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GLOW Interferometry School, Bielefeld 2011
1. Fourier optics
2. Fourier theorems
3. Visibility function
4. Aperture synthesis
5. Coordinates
6. Correlation interferometer
7. Imaging
8. Cleaning
Radio Interferometry Basics
U. Klein
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GLOW Interferometry School, Bielefeld 2011
Fundamental understanding of the functionality of radio interferometry
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GLOW Interferometry School, Bielefeld 2011
1. Fourier optics
Aperture with constant extent perpendicular to screen, with extent a in this y-direction, constant current Gy(x) in y-direction. Infinitesimal far-field element dE resulting from an E-field component E(x) along infinitesimal element dx
and integrating over y and x leads to
This is Huygens’ Principle for the far-field: superposition of field components along the aperture Far-field: R x
(… ties up to Jackson, Chapt. 14)
dydx eR
E(x) i E d rki
22
dxE(x) e R
a i dE rki
2
sin xR r
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GLOW Interferometry School, Bielefeld 2011
Normalising by wavelength
Lumping constants and R dependence into
leads to simple Fourier intergral (x = x / , direction cosine ξ = sin )
xx
ikR
deER
eai E
x sin2
2
xikR E eR
a i E
2
deE xE
dxexE E
xi
xi
2
2
)(
)(
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GLOW Interferometry School, Bielefeld 2011
slit or uniformly illuminated aperture: sinc function
otherwise xE
Dxforconst
0)(
2
)(sinc)sin(
)(
DD
DE
circular aperture: Bessel function
antenna diagram: P(,) = |E(, )|2
D
DJE
)(2)( 1
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GLOW Interferometry School, Bielefeld 2011
2. Fourier theorems
Fourier transform F(s) of function f(x)
with inverse transform
f(x)dxexf sF xsi F
2)(
)()( 2 sFdsesF xf xsi F
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GLOW Interferometry School, Bielefeld 2011
Convolution:
)()()()( xgxfduuxguf xh
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GLOW Interferometry School, Bielefeld 2011
Consider the two domains:
x s application
time t [s] frequency [Hz]
acoustics
angle [rad] spatial frequ. u [rad-1]
radio astronomy
Oboe: waves Oboe: spectrum
LMC: radio brightness LMC: spatial frequ. spectrum
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GLOW Interferometry School, Bielefeld 2011
Let F f(x) = F(s), F g(x) = G(s)
addition theorem
)()()]()([ sGsFxgxf F
a
sF
axaf
1)(F
example: an aperture has beam size
/D
similarity theorem
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GLOW Interferometry School, Bielefeld 2011
shift theorem
sFeaxf sai 2)( Fexample: electronic steering
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GLOW Interferometry School, Bielefeld 2011
convolution theorem
example: beam smearing
)()()]()([ sGsFxgxf F
convolution with broader beam
multiplication with narrower ‘filter‘
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GLOW Interferometry School, Bielefeld 2011
autocorrelation theorem
2* )()( sFduxufuf
F
example: antenna diagramme
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GLOW Interferometry School, Bielefeld 2011
convolution and auto-/cross-correlation theorem: reception pattern of an interferometer
dxuxEuExP
)()()( * )()()( * EEP
)2cos()()( DPP SD
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GLOW Interferometry School, Bielefeld 2011
sampling theorem: how to properly convert continuous into discretised functions
comb or „Sha“ function:
multiplication
f(x) · III(x)
converts continuous
into
discretised function
)( xnxxx
x
n
III
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GLOW Interferometry School, Bielefeld 2011
A function whose Fourier transform is zero for |s| > sc is fully specified by values spaced at equal intervals not exceeding ½ sc
-1 save for any harmonic term with zeros at the sampling points.
‘aliasing‘!
