Principles of Interferometry I

CASS Radio Astronomy SchoolR. D. Ekers24 Sep 2012

292Sep 2012 R D Ekers 2

WHY?

Importance in radio astronomy– ATCA, VLA, WSRT, GMRT, MERLIN, IRAM...– VLBA, JIVE, VSOP, RADIOASTON– ALMA, LOFAR, MWA, ASKAP, MeerKat, SKA

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Cygnus region - CGPS (small)

Radio Image ofIonised Hydrogen in Cyg XCGPS (Penticton)

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Cygnus A

Raw dataVLA continuum

Deconvolutioncorrecting for gaps between

telescopes

Self Calibrationadaptive optics

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WHY?

Importance in radio astronomy– ATCA, VLA, WSRT, GMRT, MERLIN, IRAM...– VLBA, JIVE, VSOP, RADIOASTRON– ALMA, LOFAR, ASKAP, SKA

AT as a National Facilityeasy to use don’t know what you are doing

Cross fertilization Doing the best science

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Indirect Imaging Applications

Interferometry– radio, optical, IR, space...

Aperture synthesis– Earth rotation, SAR, X-ray crystallography

Axial tomography (CAT)– NMR, Ultrasound, PET, X-ray tomography

Seismology Fourier filtering, pattern recognition Adaptive optics, speckle Antikythera

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Doing the best science

The telescope as an analytic tool– how to use it– integrity of results

Making discoveries– Most discoveries are driven by instrumental developments– recognising the unexpected phenomenon– discriminate against errors

Instrumental or Astronomical specialization?

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HOW ?

Don’t Panic!– Many entrance levels

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Basic concepts

Importance of analogies for physical insight Different ways to look at a synthesis telescope

– Engineers model» Telescope beam patterns…

– Physicist em wave model» Sampling the spatial coherence function» Barry Clark Synthesis Imaging chapter1» Born & Wolf Physical Optics

– Quantum model» Radhakrishnan Synthesis Imaging last chapter

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Spatial Coherence

van Cittert-Zernike theoremThe spatial coherence function is the Fourier Transform

of the brightness distribution

P1 P2

Q1 Q2

P1 & P2 spatially incoherent sources

At distant points Q1 & Q2 The field is partially coherent

• 11

Physics: propagation of coherence• Radio source emits independent noise from each

element• Electrons spiraling around magnetic fields• Thermal emission from dust, etc.

• As electromagnetic radiation propagates away from source, it remains coherent

• By measuring the correlation in the EM radiation, we can work backwards to determine the properties of the source

• Van Cittert-Zernicke theorem states that the• Sky brightness and Coherence function are a Fourier

pair• Mathematically: dmdlvmuljemlIvuV ..2.),(),(

• 12

Physics: propagation of coherence

• Correlate voltages from the two receiversV1

r .V2r Va

e.g1a Vb

e.g1b Va

e.g2a Vb

e.g2b

Vae.Va

e .g2a .g1

a Vbe.Vb

e .g2b .g1

b

Ia .g2a .g1

a Ib .g2b .g1

b

beb

aea

r

beb

aea

r

gVgVV

gVgVV

222

111

..

..

• Simplest example– Put two emitters (a,b) in a plane– And two receivers (1,2) in another plane

• Correlation contains information about the source I• Can move receivers around to untangle information

in g’s

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Analogy with single dish

Big mirror decomposition

14

15

16

( Vi )2

Free space

Guided

17

( Vi )2

Free space

Guided

18( Vi )2Phased array

Free space

Guided

Delay

19( Vi )2

Free space

Guided

Phased array

Delay

20( Vi )2 = (Vi )2 + (Vi Vj )

Free space

Guided

Phased array

Delay

21( Vi )2 = (Vi )2 + (Vi Vj )

Free space

Guided

Phased array

Delay

Correlation array

Ryle & Vonberg (1946) phase switch

22

24I

PhasedArray

x2 x2 x2 x2 x2 x2Split signalno S/N loss

t

Phased array ( Vi )2

I()

