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Halftoning for High-Contrast Imaging

P. Martinez1

C. Dorrer2, E. Aller Carpentier1, M. Kasper1, A. Boccaletti3, and K. Dohlen4

1European Southern Observatory 2Aktiwave – Rochester N.Y 3LESIA – Paris Observatory

4LAM – Marseille Observatory

This activity is supported by the European Community under its Framework Programme 6, ELT design study.

New Technologies for Probing the Diversity of Brown Dwarfs and Exoplanets – Shanghai 2009

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Halftoning principle & key parameters

dot => 0% transmission (substrate + opaque metal)no dot => 100% transmission (substrate only)

Key parameters (application-dependent)

-shapeof the dots (square, hexagone…)- size of the dots [p], i.e. sampling problem (p >)-metal layer (Cr, Al…)-opticaldensity, i.e. opacity of the dots, OD()-algorithm used for dots distribution

Components are based on metallic micro-dots to generate spatially-varying transmission ® displaying continuous-tonefilters with only black (opaque) and white (transparent) dots

Free-spacepropagation

Impact the power spectrum of a microdot filter

Image from Dorrer et al. 2007, JOSA

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Halftoning for coronagraphy

Band-Limited coronagraph, BL (Kuchner et Traub, ApJ 2002)(see talk of M. Kuchner this afternoon)

Apodized Pupil Lyot Coronagraph, APLC (Soummer et al., A&A 2003)(Idem for conventional pupil apodization coronagraph or Dual Zone coronagraph)

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Diffraction stray-light: APLC pupil apodizer

Pupil plane (3mm)prototypes

APLC coronagraphic images

Laboratory validation (Martinez et al. 2009b, A&A 500)

The smaller the dots are, the better the transmission profile matches the desired one (i.e. sampling problem, S =pupil diameter / dot size)

The APLC coronagraphic image is affected by:

- Deterministic effect: diffraction peaks (dot scatters light)- Stochastic effect: speckles will border diffraction peaks (dots distribution is not regular)

theoretical derivations in Martinez et al. 2009a, A&A 495

15 microns 30 microns 60 microns 120 microns 240 microns Dot size (μm) S = 200 S = 100 S = 50 S = 25 S = 12.5

1.2”

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Diffraction stray-light: BL focal plane mask

Metric: s = (F#xλmin) / pFurther details in Martinez et al. 2009c, ApJ submitted

Ok if IWA > 3 λ/DContrast 10-8 (IWA) to 10-10

Ψpupil plane = [ FT( Mask BL) ✪ pupil ] ✖ pupil-stop

Ideal mask Microdot mask

Numerical noise

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Final prototype: APLC pupil apodizer

Filter shape: Prolate-like function

Diameter pattern: 3 mmMetal layer: CrOD 4(standard)Dot size: 4.5 micronsS = 660Local profile accuracy: 3% (new proto => 2%)

Achromaticity transmission 1%(J and H-band)

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Final prototype: BL focal plane mask

Filter shape: 1 - sinc

Diameter pattern: 10 mmMaterial layer: AlOD 8+ in near-IRDot size: 5 micronss = 16Local profile accuracy: ~ 5%(new proto => 3%)

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APLC & BL laboratory tests Band-Limited laboratory testsAPLC laboratory tests

Metrics IWA τ0 CIWA C0.1” C0.5”

APLC 2.3 λ/D 700 2.0 10-4 2.3 10-6 1.2 10-6

BL5 5.0 λ/D 2550 3.7 10-5 5.6 10-7 2.7 10-8

BL10 10 λ/D 97400 1.5 10-7 7.7 10-8 3.7 10-8

Laboratory experiment demonstrate correct behavior of the coronagraphs

H-bandΔλ/λ = 24%

Δλ/λ = 1.4%Δλ/λ = 24%

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XAO APLClaboratory test on HOT

The High-Order Testbench (HOT)AO bench developed at ESO(Aller Carpentier et al., 2008 SPIE)Seeing: 0.5”DM: 31 x 31 actuatorsAO cut-off frequency: 15 λ/D (0.6”)

APLC Contrast 5σ (HPF):

- 2.5 10-4 @ 0.1”- 2.2 10-5 @ 0.5”

see poster: XAO coronagraphy with HOT

APLCPSF

High-Pass Filtered APLC

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Conclusion

It works: APLC and BLs using halftoning demonstrate correct behavior

Laboratory limitations originate from external error sources

Accurate

Achromatic

Reproducible

Cheep : 2-3k€

Validated for the APLC on the SPHERE (VLT) & GPI (Gemini) instruments

Baseline for EPICS (E-ELT)

Not only for coronagraphy (e.g. Laser beam shaping: Dorrer et al. 2007, JOSA – B, vol. 24)