Fully achromatic nulling interferometer (FANI) for high SNR exoplanet characterization
Fully achromatic nulling interferometer (FANI) for high SNR exoplanet characterization
François Hénault Institut de Planétologie et d’Astrophysique de Grenoble Université Joseph Fourier Centre National de la Recherche Scientifique BP 53, 38041 Grenoble – France
Conf. 9605 – Techniques and Instrumentation for Detection of Exoplanets VII
San Diego, 08-12-15
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Fully achromatic nulling interferometer (FANI) for high SNR exoplanet characterization
Plan of presentation • Principle and Mathematics • System definition – Dimensioning the grism element – Simulated fringe patterns
• Optical design – Performance – Tolerance analysis
• Potential SNR gain • Conclusion Conf. 9605 – Techniques and Instrumentation for Detection of Exoplanets VII
San Diego, 08-12-15
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Fully achromatic nulling interferometer (FANI) for high SNR exoplanet characterization
Classical interferometer Beam relaying optics
Telescope 1
L1
Multi-axial Beam collimatng combining stage optics
L2 M1
F
X” L’1
L4
L5
L3
F’
Entrance O baseline B B
M2
Telescope 2
B’0
FD
O”
O’
FD
FC Conf. 9605 – Techniques and Instrumentation for Detection of Exoplanets VII
Multi-axial combining optics
Z
Focal plane
Exit pupil plane San Diego, 08-12-15
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Fully achromatic nulling interferometer (FANI) for high SNR exoplanet characterization
Principle and Mathematics • The transmission map of Bracewell’s nulling interferometer (two telescopes) writes as:
Tλ (r ,θ ) = AiryT (π Dr /λ ) sin 2 (π B r cos θ λ ) v Fringes envelope (single telescope PSF)
r
θ
u
Fringe pattern (chromatic)
• Having a variable baseline B(λ ) = B0 λ λ 0 would cancel the 1/λ chromatism inherent to diffraction Conf. 9605 – Techniques and Instrumentation for Detection of Exoplanets VII
San Diego, 08-12-15
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Fully achromatic nulling interferometer (FANI) for high SNR exoplanet characterization
Fully achromatic nulling interferometer Beam relaying optics
Telescope 1
L1
L2
Beam dispersing optics
M1
F
X” Grism lens
L3
L4
L5
F’ M2
B’0
Telescope 2
Entrance O baseline B B
Grism lens
FD
O”
O’
FD
Grism mirrors FC
Conf. 9605 – Techniques and Instrumentation for Detection of Exoplanets VII
Multi-axial combining optics
Z
Focal plane
Exit pupil plane San Diego, 08-12-15
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Fully achromatic nulling interferometer (FANI) for high SNR exoplanet characterization
Fully achromatic nulling interferometer Dispersed exit pupil
Entrance pupil
Y
O
Y’
X
B Conf. 9605 – Techniques and Instrumentation for Detection of Exoplanets VII
O’
X’
B’(λ0) San Diego, 08-12-15
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Fully achromatic nulling interferometer (FANI) for high SNR exoplanet characterization
Dimensioning the grism element h′
a α
β (λ)
Grism
β (λ)
λR λ0
h′(λ ′) Collimating lens L4
h’(λ0) = B’(λ0)/2
h′ FD
Grism equation Ray impact on L4
β (λ ) ≈ −
tan α
λ0
λB λ0
{λ0 n(λ ) − λ n(λ0 ) + λ − λ0 }
β (λ ) ≈
h′ λ − λ0 dh′(λ ) + FD λ0 FD
Conf. 9605 – Techniques and Instrumentation for Detection of Exoplanets VII
San Diego, 08-12-15
7
Fully achromatic nulling interferometer (FANI) for high SNR exoplanet characterization
Dimensioning the grism element • The grism is optimized using first-order dispersion law of its n(λ ) = n0 + ν 0 (λ −λ0 ) + dn (λ ) material refractive index • Application to mid-IR materials (TPF-I or Darwin-like missions) Material CdTe Csi KBr KCl KRS5 NaCl ZnS ZnSe
Grism angle
Refractive index at λ0 2.700 1.747 1.548 1.496 2.392 1.526 2.336 2.470
Spectral slope ν0 (µm-1) -2.7E-03 -8.2E-04 -2.3E-03 -4.0E-03 -2.2E-03 -1.2E-05 -1.4E-02 -6.5E-03
tan α =
Grism angle (°) 6.710 14.982 20.018 21.934 8.174 20.802 8.488 7.741