( Vi )2

Tied arrayBeam former

25

<Vi Vj>

t

= t/

correlator

Fourier Transform

I(r)

van Cittert-Zernike theorem

Synthesis Imaging

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Analogy with single dish

Big mirror decomposition Reverse the process to understand imaging with a

mirror– Eg understanding non-redundant masks– Adaptive optics

Single dishes and correlation interferometers– Darrel Emerson, NRAO– http://www.gb.nrao.edu/sd03/talks/whysd_r1.pdf

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Filling the aperture

Aperture synthesis– measure correlations sequentially– earth rotation synthesis– store correlations for later use

Redundant spacings– some interferometer spacings twice

Non-redundant aperture Unfilled aperture

– some spacings missing

1 2 3 4 5 6

1unit 5x2units 4x3units 3x4units 2x5units 1x 15n(n-1)/2 =

Redundancy

1 2 3 4 5 6 7 8

1unit 1x2units 1x3units 1x4units 1x5units 0x6units 1x7units 1xetc

Non Redundant

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Basic Interferometer

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Storing visibilities

Storage

Can manipulate the coherence function and re-image

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Fourier Transform and Resolution

Large spacings– high resolution

Small spacings– low resolution

Fourier Transform Propertiesfrom Kevin Cowtan's Book of Fourier

http://www.ysbl.york.ac.uk/~cowtan/fourier/fourier.html

FT

FT

Fourier Transform Properties

http://www.general.uwa.edu.au/u/vpatrick/fourier/magic.html

FT

10% data omitted in rings

Fourier Transform Properties

http://www.general.uwa.edu.au/u/vpatrick/fourier/magic.html

Amplitude of duckPhase of cat

FT

Fourier Transform Properties

http://www.general.uwa.edu.au/u/vpatrick/fourier/magic.html

Amplitude of catPhase of duck

FT

• 37

In practice…1. Use many antennas (VLA has 27)2. Amplify signals3. Sample and digitize4. Send to central location5. Perform cross-correlation6. Earth rotation fills the “aperture”7. Inverse Fourier Transform gets

image8. Correct for limited number of

antennas9. Correct for imperfections in the

“telescope” e.g. calibration errors10.Make a beautiful image…

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Aperture Array or Focal Plane Array?

Why have a dish at all?– Sample the whole wavefront– n elements needed: n Area/( λ/2)2

– For 100m aperture and λ = 20cm, n=104

» Electronics costs too high!

Phased Array Feeds– Any part of the complex wavefront can be used– Choose a region with a smaller waist– Need a concentrator

Computer

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Find the Smallest Waistuse dish as a concentrator

D1 < D2

n1 < n2

D1

D2

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Radio Telescope Imagingimage v aperture plane

Computer

Computer

Dishes act as concentratorsReduces FoV

Reduces active elementsCooling possible

λ

Increase FoV Increases active elements

Active elements ~ A/λ2

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Analogies

RADIO

grating responses

primary beam directionUV (visibility) plane

bandwidth smearing

local oscillator

OPTICAL

Û aliased orders

Û grating blaze angleÛ hologram

Û chromatic aberration

Û reference beam

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Terminology

RADIO

Antenna, dish

Sidelobes

Near sidelobes

Feed legs

Aperture blockage

Dirty beam

Primary beam

OPTICALÛ Telescope, elementÛ Diffraction patternÛ Airy ringsÛ SpiderÛ VignettingÛ Point Spread Function (PSF)Û Field of View

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Terminology

RADIO

Map

Source

Image plane

Aperture plane

UV plane

Aperture

UV coverage

OPTICALÛ ImageÛ Object

Û Image planeÛ Pupil planeÛ Fourier planÛ Entrance pupilÛ Modulation transfer function

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Terminology

RADIO

Dynamic range

Phased array

Correlator

no analog

Receiver

Taper

Self calibration

OPTICAL Û ContrastÛ Beam combinerÛ no analogÛ CorrelatorÛ DetectorÛ ApodiseÛ Wavefront sensing (Adaptive optics)