – CMV 2009/09/01
CEntral Processing Facility
25000 Tbyte/day
10 Tbyte/day
250 Tbyte/ day
BDT Radio – 1a – CMV 2009/09/01
BDT Radio – 1a – CMV 2009/09/01
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