h′ FD (n0 − 1 − ν 0λ0 )
Distortion wrt linear dependence in λ Conf. 9605 – Techniques and Instrumentation for Detection of Exoplanets VII
Groove RMS period (µm) distortion (%) 53.77 2.1E-02 54.98 1.7E-02 58.40 7.8E-02 61.85 1.6E-01 53.92 2.2E-02 56.17 1.3E-03 59.69 2.6E-01 55.55 9.2E-02
dh′(λ ) dn(λ ) = h′ n0 − 1 − ν 0λ0 San Diego, 08-12-15
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Fully achromatic nulling interferometer (FANI) for high SNR exoplanet characterization
Simulated fringe patterns
4 telescopes
Spectral range 7-14 µm Entrance baseline B = 20 m Telescope diameter D=5m Compression factor m = 1/500 Dispersive material ZnSe Fizeau interferometer at λ = 10.5 µm
8 telescopes
Specifications
2 telescopes
Monochromatic PSF
π
Wideband PSF
Corrected PSF at centre
0
π
0 1”
π
0 0
π
π 0
0 π
π
0
Conf. 9605 – Techniques and Instrumentation for Detection of Exoplanets VII
San Diego, 08-12-15
9
Fully achromatic nulling interferometer (FANI) for high SNR exoplanet characterization
Preliminary optical design • All-reflective design excepting grism • Well below diffraction limit of one individual telescope Achromatic phase shifter (APS) Compressed beam from telescope
Spot-diagram Focusing Mirror (L3)
Multi-axial combining mirror
+λ/5
Spot-diagram
FD
Grism mirror
OPD fans
Diffraction limit
OPD fan
Deformable collimating Mirror (L4)
-λ/5
Focusing mirror
Grism mirror
X”
B’0/2
Collimating mirror
O’ Exit pupil plane
O” F’
Z
Focal plane
Conf. 9605 – Techniques and Instrumentation for Detection of Exoplanets VII
Focal point
San Diego, 08-12-15
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Fully achromatic nulling interferometer (FANI) for high SNR exoplanet characterization
Preliminary optical design • Achromatic phase shifter (APS) Couples of ZnSe/ZnS wedge plates • Null depth ≤ 10-6 over 7-13.5 µm range • Alignment and manufacturing tolerances of dispersive element are mild, demonstrate their feasibility Geometrical parameter Grism mirror translation along Z-axis Grism mirror decenter (along X’ and Y’ axes) Grism mirror tilt around X’-axis Grism mirror tilt around Y’-axis Grism mirror roll angle(around Z-axis) Grism thickness at centre Grism angle α Conf. 9605 – Techniques and Instrumentation for Detection of Exoplanets VII
Tolerance ≤ 0.1 mm ≤ 1 mm ≤ 5 degs. ≤ 1 deg. ≤ 5 degs. ≤ 0.1 mm ≤ 1 deg. San Diego, 08-12-15
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Fully achromatic nulling interferometer (FANI) for high SNR exoplanet characterization
Potential SNR gain • Planet detection possible on all bright fringes Higher Signal • If used as a widebabd imaging stellar interferometer, SNR gain ≈ n for read noises Low dispersion spectrograph
Slit or multi-object spectrograph
Single mode fiber
T(u,v)
Detector array
Star
Single mode fibers
Detector array
Planet
u Broadband interferogram
FANI interferogram
Conf. 9605 – Techniques and Instrumentation for Detection of Exoplanets VII
San Diego, 08-12-15
12
Fully achromatic nulling interferometer (FANI) for high SNR exoplanet characterization
Conclusion • Setting dispersive elements at intermediate image planes allows full achromatization of the fringe pattern created by an interferometer • The core dispersive element can be a grism mirror • Manufacturing and alignment tolerances of dispersive optics are reasonable • High SNR gains for exoplanets characterization are expected • The principle is also applicable to imaging stellar interferometers Conf. 9605 – Techniques and Instrumentation for Detection of Exoplanets VII
San Diego, 08-12-15
13
Fully achromatic nulling interferometer (FANI) for high SNR exoplanet characterization
• Would you like to help us building a Science Case for FANI ? Please contact :
[email protected]
Conf. 9605 – Techniques and Instrumentation for Detection of Exoplanets VII
San Diego, 08-12-15
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