Transcript Project Overview - Amateur Radio Astronomy Observatory

Radio Astronomy
 An Amateur Radio Astronomy Observatory
David Morgan
Part 2
Interferometers & Aperture Synthesis
From amateur equipment to future global systems
7/17/2015
Website - dmradas.co.uk
1
Radio Astronomy
7/17/2015
Website - dmradas.co.uk
2
Radio Astronomy
 Total Power Receiver systems - Part 1
 Interferometers - Part 2 (A&B)
A
B
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Radio
Window
Origin of Galaxies
Basic concept
Observing ‘point sources’
Spatial resolution and sensitivity
Multiple baselines & aperture synthesis
Fringe visibility functions
Cosmic hydrogen distribution
Today’s best instruments
< 300,000years after big bang
The future global radio telescope - The SKA
Mk1 at Jodrell Bank
VLA New Mexico
Website - dmradas.co.uk
SKA 2020
3
Radio Astronomy - Interferometer Basics
 Adding two waves
same signal
from source
wave crests sometimes ‘instep’
sometimes out of step
depending on arrival angle
source moves
across sky
Arrival angle q changes
phase difference changes
output
7/17/2015
Website - dmradas.co.uk
4
Radio Astronomy
 Adding signals together - Phasing of two waves
+1
Signal 1
0
-1
+1 +1
moved l/2
0
0
-1
-1
Signal 2
+2
The resulting amplitude varies between 2 and 0 depending the
‘Phase’
thesignal
two 2signals
Peaks
on relationship
signal 1 cancelbetween
troughs on
0 0
Result = ZERO
-2
7/17/2015
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5
Radio Astronomy
MHz
The BEAT or ‘Fringe’ frequency
depends on Earth’s rotation & antenna baseline
Moves
slowly
Moves
quickly
2
BEAT AMPLITUDE
This is the signal
that gets recorded
Sub Hz
0
7/17/2015
Time
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6
Radio Astronomy
 Combined signal - omni directional antennas
beat frequency signal
position / time
equally sensitive
in all directions
 Combined signal - directional antennas
beat frequency signal
sidelobes
position / time
7/17/2015
Website - dmradas.co.uk
sensitive only in
forward direction
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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 q changes the signals
go in and out of phase so that
signal strength varies with angle
and therefore time
Wavelength l
Antenna # 1
Baseline b
Antenna # 2
Example of Interferometer fringes
Response of single antenna
sidelobe
sidelobe
Angular resolution
Response of two antennae
Time
7/17/2015
Website - dmradas.co.uk
8
Radio Astronomy
 Why use an Interferometer ?
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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
30m East – West Baseline
Frequency = 408MHz
l= 0.735m & baseline =30m
Dq = 0.735/ 30 = 0.0245 rads
or 1.4 degrees
Earth rotation angular velocity
= 150 / hr
Fringe frequency = 1.4 /15 hrs
= 5.6mins
My Amateur Radio Telescope Interferometer
7/17/2015
Website - dmradas.co.uk
This time is > signal averaging TC
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Radio Astronomy
 My twin 15 element Quagi antenna 30m E-W Interferometer
30m apart = 41 wavelengths @ 408MHz
West Tower
7/17/2015
East Tower
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10
Radio Astronomy
 Twin Quagi Antenna responses
• Each twin Yagi unit has a response shown below
Polar Antenna response
Antenna output
Cartesian Antenna response
-900
00
+900
Angle from antenna bore sight
14dB Gain & 170 Beamwidth
When two are used as an interferometer Beamwidh < 1.40
7/17/2015
Website - dmradas.co.uk
11
Radio Astronomy
 Moving through interferometer ‘beams’
Antenna
pattern
Source moves through beams
beat frequency signal
position / time
7/17/2015
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12
Radio Astronomy
 Observable discrete ‘Point’ sources
(northern hemisphere)
Cass A 3C461
Cygnus A 3C405
RA 23:23:21, DEC +58:49:59
RA 19:59:28, DEC +40:44:00
Virgo A
Taurus A
Taurus
Virgo
7/17/2015
Taurus A 3C144 (crab)
Virgo A 3C274 (M87)
RA 05:34:30, DEC +22:00:57
RA 12:30:48, DEC +12:22:59
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13
Radio Astronomy
 Taurus signal embedded in galaxy background
System Data
Taurus
Galactic background
7/17/2015
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14
Radio Astronomy
 Extracted Signal from Taurus A - ‘The Crab’
Signal ‘fringes’
Taurus A
Crab Nebula
NGC 1952
6,300Ly
SNR
AD 1054
30m separation
7/17/2015
Website - dmradas.co.uk
15
Radio Astronomy
Plot of Fringe amplitude
Transit was ‘bang on schedule’
This fringe amplitude plot
was derived by cross -
correlation of the signal
on the previous graph
with the theoretical
interferometer
fringe frequency
Fringe frequency 1/ 5.6mins
Taurus A
Source strength = 1200Jy
1Jy = 10-26W/m2/Hz
So we receive from the Crab about
1.2x10-23W/m2/Hz
This produces about 0.01mV in the
antenna
7/17/2015
Website - dmradas.co.uk
16
Radio Astronomy
 Cross Correlation (are you like me ?)
Signal from Taurus A 16:00 – 01:00GMT 25/1/08
Calculated
fringe signal
period =5.6min
Cross Correlation
Function
7/17/2015
Website - dmradas.co.uk
Transit
17
Radio Astronomy
 Virgo A M87
Good example of how an interferometer can distinguish
compact from diffuse Radio sources
Radio Galaxy
Looking toward
Galactic N Pole
7/17/2015
Virgo A is a compact
Radio Source
Looking toward centre
of galactic plane
Website - dmradas.co.uk
M87 Virgo A NGC4486
Giant Elliptical Galaxy with
intense relativistic jet
~ 60 million LY distant
18
Radio Astronomy
This is close to the limit of what
can be measured with my
2 antenna interferometer
Another view of the
energetic jet in M87
Virgo A
Virgo A Radio
Spectrum
Flux =klx (x=spectral index)
7/17/2015
Website - dmradas.co.uk
19
Radio Astronomy
Virgo A M87
Fringe Amplitude
This fringe visibility plot was derived by cross - correlation of the signal
with the theoretical interferometer fringe frequency
There are no more sources visible from the northern hemisphere at this level
Pulsars are < 100Jy and would require a very costly 10m dia dish
7/17/2015
Website - dmradas.co.uk
20
Radio Astronomy
 Estimating the size of a radio source
• If the source produces fringes then we know that its angular
diameter is less than l/b (l= 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)
Small source of angular size < l/b
S
S1
S2
Wavelength = l
S3
S4
Multiple sources ‘fill in’ fringes leading to ‘flat line’
7/17/2015
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Produces a clear ‘fringe pattern’
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Radio Astronomy
 Example of distributed & ‘point’ sources
7/17/2015
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22
Radio Astronomy
 Comparison of strengths of ‘point’ radio sources
SNR
AD 1667
CASS A
Cygnus A
Jet
Massive
Black
Hole ?
Elliptical
Galaxy
Virgo A
7/17/2015
Website - dmradas.co.uk
23
Radio 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
7/17/2015
Website - dmradas.co.uk
24
Radio Astronomy – Part 2 B Aperture 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
7/17/2015
Website - dmradas.co.uk
SKA
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