an amateur radio astronomy observatory david morgan
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Radio Astronomy. An Amateur Radio Astronomy Observatory David Morgan. Part 2 Interferometers & Aperture Synthesis From amateur equipment to future global systems. Radio Astronomy. Radio Astronomy. A. B. Total Power Receiver systems - Part 1 Interferometers - Part 2 (A&B) - PowerPoint PPT PresentationTRANSCRIPT
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An Amateur Radio Astronomy Observatory
David Morgan
Radio AstronomyRadio Astronomy
Part 2 Interferometers & Aperture Synthesis
From amateur equipment to future global systems
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Radio AstronomyRadio Astronomy
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Radio AstronomyRadio Astronomy
Total Power Receiver systems - Part 1
Interferometers - Part 2 (A&B)• Basic concept
• Observing ‘point sources’
• Spatial resolution and sensitivity
• Multiple baselines & aperture synthesis
• Fringe visibility functions
• Today’s best instruments
• The future global radio telescope - The SKA
Radio Window
VLA New MexicoMk1 at Jodrell Bank SKA 2020
Cosmic hydrogen distribution< 300,000years after big bang
A
B
Origin of Galaxies
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Radio Astronomy - Radio Astronomy - InterferometerInterferometer BasicsBasics
same signalfrom source
source moves across sky
Arrival anglechanges
phase difference changes
output
wave crests sometimes ‘instep’sometimes out of step
depending on arrival angle
Adding two waves
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Radio Astronomy Radio Astronomy
Adding signals together - Phasing of two waves
Signal 1
Signal 2
0
+1
-1
0
+1
-1
0
+2
-2
0
+1
-1
moved /2
0
Peaks on signal 1 cancel troughs on signal 2Result = ZERO
The resulting amplitude varies between 2 and 0 depending the ‘Phase’ relationship between the two signals
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Radio AstronomyRadio Astronomy
Time
BE
AT
AM
PL
ITU
DE
Moves quickly
The BEAT or ‘Fringe’ frequency The BEAT or ‘Fringe’ frequency depends on Earth’s rotation & antenna baselinedepends on Earth’s rotation & antenna baseline
2
0
This is the signalthat gets recorded
MHz
Moves slowly
Sub Hz
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Radio AstronomyRadio Astronomy
Combined signal - omni directional antennas
equally sensitivein all directions
beat frequency signal
position / time
sensitive only inforward directionposition / time
Combined signal - directional antennas
beat frequency signal
sidelobes
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Baseline b
Pha
se d
iffer
ence
Antenna # 1 Antenna # 2
Wavelength
Radio Astronomy Radio Astronomy
Radio Interferometry - ‘enables detection of small sources’
• Signals drift in and out of phase as the angle to the source line of sight from the baseline changes over time (Right Ascension)
As changes the signalsgo in and out of phase so thatsignal strength varies with angleand therefore time
Response of single antenna
Response of two antennae
Angular resolution
Example of Interferometer fringes
Time
sidelobe sidelobe
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Radio AstronomyRadio Astronomy
Why use an Interferometer ?• Higher spatial resolution than a Total Power system using only one antenna • Pick out small diameter sources against a general bright radio background• The hardware is cheaper (small antennae spaced apart v single very large
antenna)• Interferometer more gain stable than a total power system• But more processing is required to recover the source brightness ‘picture’• Most modern professional Radio Telescopes are Interferometers
My Amateur Radio Telescope InterferometerMy Amateur Radio Telescope Interferometer
30m East – West Baseline
Frequency = 408MHz= 0.735m & baseline =30m
= 0.735/ 30 = 0.0245 radsor 1.4 degrees
Earth rotation angular velocity= 150 / hr
Fringe frequency = 1.4 /15 hrs = 5.6mins
This time is > signal averaging TC
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Radio AstronomyRadio Astronomy
My twin 15 element Quagi antenna 30m E-W Interferometer
East TowerWest Tower
30m apart = 41 wavelengths @ 408MHz
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Radio AstronomyRadio Astronomy
Twin Quagi Antenna responses• Each twin Yagi unit has a response shown below
Angle from antenna bore sight
00 +900-900
An
ten
na
ou
tpu
t
Cartesian Antenna response Polar Antenna response
14dB Gain & 170 Beamwidth
When two are used as an interferometer Beamwidh < 1.40
