an amateur radio astronomy observatory david morgan

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06/15/22 Website - dmradas.co.uk 1 An Amateur Radio Astronomy Observatory David Morgan Radio Astronomy Radio Astronomy Part 2 Interferometers & Aperture Synthesis From amateur equipment to future global syste

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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 Presentation

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Page 1: An Amateur Radio Astronomy Observatory David Morgan

04/19/23 Website - dmradas.co.uk 1

An Amateur Radio Astronomy Observatory

David Morgan

Radio AstronomyRadio Astronomy

Part 2 Interferometers & Aperture Synthesis

From amateur equipment to future global systems

Page 2: An Amateur Radio Astronomy Observatory David Morgan

04/19/23 Website - dmradas.co.uk 2

Radio AstronomyRadio Astronomy

Page 3: An Amateur Radio Astronomy Observatory David Morgan

04/19/23 Website - dmradas.co.uk 3

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

Page 4: An Amateur Radio Astronomy Observatory David Morgan

04/19/23 Website - dmradas.co.uk 4

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

Page 5: An Amateur Radio Astronomy Observatory David Morgan

04/19/23 Website - dmradas.co.uk 5

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

Page 6: An Amateur Radio Astronomy Observatory David Morgan

04/19/23 Website - dmradas.co.uk 6

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

Page 7: An Amateur Radio Astronomy Observatory David Morgan

04/19/23 Website - dmradas.co.uk 7

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

Page 8: An Amateur Radio Astronomy Observatory David Morgan

04/19/23 Website - dmradas.co.uk 8

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

Page 9: An Amateur Radio Astronomy Observatory David Morgan

04/19/23 Website - dmradas.co.uk 9

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

Page 10: An Amateur Radio Astronomy Observatory David Morgan

04/19/23 Website - dmradas.co.uk 10

Radio AstronomyRadio Astronomy

My twin 15 element Quagi antenna 30m E-W Interferometer

East TowerWest Tower

30m apart = 41 wavelengths @ 408MHz

Page 11: An Amateur Radio Astronomy Observatory David Morgan

04/19/23 Website - dmradas.co.uk 11

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

Page 12: An Amateur Radio Astronomy Observatory David Morgan

04/19/23 Website - dmradas.co.uk 12

Radio AstronomyRadio Astronomy

Moving through interferometer ‘beams’

position / time

beat frequency signal

Source moves through beams

Antennapattern

Page 13: An Amateur Radio Astronomy Observatory David Morgan

04/19/23 Website - dmradas.co.uk 13

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

Page 14: An Amateur Radio Astronomy Observatory David Morgan

04/19/23 Website - dmradas.co.uk 14

Radio AstronomyRadio Astronomy

Taurus signal embedded in galaxy background

Taurus

Galactic background

System Data

Page 15: An Amateur Radio Astronomy Observatory David Morgan

04/19/23 Website - dmradas.co.uk 15

Radio AstronomyRadio Astronomy

30m separation

Taurus ACrab NebulaNGC 1952

6,300Ly

Signal ‘fringes’

SNRAD 1054

Extracted Signal from Taurus A - ‘The Crab’

Page 16: An Amateur Radio Astronomy Observatory David Morgan

04/19/23 Website - dmradas.co.uk 16

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

Page 17: An Amateur Radio Astronomy Observatory David Morgan

04/19/23 Website - dmradas.co.uk 17

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

Page 18: An Amateur Radio Astronomy Observatory David Morgan

04/19/23 Website - dmradas.co.uk 18

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

Page 19: An Amateur Radio Astronomy Observatory David Morgan

04/19/23 Website - dmradas.co.uk 19

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)

Page 20: An Amateur Radio Astronomy Observatory David Morgan

04/19/23 Website - dmradas.co.uk 20

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

Page 21: An Amateur Radio Astronomy Observatory David Morgan

04/19/23 Website - dmradas.co.uk 21

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 =

Page 22: An Amateur Radio Astronomy Observatory David Morgan

04/19/23 Website - dmradas.co.uk 22

Radio AstronomyRadio Astronomy

Example of distributed & ‘point’ sources

Page 23: An Amateur Radio Astronomy Observatory David Morgan

04/19/23 Website - dmradas.co.uk 23

Radio AstronomyRadio Astronomy

Comparison of strengths of ‘point’ radio sources

Cygnus A

CASS A

SNR AD 1667

Virgo A

Jet MassiveBlack

Hole ?EllipticalGalaxy

Page 24: An Amateur Radio Astronomy Observatory David Morgan

04/19/23 Website - dmradas.co.uk 24

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

Page 25: An Amateur Radio Astronomy Observatory David Morgan

04/19/23 Website - dmradas.co.uk 25

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