Transcript Project Overview

Radio Astronomy
Radio Telescope Interferometers
(from amateur to awesome)
David Morgan
17-Jul-15
www.dmradas.co.uk
Website - dmradas.co.uk
1
Contents
Simple Interferometer configuration
Radio source
signal
Phase difference
 Interferometer Basics
normal to baseline
Single
antenna
beam

West Antenna
baseline
East Antenna
Receiver output
 Amateur observation of Virgo A
 Aperture Synthesis – Radio Imaging
 Current & Future Radio Telescopes
17-Jul-15
Website - dmradas.co.uk
2
Interferometer basics
 Coherent combination of signals from two or more
antennas spaced some distance apart
Simple Interferometer configuration
Antenna gain
Radio source
Interferometer Beam Pattern
signal
Phase difference
normal to baseline
West Antenna
baseline
East Antenna
East – West beam
Output to receiver
Signals add, producing peaks and nulls with period
proportional to length of baseline
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17-Jul-15
E-W beam angle
North – South beam

beamwidth of
a single antenna
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q =57.3 l/ D
Where q = beam angular resolution
l= wavelength in m
D= length of baseline in m
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Interferometer basics
 Two arms of an amateur 408MHz interferometer separated
by 30m
West arm
Wavelength l= 0.74m
30m
East arm
Baseline length in wavelengths = 30/0.74
= 40 wavelengths
Angle between maxima = 1.40 Earth rotates at 150 / hour Fringe frequency = 60 * 1.4/15 = 5.6 min
17-Jul-15
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408MHz receiver components
 Signals from stacked Yagis combined into cooled LNA
 Signals from both arms combined and fed to receiver
ICOM IC-R7000 Receiver
FunCube Dongle SDR
17-Jul-15
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5
Interferometer beam pattern
 Fringe pattern at 406.5MHz of the Crab Nebula radio source
for 40l baseline
0.7
0.6
0.5
Linear Scale
406.5MHz Interferometer Beam Pattern
0.4
0.3
0.2
0.1
360 Deg / 24 hours/ Transit
0
Fringe frequency a baseline length
11th /12 Feb
08
17-Jul-15
Measurement limits
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Amateur measurement of Virgo A
 Virgo A is 53M ly distant in Virgo Cluster
• 200x mass of Milky Way
• Has an AGN, probably a super massive black hole (6.6 ± 0.4)×109 solar mass
• 4000ly relativistic plasma jet
M87 Giant Elliptical Galaxy
Jet
17-Jul-15
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Making a measurement of Virgo A
 Using ‘Radio Eyes’
Right Ascension hrs
Main beam
Radio Eyes 408MHZ
All Sky map
Taurus A
Horizon
500 Elevation
Declination 0
Virgo A
Beam also
transits Taurus
Beam moves this way
18/8/2012 17-Jul-15
www.dmradas.co.uk
Website - dmradas.co.uk
8
Making a measurement of Virgo A
 Beam pattern sweeps across M87 in a transit scan
150
12:45hr
12:30hr
Right Ascension
12:15hr
12hr
Declination
13hr
+200
+150
150
M87
+100
10
HPBW = 36’
+50
17-Jul-15
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Making a measurement of Virgo A
Signal Amplitude (lin)
 Output signal from the interferometer with 40l baseline
Taurus A
Virgo A
18
17
16
15
14
13
12
11
10
9
8
7
6
5
4
3
2
Right Ascension (hrs)
17-Jul-15
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Making a measurement
 Can distinguish between diffuse and point sources
Diffuse source
Pn
P3
Summed signal fringes
Smooth background
Signal level
P2
P1
Fringes from point source
Source components
Antenna 1
Signal out
Antenna 2
Time during transit scan
 Only a ‘point source’ produces a fringe pattern
17-Jul-15
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Making a measurement of Virgo A
 Signal from ‘point source’ Virgo A in the diffuse background
Virgo A,406.5MHz FM , 50El,180Az,Twin Quagi Antenna
Interferometer, Records of 12th& 13th Feb 08 Summed
1.4
Icom-R7000FM,Wilk.Comb. 6db N type Att, HP Att=6dB,Orig SPU Gain=Max,Amp=7,Input Offset=2.4,TC=6
Interferometer Fringe Signal
Noise in the signal is reduced by summing the records on two days
1.2
Full data record (blue)
1
Average signal (yellow)
0.8
Expected
transit
Increase
in signal
due to Virgo A
0.6
Represents a signal level of
~560Jy, 5.6x10-24W/m2/Hz
0.4
0.2
0
02:00
03:00
04:00
Time GMT (hrs)
So - How are these fringes used to make a radio image ?
17-Jul-15
Website - dmradas.co.uk
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Aperture Synthesis – Radio Imaging
 Have seen how interferometers produce ‘fringes’
 Now examine a technique known as aperture synthesis
 Uses data at many baselines and angles across the source
 Complex and computationally intensive
An example radio source
Interferometer effectively ‘samples’
the wavefront from the source at various angles
with many different baseline lengths.
17-Jul-15
Website - dmradas.co.uk
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Interferometer imaging
 Consider measurement at 3 baselines and at one angle
Source of finite size is moving
East to West
(b0)
A
Amplitude
W
Amplitude
E
Short baseline
b0
Time
fringe Frequency /
baseline length
Baseline b0
Amplitude
fringe amplitude at baseline bn
fringe amplitude at very Medium
shortbaseline
baseline
Amplitude
Fringe Visibility g =
(b1)
Baseline b1
B
b1
Time
fringe Frequency /
baseline length
Amplitude
Amplitude
Long baseline
(b2)
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b2
Time
Baseline b2
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C
fringe Frequency /
baseline length
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Interferometer imaging
Amplitude
 Generating a fringe visibility graph
A
Example Visibility Function
Fringe Frequency
B/A = 0.8
Fringe Visibility g
Amplitude
Baseline b0 (Short)
A/A =1
1
B
C/A= 0.6
Fringe Frequency
0
Amplitude
Baseline b1 (longer)
Short
longer Longer still
longest
Baseline length bn
C
The shape of this graph tells us about the brightness
as a function of position across the source
Fringe Frequency
17-Jul-15
Baseline b2 (longer still)
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Interferometer imaging
 Fringe visibility function
Chosen source axis
g determined by source brightness function
Orthogonal source axis
Single point source
Unresolved at any baseline spacing
Unresolved at any baseline spacing
Circularly symmetric Gaussian source
Single curve falling to 0 at some
long baseline spacing
Single curve falling to 0 at some
long baseline spacing
Sin x /x type curve with zeros at
various baseline spacing
Sin x /x type curve with zeros at
various baseline spacing
Repeating fixed amplitude maxima
and minima at various baseline
spacings
Unresolved at any baseline spacing
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H P Palmer (JB 1962) Website - dmradas.co.uk
Uniformly bright circular source
with sharp edges
Two point sources
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Early angular size measurements
 The technique just described was used in early 1960s to determine
first angular sizes of radio sources
Radio Object
Size
Comment
Cassiopeia A
3’ x 3.8’
Cygnus A
30’’ x 51’’
Taurus A
3.3’ x 3.7’
Centaurus A
3’ x 6.5’
Halo 30 x 80
Virgo A
2.5’ x 5’
Halo + double core
double source
Interferometry has developed greatly since that time
to produce greater accuracy and radio images
17-Jul-15
Website - dmradas.co.uk
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Interferometer imaging
 To get all the spatial frequency components, we need data
at many baselines & various angles
 Could take a long time!
Arbitrary shaped radio source
Multiple interferometer ‘scans’
across source at varying angles
with varying baseline lengths
Very Large Array (VLA) New Mexico
 Use multiple antennas at many positions to minimise time needed
17-Jul-15
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Interferometer imaging
 Build up a 2D g plot of spatial frequencies (baselines) in u,v plane
Example of 2D
g
v
spatial frequency (b/l) plots (from various baselines on different axes)
Fringe Visibility
Orthogonal N-S Axis v (by/l)
g
g values are the white levels
Fourier Transforms
to a sharp edged source
The source image is calculated from the
2D Fourier Transform
of the g plot
Can leave gaps !
`
Spatial frequency b/l (for Axis 1)
Source brightness
function (Top Hat)
Repeat measurements for
each axis
17-Jul-15
E-W Axis u (bx/l)
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Tracking Interferometers
 Use rotation of Earth to form baseline tracks with multiple antennas
 Baseline changes when tracking a source
meridian
b2
b1
b2
q
b0
Baseline length b’
q
b3
b1
b3
b0
t0
t1
t2
t3 time
Fringe frequency = (2p/86400). b’/l
Fringe frequency depends on source hour angle
Baseline length changes with time
and helps ‘fill’ the u,v plane
17-Jul-15
Website - dmradas.co.uk
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Example baseline coverage - of a point source




