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A Single-Mode Nulling RotatingCoronagraph for High Contrast

Ground Based Imaging

Bertrand MennessonGene Serabyn, Stefan Martin and Kurt Liewer

Caltech / Jet Propulsion Laboratory

Spirit of Lyot ConferenceBerkeley, June 4-8, 2007

“Batiment” Lyot at the Paris Observatory(Meudon site)

Where COME-ON, FLUOR, NAOS etc were conceived, tested and partly assembled

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Current limits for high contrast / high resolutionground based imaging: single telescopes

Classical Lyot coronagraphs: central 4-5 λ/D is not accessible

Apodized coronagraphs and phase mask coronagraphs: cantheoretically image companions at λ/DHigh dynamic range inside 3-4 λ/D only accessible with very highStrehl (> 90% restricted to small D or future ExAO systems)

Speckle interferometry: imaging at λ/D, dynamic range of a few tens

Aperture masking (Keck, Palomar): allows imaging within λ/D butwith dynamic range still limited by V accuracy (fibers would help)

Current off axis point source detection limit:~1000:1 at 5λ/D, degrading to ~100:1 at λ/D

Current limits of high contrast /high resolutionground based imaging: long baseline

interferometers

Very high angular resolution (down to ~1mas in thenear IR), but dynamic range limited to:

20-30:1 for conventional (bulk optics) interferometers

~100:1 using single-mode waveguide recombination andphotometric calibration (FLUOR)

~100:1 or better using nulling in the mid–IR (MMT, KECK,LBTI). Nulling brings extra spatial resolution

Contrast limit for detection of off-axis sourcesis ~100:1

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Bridging the gap: near IR fiber nulling ona single-telescope

Principle: rotating nuller feeding a single-mode fiber

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FIBER NULLER POSSIBLE IMPLEMENTATIONS

Multi-Axial Co-Axial

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Possible Nulling Beam Combinations

Co-axial recombination:+ Variety of solutions provide achromatic nulls:RSI: rooftops, periscopes, LSI: dielectric plates + MMZ+ Proven techno (Wallace, Serabyn, Mennesson etc)

Multi-axial recombination:Beam combiner = focusing element + fiber+ Easier to implement on telescope+ Adaptability to a large number of sub-beams- Deep nulls unproven before 2005- Fiber injection efficiency lower

“A la FLUOR or IONIC” Would need zero dispersion waveguides e.g. photonic fibers

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The Multi-axial (Fiber) Combiner

SM Fiber provides 2 in 1: Beam Combination and Spatial Filtering

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Lab results: visible LASER

Lab results: visible LASER

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Lab results: visible LASER

Best nulls are 1.3x10-6 (from Haguenauer & Serabyn, Applied Optics, 2006

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Lab results: white lightDielectric Plates

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Lab results: white light

Dual Polarization Stable White Light (1.5 to 1.8 microns) Null Level: ~6x10-5

Multi-axial recombination (Mennesson et al. 2007 in prep)

Key Advantages High contrast accessible without high Strehl (fiber corrects

individual wavefronts)

Ultimate contrast fixed by OPD rms over unit integration time K band Nulls of 1e-3 readily accessible using 10ms sampling

Spatial resolution = λ/2B (56 mas in K with B=4m), half transmissionpoint is λ/4B =28 mas

Null level is constant over FOV. Achievable detection limit is fixedby fringe pattern Tr, not by field dependent residuals

Can theoretically achieve 500:1 detection at λ/D in the near IR

Much simpler to implement than long baseline interferometry;baseline can rotate much faster

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Limitations• FOV limited to diffraction limit of sub-aperture λ/d ~ 3-4λ/D. Really

tailored to high angular resolution.Could work in conjunction with conventional coronagraph accessinglarger separations

• Sensitivity loss wrt to full telescope• Both limitations reduced when using more small sub-apertures

Injection efficiency into a singlemode fiber using b/d =3.5m/1.5m(Multi-axial combination, nodensification, Palomar 200 inch) at2.2 microns. Plain curve:constructive case. Dotted curve:nulling case. A denser pupil, asavailable on Keck or Subaru, willallow better on-axis couplingefficiency, up to 62% in the idealcase where b=d Dotted curve:nulling case.

Simulated Performance of Palomar system:two 1.5x3m sub-apertures 3.5m apart

K band signal detected over 60 baseline rotations (5s each, sampled at 100Hz)

Left: mK=3 star with 500 times dimmer companion 40 mas away

Right: same but with companion 180 mas away

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Objectives of Fiber Nuller Set-up atPalomar 200”

Technical goals:- Prove feasibility of the approach on the sky- Demonstrate detection of companion at the 500:1 level within100 mas of primary (e.g. composite spectrum stars)- Test TPF-I companion detection approach on the sky underfavorable conditions

Science goals:- Stellar diameters, atmospheric layers, asymmetries (evolvedstars easily resolved)- Survey of hot material around mK

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Palomar layout

Palomar Layout

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Palomar 1st Engineering run (May 07)

• Mounted FN bread-board in 200” Cassegrain cage,behind PALAO system

• Injected starlight from both beams into K band SM fiber

• Checked angle tracker (quad cells), photometriccalibration wheel and K photometer sensitivity (presentlylimited to mK=5 at constructive peak)

• Under Jet Stream: strong winds, bad/fast seeing (~2”)made fiber injection unstable

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Future Developments• November 07 engineering run goals:

- Measure first nulls- Enable aperture rotation

• Key upgrades (2008 = present limit of funding):- Low noise fast read IR camera- Enable fringe tracker to further decrease opd

residuals left after AO (larger sub-apertures than AO)- Install refraction corrector- Use dispersion and spectral differential nulling- Use >2 sub-apertures (deeper central nulls, or try nulling + DP)

• > 2008: Get to larger telescope at a prime astronomical site. Southhemisphere preferred to survey nearby young stars

• Possible FN in space on a 1.5m vis/near IR astrometric telescope(Pravdo et al. 2007)

Related presentations:

• Talk by M. Shao (Tuesday 11:30 am):Visible nulling coronagraphy

• Poster by G. Perrin et al. :Diffraction limited high dynamic range imaging from thevisible to the infrared

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Back up slides

Advantages

• High contrast accessible without highStrehl (fiber corrects individual wavefronts)

• Ultimate contrast fixed by rms of opticalpath difference over unit integration time:

• N=1/4 (t2 + σ2φt) N=1e-3 readilyaccessible with 10ms sampling in K

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Palomar layout

Palomar Layout

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Multi-axial Beam Combination

• After densification, can adapt to the case fmany-sub-apertures:

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Expected performance at Palomar