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Optics in Astronomy- Interferometry -
Oskar von der LüheKiepenheuer-Institut für Sonnenphysik
Freiburg, Germany
September 2002 Optics in Astronomy 2
Contents
• Fundamentals of interferometry• Concepts of interferometry• Practical interferometry
• Fundamentals of interferometry– Why interferometry?– Diffraction-limited imaging– A Young‘s interference experiment– Propagation of light and coherence– Theorem of van Cittert - Zernike
September 2002 Optics in Astronomy 3
What is interferometry?• superposition of electromagnetic waves
– at (radio and) optical wavelengths– infrared: λ = 20 µm ... 1µm– visible: λ = 1 µm ... 0.38 µm
• which emerge from a single source• and transverse different paths• to measure their spatio-temporal coherence properties
• Why interferometry?– to increase the angular resolution in order to– compensate for atmospheric and instrumental aberrations
• speckle interferometry• diluted pupil masks
– overcome the diffraction limit of a single telescope by coherentcombination of several separated telescopes
• Topic of this lecture on interferometry?– to increase the angular resolution in order to– compensate for atmospheric and instrumental aberrations
• speckle interferometry• diluted pupil masks
– overcome the diffraction limit of a single telescope by coherentcombination of several separated telescopes
September 2002 Optics in Astronomy 4
Diffraction limited imaging
D
DFX λ44.2=∆
telescope aperture
focal length F
September 2002 Optics in Astronomy 5
A Young‘s interference experiment������
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SourceMask
Screen
diameter D
baseline B
focal length F
wavelength λλλλ
BFx λ=∆
DFX λ44.2=∆
September 2002 Optics in Astronomy 10
Dependence on source position������
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September 2002 Optics in Astronomy 12
Dependence on internal delay������
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delay δδδδ
September 2002 Optics in Astronomy 14
Monochromatic e. m. waves
[ ]tjrUtrV ωω −= exp)(),(Time-dependent, monochromatic e.m. field:
πνω 2frequency circular time
spacein position
=t
r
Time-independent e.m. field fulfils Helmholz equation: λ
πωω
2;0)()( 22 ===+∇c
krUk
∈VU , Ī
September 2002 Optics in Astronomy 15
dsrjr
rUj
rU o ∫∫∑
= )(2exp1)(1)( 1 ϑχ
λπ
λ ωω
Propagation of field
Kirchhoff-Fresnel integral, based on Huyghens‘ principle
Σ
0r
1rr
September 2002 Optics in Astronomy 16
Non-monochromatic waves
[ ] ωωπ ω
οdtjrUtrV −= ∫
∞exp)(1),(Spectral decomposition:
∫∫∑
−= ds
crcrtrV
ttrV
πϑχ
∂∂
2)(,),( 10
Propagation of a general e.m. wave:
ϖω<<∆Condition of quasi-monochromatism:
Propagation of a quasi-monochromatic wave:
∫∫∑
−= ds
crtrV
rjtrV o )(,1),( 1 ϑχ
λ
September 2002 Optics in Astronomy 17
Intensity at point of superposition
{ }),(),(Re2),(),(
),(),(),(
*2121
221
trVtrVtrItrI
trVtrVtrI
++=
=+=
The intensity distribution at the observing screen of the Young‘s interferometer is given by the sum of the intensities originating from the individual apertures plus the expected value of the cross product (correlation) of the fields.
September 2002 Optics in Astronomy 18
),(),(:),,,( 22*
112121 trVtrVttrr ⋅=Γ
Concepts of coherence I(terminology from J. W. Goodman, Statistical Optics)
mutual intensity:
temporally stationary conditions, with τ = t2 - t1 )(:
),,,(),,,(
12
11212121τ
τΓ=
+Γ=Γ ttrrttrr
[ ]( ) ( )
( ) ( )212
211
12*
11
2/12211
1212
,,
,,
)0()0()(:)(
τ
τ
ττγ
+
+⋅=
Γ⋅ΓΓ=
trVtrV
trVtrV
complex degree of coherence:
September 2002 Optics in Astronomy 19
Concepts of coherence II
( )),(),(
),,(:
11*
11
1111
τ
ττ
+⋅=
Γ=Γ
trVtrV
rr
)(11 τγ
self coherence (temporal coherence):
complex degree of self coherence:
( )),(),(
0
12*
11
1212
trVtrV
J
⋅=
Γ=
)0(1212 γµ =
mutual intensity (spatial coherence):
complex coherence factor:
September 2002 Optics in Astronomy 20
Concepts of coherence III
• Coherence is a property of the e.m. field vector!
