Fully achromatic nulling interferometer (FANI) for high SNR exoplanet characterization

Transcription

1 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, 3841 Grenoble France 1

2 Plan of presentation Principle and Mathematics System definition Dimensioning the grism element Simulated fringe patterns Optical design Performance Tolerance analysis Potential SNR gain Conclusion 2

3 Classical interferometer Telescope 1 F Beam relaying optics L1 L2 M1 L3 Multi-axial combining stage Beam collimatng optics L 1 L4 L5 F X Entrance O baseline B B M2 B O O Z Telescope 2 F D F D Multi-axial combining optics Focal plane F C Exit pupil plane 3

4 T λ ( r, θ ) = Airy Fully achromatic nulling interferometer (FANI) Principle and Mathematics The transmission map of Bracewell s nulling interferometer (two telescopes) writes as: T ( ) 2 πdr/λ sin ( πbr cosθ λ) v Fringes envelope (single telescope PSF) r θ u Fringe pattern (chromatic) ( ) Having a variable baseline B λ = Bλ λ would cancel the 1/λ chromatism inherent to diffraction 4

5 Fully achromatic nulling interferometer Beam relaying optics Telescope 1 F L1 L2 M1 L3 Beam dispersing optics Grism lens L4 L5 F X Entrance O baseline B B M2 B O O Z Telescope 2 F D F D Multi-axial combining optics Focal plane Grism lens Grism mirrors F C Exit pupil plane 5

6 Fully achromatic nulling interferometer Entrance pupil Dispersed exit pupil Y Y O X O X B B (λ ) 6

7 Dimensioning the grism element a h λ R λ α β (λ) Grism β (λ) h ( λ ) h (λ ) = B (λ )/2 F D Collimating lens L4 λ h B λ Grism equation Ray impact on L4 β tan α λ ( λ) { λ n( λ) λ n( λ ) + λ λ } β ( λ) h F λ λ λ D F D dh + ( λ) 7

8 Dimensioning the grism element The grism is optimized using first-order dispersion law of its material refractive index n λ) = n + ν ( λ λ ) + dn( ) Application to mid-ir materials (TPF-I or Darwin-like missions) Material Grism angle Refractive index at λ Spectral slope ν (µm -1 ) Distortion wrt linear dependence in λ Grism angle ( ) Groove period (µm) RMS distortion (%) CdTe E E-2 Csi E E-2 KBr E E-2 KCl E E-1 KRS E E-2 NaCl E E-3 ZnS E E-1 ZnSe E E-2 tan α = F ( λ h D ( n 1 ν λ ) dh ( λ) dn( λ) ν λ 1 8 h = n

9 Simulated fringe patterns Monochromatic PSF Wideband PSF Corrected PSF at centre Specifications Spectral range 7-14 µm Entrance baseline B = 2 m Telescope diameter D = 5 m Compression factor m = 1/5 Dispersive material ZnSe Fizeau interferometer at λ = 1.5 µm 8 telescopes 4 telescopes 2 telescopes π π π π π π π 1 9

10 Preliminary optical design All-reflective design excepting grism Well below diffraction limit of one individual telescope Achromatic phase shifter (APS) Spot-diagram OPD fans Compressed beam from telescope Focusing Mirror (L3) Spot-diagram +λ/5 Grism mirror F D Diffraction limit OPD fan -λ/5 Multi-axial combining mirror Deformable collimating Mirror (L4) Focusing mirror B /2 X Grism mirror Exit pupil plane O F O Focal plane Z Focal point Collimating mirror 1

11 Preliminary optical design Achromatic phase shifter (APS) Couples of ZnSe/ZnS wedge plates Null depth 1-6 over 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 Tolerance.1 mm 1 mm 5 degs. 1 deg. 5 degs..1 mm 1 deg. 11

12 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) Star Detector array Single mode fibers Detector array Planet Broadband interferogram FANI interferogram u 12

13 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 13

14 Would you like to help us building a Science Case for FANI? Please contact : francois.henault@obs.ujf-grenoble.fr 14