Halftoning for High-Contrast Imaging P. Martinez 1 C. Dorrer 2, E. Aller Carpentier 1, M. Kasper 1, A. Boccaletti 3, and K. Dohlen 4 1 European Southern Observatory 2 Aktiwave – Rochester N.Y 3 LESIA – Paris Observatory 4 LAM – Marseille Observatory This activity is supported by the European Community under its Framework Programme 6, ELT design study. 1 New Technologies for Probing the Diversity of Brown Dwarfs and Exoplanets – Shanghai 2009

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-space propagation 2 Impact the power spectrum of a microdot filter Image from Dorrer et al. 2007, JOSA

Halftoning for coronagraphy 3 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)

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

Diffraction stray-light: BL focal plane mask Metric: s = (F # xλ min ) / p Further details in Martinez et al. 2009c, ApJ submitted 5 Ok if IWA > 3 λ/DContrast (IWA) to Ψ pupil plane = [ FT( Mask BL) ✪ pupil ] ✖ pupil-stop Ideal maskMicrodot mask Numerical noise

Final prototype: APLC pupil apodizer Filter shape: Prolate-like function Diameter pattern: 3 mm Metal layer: Cr OD 4(standard) Dot size: 4.5 microns S = 660 Local profile accuracy: 3% (new proto => 2%) Achromaticity transmission 1% (J and H-band) 6

Final prototype: BL focal plane mask Filter shape: 1 - sinc Diameter pattern: 10 mm Material layer: Al OD 8+ in near-IR Dot size: 5 microns s = 16 Local profile accuracy: ~ 5% (new proto => 3%) 7

APLC & BL laboratory tests Band-Limited laboratory testsAPLC laboratory tests MetricsIWA τ0τ0 C IWA C 0.1” C 0.5” APLC2.3 λ/D BL55.0 λ/D BL1010 λ/D Laboratory experiment demonstrate correct behavior of the coronagraphs H-band Δλ/λ = 24% Δλ/λ = 1.4% Δλ/λ = 24%

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 actuators AO cut-off frequency: 15 λ/D (0.6”) APLC Contrast 5σ (HPF): ” ” see poster: XAO coronagraphy with HOT APLCPSF High-Pass Filtered APLC 9

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) 10