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GLOW Interferometry School, Bielefeld 2011
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GLOW Interferometry School, Bielefeld 2011
3. Visibility function
Correlated power received from source with brightness B
where (D = D/)
D is the baseline. Integration yields
dBAdP )(cos)(
sDss 2,0
Ωd)σ · Dπ() · σ) · B(σA() sDπ(- Δ
Ωd)σ · Dπ() · σ) · B(σA() sDπ( Δ
dΩ)σs(Dπ)σB()σA(Δν)s,DP(
Source
λ
Source
λ
Source
λλ
2sin2sin
2cos2cos
2cos
0
0
00
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GLOW Interferometry School, Bielefeld 2011
With
it is convenient to introduce complex visibility
such that
)()0()0(
)()0()(
NAAF
FAA
dΩe)σB()σ(AeVVSource
σDπiN
i λ 2
VdΩ)σDπ()σB()σ(AφV
VdΩ)σDπ()σB()σ(AφV
Source
λN
Source
λN
Im
Re
2sinsin
2coscos
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GLOW Interferometry School, Bielefeld 2011
Since AN 1 (the single-antenna diagramme is very broad), and writing
we obtain
This is the basic relation for aperture synthesis:
Comparing with 1st version of P we arrive at
dude)V(u)B(
dde)B(uV
uπi
uπi
)(2
)(2
,,
,),(
In order to retrieve the brightness B(ξ,η), we need to measure the visibility V(u,v) for as many baselines Dλ= (u,v) as possible and Fourier-transform it.
)2cos(),( 000 VsDVAsDP
uD
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GLOW Interferometry School, Bielefeld 2011
example:
double source (schematically):
apply convolution theorem
source brightness = convolutionof -function with two -functions
visibility = multiplication of cosinewith sinc-function
Now observe with different base-lines
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GLOW Interferometry School, Bielefeld 2011
astronomical telescope = spatial filter:
single dish: low-pass filter
interferometer: high-pass filter
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GLOW Interferometry School, Bielefeld 2011
4. Aperture synthesis
need many D = (u,v)
consider 4 4 apertures, producing n (n-1) 2 = 120 baselines, most of which are redundant, while we need only 24 of them to cover the (u,v)plane
aim: preferably little redundancy!
ddueuV)(B uπiobsobs )(2),(,
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GLOW Interferometry School, Bielefeld 2011
Earth-rotation synthesis
need good sampling of (u,v) plane with many (non-redundant) baselines
earth-rotation synthesis helps a lot
due to Sir Martin Ryle (Nobel Prize 1974 with A. Hewish)
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GLOW Interferometry School, Bielefeld 2011
144 m 144 m
9144 m
36 m
300 m 180 m9144 m
0 1 2 3 4 9 A B C D
east-west array: e.g. WSRT
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GLOW Interferometry School, Bielefeld 2011
earth rotation produces a goodtransfer function: (u,v) tracks
e.g. pure east-west interferometer
FT antenna diagramme („dirty beam“)
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GLOW Interferometry School, Bielefeld 2011
for the kth ring, the grating rings have functional form
where sinc1/2 implies the “half-order derivative“ of the sinc function
derívative theorem:
and from this:
(intrigued? Bracewell & Thompson, Ap.J. 188, 77, 1973)
kDN)F( 22
2
12sinc, 2
1
)(2)( sFsixfdxd F
)(2)(21
21sFsixf
dx
d F
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GLOW Interferometry School, Bielefeld 2011
actually need max. 12 hrs. to obtain full (u,v) coverage
VLA: 8 hrs. (Y-shape)
V(u,v) is Hermitean, since thebrightness is a real quantity.
… we get the other half for free!
),(),( * uVuV
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GLOW Interferometry School, Bielefeld 2011
5. Coordinates
Complex visibility reads
Define coordinate system (u,v,w), whichare related to unit vectors and baseline:
We may have to deal with non-zero w-component!
dΩe)σB()σ(AeVVSource
σDπiN
i λ 2
d
ddddd
wusλ
D
wsλ
D
22
22
0
1
1
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GLOW Interferometry School, Bielefeld 2011
, , are direction cosines
and the visibility becomes
cos
cos
cos
dde
)B()(AwuV
wuπi
N
22
)11(2
1,,),,(
22
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GLOW Interferometry School, Bielefeld 2011
Coordinate system (,) corresponds to projection of celestial sphere onto a tangential plane at the origin of the field centre.