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Radio AstronomyRadio Astronomy
Moving through interferometer ‘beams’
position / time
beat frequency signal
Source moves through beams
Antennapattern
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Radio AstronomyRadio Astronomy
Observable discrete ‘Point’ sources (northern hemisphere)
Taurus A Taurus
Virgo
Virgo A
Cass A 3C461 RA 23:23:21, DEC +58:49:59
Cygnus A 3C405RA 19:59:28, DEC +40:44:00
Taurus A 3C144 (crab)RA 05:34:30, DEC +22:00:57
Virgo A 3C274 (M87)RA 12:30:48, DEC +12:22:59
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Radio AstronomyRadio Astronomy
Taurus signal embedded in galaxy background
Taurus
Galactic background
System Data
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Radio AstronomyRadio Astronomy
30m separation
Taurus ACrab NebulaNGC 1952
6,300Ly
Signal ‘fringes’
SNRAD 1054
Extracted Signal from Taurus A - ‘The Crab’
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Radio AstronomyRadio Astronomy
Taurus A Source strength = 1200Jy1Jy = 10-26W/m2/Hz
So we receive from the Crab about1.2x10-23W/m2/Hz
This produces about 0.01V in the antenna
Transit was ‘bang on schedule’
This fringe amplitude plot
was derived by cross - correlation of the signal on the previous graphwith the theoretical interferometerfringe frequency
Plot of Fringe amplitude
Fringe frequency 1/ 5.6mins
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Radio AstronomyRadio Astronomy
Cross Correlation (are you like me ?)
Calculated fringe signal
period =5.6min
Cross CorrelationFunction
Transit
Signal from Taurus A 16:00 – 01:00GMT 25/1/08
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Radio AstronomyRadio Astronomy
Looking toward Galactic N Pole
Looking toward centre of galactic plane
Virgo A is a compact Radio Source
Good example of how an interferometer can distinguish compact from diffuse Radio sources
M87 Virgo A NGC4486Giant Elliptical Galaxy with
intense relativistic jet~ 60 million LY distant
Radio Galaxy
Virgo A M87
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Radio AstronomyRadio Astronomy
Virgo A RadioSpectrum
This is close to the limit of whatcan be measured with my 2 antenna interferometer
Another view of theenergetic jet in M87
Virgo A
Flux =kx (x=spectral index)
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Radio AstronomyRadio Astronomy
This fringe visibility plot was derived by cross - correlation of the signal with the theoretical interferometer fringe frequency
Virgo A M87
There are no more sources visible from the northern hemisphere at this levelPulsars are < 100Jy and would require a very costly 10m dia dish
Fringe Amplitude
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Radio AstronomyRadio Astronomy
Estimating the size of a radio source• If the source produces fringes then we know that its angular
diameter is less than /b (= wavelength, b = baseline)
• The longer the baseline the smaller the source diameter that can be measured
• Large distributed sources don’t produce fringes
Distributed source (made up of many point sources)
Multiple sources ‘fill in’ fringes leading to ‘flat line’
S1S2
S3 S4
S
Singl
e de
fined
phas
e di
ffere
nce
Produces a clear ‘fringe pattern’
Small source of angular size < /b
Wavelength =
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Radio AstronomyRadio Astronomy
Example of distributed & ‘point’ sources
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Radio AstronomyRadio Astronomy
Comparison of strengths of ‘point’ radio sources
Cygnus A
CASS A
SNR AD 1667
Virgo A
Jet MassiveBlack
Hole ?EllipticalGalaxy
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Radio AstronomyRadio Astronomy
Amateur capability• Possible to detect point sources within the Milky Way - Taurus A
• Possible to detect other Galaxies - Virgo A (60MLy)
• Easily possible to detect powerful Radio Galaxies
- Cygnus A 700MLy
• Limiting sensitivity ~ 100Jy or 10-24 W/m2/Hz
• Pulsar detection requires 100x increase in sensitivity
• This would need a larger antenna array
Cygnus A Radio Galaxy
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Radio Astronomy – Radio Astronomy – Part 2 BPart 2 B Aperture SynthesisAperture Synthesis
Obtaining radio ‘pictures’ By using multiple antennas with variable baselines it is possible
to ‘synthesise’ the performance of a very large single dish Radio Telescopes use Aperture Synthesis to give ‘Radio Pictures’
The ultimate system is the Square Kilometre Array SKA
Partly operational in 2015 Fully on line in 2020
This is subject of Part 2 B
SKA