The Very Large array has 27 antennas
For N antennas we have N(N-1) / 2 baselines in this u, v plane’
Gives 351 baselines - (baseline length & angle)
Earth rotation creates more baselines and angles
31hours
hour
Fringe pattern
g
17-Jul-15
Website - dmradas.co.uk
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Image – Object transform
 Interferometers assemble spatial frequency data in the u,v plane
 Fourier Transform yields the image in the x,y plane
Example of image formation
2D FT
radio image of object
Object plane (x,y)
17-Jul-15
data collection plane (u,v)
(resolution plane)
Website - dmradas.co.uk
Spatial frequency data
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Imaging Radio Telescopes
 Multiple baseline interferometers - today’s Radio Telescopes
• Measure g automatically for many orientations and baseline spacings
• Large amounts of coherent data are gathered and processed
• Large 2D transforms calculated to produce high resolution images
 Aperture synthesis simulates a very large sensitive ‘dish type’ antenna
The Very Large Array in New Mexico
 Most current imaging radio
telescopes are interferometers
 The new LOFAR and SKA telescopes
are extremely large interferometers
 They promise exquisite sensitivity
and resolution
Multiple Interferometer ‘synthesises’ a very large
antenna aperture
17-Jul-15
Website - dmradas.co.uk
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Instrument Resolution
Instrument
Resolution
 Human eyes
 Optical Telescopes (1990)
 Optical telescopes VLT (2012)
a few arc seconds
<1 arc second (turbulence)
0.001 arc sec
 VLA (USA)
 eMERLIN (Europe)
 VLBA (Worldwide)
0.13 arc seconds
0.01 arc seconds
<0.001 arc seconds
VLA
17-Jul-15
eMERLIN
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VLBA
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Instrument Resolution - an example
 The VLA + VLBA has produced some of the finest radio images
 Hi resolution image of M87 - Virgo A – 53Mly away
0.0005arc sec
17-Jul-15
Virgo A jet
Website - dmradas.co.uk
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New Radio Telescopes
 Telescopes have moved away from single large dish structures
 Now use many simple antennas in a very large phased array
 Complexity increased - computation & data communications
10000
SKA
100
1000
Example telescope
improvement
factors
10
Average 5 to 10x
1
Performance multiple
 New LOFAR / SKA require massive fibre optic communications &
computing power
1600
1700
1800
1900
2000
2010
year
17-Jul-15
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SKA Sensitivity
 Order of magnitude increase in sensitivity
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SKA will look back into the Dark Ages
 What size of telescope is needed to see back to the ‘dark ages’ ?
Evolution
Hot Big Bang Dark Ages
First Galaxies Age of Galaxies
102 = 100 x bigger
= 1sq km
10,000sqm
1,000sqm
Size of telescope required
17-Jul-15
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LOFAR
 Low frequency telescope 10 – 240MHz
 Part of / Pathfinder project for SKA
 Multiple simple antennas
 Very powerful computers
17-Jul-15
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Multi-function capability
 LOFAR / SKA can observe with many beams at many frequencies
SIMULTANEOUSLY !
 It is a very sophisticated interferometer
 The advanced communications and computing is the key
Very Large Array image at 74 MHz
 Can synthesise multiple tracking beams at various frequencies
 Must be robust to EMI within its wide band
 Has to be affordable
 First results have been obtained recently
Westerbork Synthesis Radio telescope
image at 325 MHz
17-Jul-15
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SKA in outline