possible projections:
• tan: optical astronomy• arc: Schmidt telescopes, single dishes• sin : aperture synthesis
(more: Calabretta & Greisen, A&A 395, 1077, 2002)
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GLOW Interferometry School, Bielefeld 2011
direction cosines and their relation to other coordinate systems:
spherical coordinates ,
equatorial coordinates ,
cossin
sinsin
arctan
arcsin 22
22
00
022
0
0
1sincos
sin1cosarctan
)cos(sincoscossin
)sin(cos
0000
0
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GLOW Interferometry School, Bielefeld 2011
Using polar coordinates , , the solid-angle element d reads
The releation between the infinitesimal elements is given by
where |J| is the determinant of the Jacobi matrix
which leads to
ddd sin
221
dd
d
ddJdd
d
d
d
dd
d
d
d
J ),(
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GLOW Interferometry School, Bielefeld 2011
Antenna coordinates and baseline components
defines a double ellipse:
Z
Y
X
L
L
L
hh
hh
hh
w
u
sinsincoscoscos
cossinsincossin
0cossin1
2
22
2
2
sin
cos
YX
Z
LLL
u
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GLOW Interferometry School, Bielefeld 2011
Coverage of (u,v) plane
e.g. VLA e.g. LOFAR
a bit weird: component LZ gives rise to non-coplanar baselines not simple Fourier integral anymore!
dde
)B()(AwuV
wuπi
N
22
)11(2
1,,),,(
22
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GLOW Interferometry School, Bielefeld 2011
6. Correlation interferometer
has power response
with phase lag
due to geometric time delay
cos)()( 0PP
Dg 22
sinc
Dg
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GLOW Interferometry School, Bielefeld 2011
heterodyne principle:
down-convert from high (RF or HF) to intermediate frequency (IF)
produces frequency spectrum including
IFLOs
IFLOi
signal frequ. (USB)
image frequ. (LSB)
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GLOW Interferometry School, Bielefeld 2011
Effect of finite bandwidth (1):
Delay compensation i indispensible in order to avoid ‚fringe washing‘.We measure correlation of two voltages V1, V2:
By definition, this is also the Fourier transform of the bandpass |H()|
Calculate power over the band, assuming rectangular bandpass and (roughly) constant power as a function of frequency:
T
T
g dttVtVT
P )()(2
1lim)( 12 T
deHP ti 22)()(
2
2
0
0
0
)2cos()(
dVAP Vgg
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GLOW Interferometry School, Bielefeld 2011
Hence finally:
So the correlated signal is modulated with a sinc function!
= 50 MHz
D = 1 km
tracking source over 1.6‘ withoutdelay compensation produces~1% of loss
needs g 1.6 ns
)2cos()sin(
)( 00 Vgg
gg VAP
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GLOW Interferometry School, Bielefeld 2011
the phases from telescopes 1 and 2 are
hence the correlated power is
where = g - i and ‘‘ refers to USB and LSB.
gIFLOg )(221
LOi 22
])(2cos[
)cos(
0
210
LOVIFgLO
V
VA
VAP
Measure complex visibility with quadrature network:
V
V
VVV
V ReIm
ImRe
arctan
22
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GLOW Interferometry School, Bielefeld 2011
Fringe rotation/stopping: earth‘s rotation modulates correlated power quasi-sinusoidally with natural fringe rate
where e = dhdt = angular rotation frequency of earth, = declination. At the VLA, the fringe frequency may exceed 150 Hz.
Not interested in fringe rate, but rather in change ofV; hence compensate fringe rotation by modulating LO phase. In the IF section, we have
So control LO phase such that
vanishes. This now requires a complex correlator in order to measure the amplitude and phase separately!
cosudt
dh
dh
dw
dt
dwe
LOVgLOLOVIFgLO 2)(221
dt
d
dt
d
dt
d LOgLO
2)( 21
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GLOW Interferometry School, Bielefeld 2011
Effect of finite bandwidth (2):
Fourier integral is precise only for monochromatic radiation! At the centre frequency 0 we have the precise relation
Away from the centre frequency we have
Generalised similarity theorem in n dimensions:
00)(2
0000),(, ddueuV)(B uπi
obsobs
0
00
00
0
00
00
1
1
uuu
a
sF
axaf n
n
1)(F
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GLOW Interferometry School, Bielefeld 2011
application to 2-D Fourier relation between visibility and brightness:
so that (“~“ indicating the influence of the bandpass)
Delay tracking is exact for 0 = 0 at = 0. For signals at frequeny arriving from direction (, ) there will be a lag error
so that the phase is imprecise by an amount
),(,,,2
000
00
2
000
uVuVB)B( oo
deHuV)(uVuπi
IF
)(22
200
00
000
000
0
)(,,~
0
00
u
)()(2
)(2 000
00
u
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GLOW Interferometry School, Bielefeld 2011
Consider point source at location 0, assume rectangular bandpass. Then
where we have assumed ( 0)2 1. Thence, with du = ( 0) du0 du0, we obtain the convolution
The Fourier transform of the sinc function is a box function
where
This effect called bandwidth smearing (chromatic aberration) is irreversible!