• It is important to consider the state of polarisation of the light as orthogonal states of polarisation do not interfere (laws of Fresnel-Arago).
• Optical designs of interferometers which change the state of polarisaiton differently in different arms produce instrumental losses of coherence and therefore instrumental errors which need calibration.
September 2002 Optics in Astronomy 21
Temporal coherence
1
2R
2Z
S2
S1
1
2
∫∞
∞−= ττγτ dc
211 )(:
coherence time: ωπτ
∆≈ 2
c
refined monochromatic condition: cc cl τδ :=<<
September 2002 Optics in Astronomy 22
Two-way interferometer VI
Extended spectral distribution of a
point source
September 2002 Optics in Astronomy 23
Two-way interferometer VII
Extended spectral distribution source
and delay errors
September 2002 Optics in Astronomy 24
Transport of coherence
∫∫ ∫∫∑ ∑
−−=
1 1
2112212
2
1
121 )(2exp),()()(),( σσ
λπ
λϑχ
λϑχ ddSSjrrJ
SSxxJ
1r
2r
1ϑ
2ϑ
1x
2x
111 rxS −=
222 rxS −=
1Σ 2Σ
September 2002 Optics in Astronomy 25
Extended sources������
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September 2002 Optics in Astronomy 26
Extended, incoherent sourcesMost astronomical sources of light have thermal origin. The processes emitting radiation are uncorrelated (incoherent) at the atomic level.
)()(),( 211
2
21 rrrIrrJ −= δπλMutual intensity at the surface of
an incoherent source:
∫∫∑
−−=
1
112121
2121 )(2exp)()()(1),( σ
λπϑχϑχ
πdSSjrI
SSxxJ
Transport of mutual intensity to the interferometer:
September 2002 Optics in Astronomy 27
( )
∫∫
∫∫
−−=
∆+⋅∆=
source
21
source21
2exp)(1
2exp)(1),(
ϑλ
ϑπϑπ
ϑδαλπϑ
π
dxxjI
dyxjIxxJ
Celestial sources have large distances S compared to their dimensions. Differences in S can be expanded in first order to simplify the propagation equation. Inclination terms χ are set to unity. Linear distances in the source surface are replaced by apparent angles . The transport equation can then be simplified:
xϑ
Theorem of van Cittert - Zernike
September 2002 Optics in Astronomy 28
Intensity distribution in the Young‘s intererometer{ }
( ){ }
( ) ( )∫∫∫∫
−=
=
⋅+=
⋅++=
++=
ϑϑϑϑλ
πϑλ
µµ
λπµ
λπ
dIdBjIB
FrBrA
FrBxxJrIrI
trVtrVrIrIrI
2exp
with
2cos1)(2
2cos,Re2)()(
),(),(Re2)()()(
12
12
2121
*2121
The complex coherence factor µ is often called the complex visibility.
It fulfils the condition 1≤µ
September 2002 Optics in Astronomy 29
Two-way interferometer VII
Spatially extended source - double star
September 2002 Optics in Astronomy 30
Two-way interferometer VII
Spatially extended source - limb
darkened stellar disk
September 2002 Optics in Astronomy 32
van Cittert - Zernike theorem
2D Fourier transform of source intensity at angular frequency B/λ (visibility function)
Instrumental cosine term
Observed Intensity
Response to a point source in direction of α
( ) ( )
( )
−=
−=
∫∫
∫∫
ααλ
παλ
π
ααλ
πα
dBjIzxBj
dzxBjIxI
2exp2expRe
2expRe
Source intensity
September 2002 Optics in Astronomy 33
What does the vCZT mean?
An interferometer projects a fringe onto the source‘s intensity distribution
The magnitude of the fringe amplitude is given by the structural content of the source at scales of the fringe spacing
The phase of the fringe is given by the position of the fringe which maximises the small scale signal
September 2002 Optics in Astronomy 34
What have we learned?• An astronomical interferometer measures spatio-temporal
coherene properties of the light emerging from a celestial source.
• The spatial coherence properties encodes the small scale structural content of the intensity distribution in celestial coordinates.
• The temporal coherence properties encodes the spectral content of the intensity distribution.
• The measured interferometer signal depends on structural and spectral content.