Three types of antennas - frequency range 70 MHz TO 10 GHz
200 square degrees between 70 to 300 MHz
1 - 200 square degrees 0.3 to 1 GHz
1 square degree between 1 to 10 GHz

resolution <0.1 arc sec
 16 384 frequency channels per band per baseline
Low frequency
17-Jul-15
Central cluster of antennas with ‘outliers’ up to three thousand miles
away
High
frequency
Medium frequency
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3000 15m dishes
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SKA hub aperture arrays
 Dense aperture arrays for three wavebands using different
types of antenna – short / medium baselines - SKA hub
• Outliers with longer baselines improve resolution
300Mhz-1GHz
1-10GHz
70-300MHz
1-10GHz
17-Jul-15
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Collecting signal information
 Antenna signals digitised at earliest opportunity – (at the antenna)
 Data time tagged preserving signal phase data
 Optical fibres used where possible
Antenna
(any type)
With thousands of antennas, the cost of
electronic and optical fibre components
must be very low
Special components have been
developed for the SKA
GS/s ADCs
17-Jul-15
Filter bank
(channels)
Website -
Data transport to
main processor
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Beam formers
(FPGAs)
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Radio Astronomy - next 10 years
 Will be based largely on array aperture synthesis
 Can expect some marvellous results towards 2020
 Science goals include
Galaxy evolution, cosmology
& dark energy
Testing gravity using pulsars
& black holes
Searching for life & planets
Probing the Dark Ages
17-Jul-15
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Origin & evolution
of cosmic magnetism
Who knows what next ?
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The power of Interferometers
 Have come a long way in this presentation
 From a simple 2 antenna armature interferometer
 To the future of terrestrial radio astronomy with SKA
Shown how the principles employed in a simple
amateur interferometer can be extended
to the development of the latest
awesome radio telescopes
17-Jul-15
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End
M87 Virgo A
thank you
17-Jul-15
Website - dmradas.co.uk
RA
= 12 : 30.8 (h:m)
DEC
= +12 : 24 (deg:m)
Distance ~ 53M ly 36
References
Radio Astronomy Today H P Palmer, R D Davies & M I Large, |Manchester University
Press , Jodrell Bank Summer school 1962
Theory of interferometry & aperture synthesis,
http://www.astro.ugto.mx/cursos/RadioAstronomy/Radioastronomy-4.pdf
VLA array configuration Amy Mioduszewski. NARO. 2008 workshop on Synthesis
imaging http://www.hartrao.ac.za/synthesis_school/Miod_Array_Design.pdf
NRAO 2008 Lecture on Interferometers Cohen, 11th synthesis imaging workshop
http://www.aoc.nrao.edu/events/synthesis/2008/lectures08.html
SKA information
17-Jul-15
www.skatelescope.org
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Fourier Transforms
 A mathematical operation that relates the components of a
function to the function in another domain
 ie Frequency spectrum (amplitude & phase) required to make a
waveform
Relates the function in u,v plane to the object function in x, y
17-Jul-15
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