0
200
00
)(2 000~
dueudue)(B uπiuπi IFsinc
2
2
22
0
0
01~
dudee)(B uπiuπi
IF
V(u)Fourier kernel
00
1
IFIF
)S( otherwise0
2
1for1 0
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GLOW Interferometry School, Bielefeld 2011
Short version of this: radiation within range of frequencies produces u-v tracks of different size produces images of different scale if referred to same frequency when doing the Fourier transform.
13‘ to fie
ld centre
radial smearing 202
0
IF
0
u
v20
20
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GLOW Interferometry School, Bielefeld 2011
7. ImagingIn practice: convert Fourier integral
into a sum and apply an FFT. This requires a retabulation of the measured visibilities onto a 2N 2M grid. In what follows, we use the integral notation for the sake of better readability of the equations. The true visibilities are strongly modified owing to:
ddueuV)B( uπi
)(2),(,
1. sampling by transfer function: S(u,v)
2. regridding onto regular grid for FFT with grid function: G(u,v)
3. weighting to shape the synthesized beam and control sensitivity: R(u,v)
4. FFT
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GLOW Interferometry School, Bielefeld 2011
transfer function: S(u,v) gives rise to ‘dirty beam‘
weighting with W(u,v)
W = R T D
R: Aeff, Tsys, ,
T: taper (shapes the beam)
D: density of visibilities
brightness (‘dirty image‘) now is
ddueuVuS)(B uπiD
)(2),(),(,
ddueuVuSuW)(B uπiD
)(2),(),(),(,
Plateau de Bure Interferometer
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GLOW Interferometry School, Bielefeld 2011
now prepare for FFT by retabulation S(u,v); needs two steps:
(i) convolve with appropriatefunction C(u,v)
(ii) multiply with grid function G(u,v) (fakir‘s bed of nails)
this is a significant modi-fication of the data
GCVSWV D ])[(
VSWCGBD FFFFF
use convolution theorem to see modification in theimage domain:
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GLOW Interferometry School, Bielefeld 2011
can ‘undo‘ gridding by dividing VD by the inverse transform of G:, yielding the ‘grid-corrected‘ dirty image:
This is the true brightness distribution, convolved with the dirty beam
C
BB
-
DDC
1F
SWGF D FFF
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GLOW Interferometry School, Bielefeld 2011
8. Cleaning
Goal: deconvolution in order to get rid of sidelobes
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GLOW Interferometry School, Bielefeld 2011
basic clean algorithm (Högbom 1974):
(i) First compute the dirty map BD(,) via FT of VD(u,v) and the dirty beam FD(,) via the FT of S(u,v) · W(u,v).
(ii) Find position of maximum brightness and subtract ·FD(,) · Bmax; “loop gain” controls speed of the cleaning process (0 < < 1); store Bmax at the corresponding position in a work array (creates “fakir’s bed of nails”).
(iii) Go to (ii) until a user-defined level is reached (e.g. number of iterations or rms noise).
(iv) Convolve the so obtained “point source model” (“fakir’s bed of nails”) with the “clean beam”, or synthesized beam, e.g. a Gaussian of width ~ / Dmax.
(v) Add this convolved model to the residual map to yield the clean image BC(,).
There are variants with major (check in the u-v domain) and minor cycles (Clark 1980, Cotton-Schwab 1984)
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GLOW Interferometry School, Bielefeld 2011
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GLOW Interferometry School, Bielefeld 2011
Zero-spacing problem
Missing short spacings: interferometer measures zero total flux!
hence
since
which cannot be measured “negative bowl”
dde)B(uV uπi )(2,),(
!,)0,0( totSdd)B(V dBS
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GLOW Interferometry School, Bielefeld 2011
need to add in single-dish measurements (in the u,v domain!)
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GLOW Interferometry School, Bielefeld 2011
Don‘t forget that the brightness distribution is multiplied by the enveloping ‘primary beam‘, i.e. the beam of the individual antennae. Need to correct forthis degradation by dividing the clean image by the primary beam:
),(
),(),(
PCP
C F
BB
Naturally gives rise to increasingnoise at radial distances largerthan the primary HPBW.
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GLOW Interferometry School, Bielefeld 2011
3C 120
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GLOW Interferometry School, Bielefeld 2011
Thanks for your attention!