ZELDA: a Zernike phase mask sensor for coronagraphic aberrations

Wavefront  sensing  and  control  for  Exoplanet  Imaging,  Pasadena,  February  12-‐13,  2015

STScI:  Mamadou  N’Diaye,  R.  Soummer,  L.  Pueyo,  and  the  HiCAT  teamLAM/ONERA:  K.  Dohlen,  T.  Fusco,  A.  CosSlle,  A.  Caillat,  F.  Madec,  L.  Mugnier,  B.  Paul,  J.-‐F.  Sauvage,  A.  ViganU.  Liège:  A.  Jolivet

Collaborators:  JPL:  J.  K.  Wallace

Work  parSally  supported  by  :-‐  Funding  from  French  CNRS-‐INSU  and  InsStute  Carnot  STAR-‐  NASA  Grants  NNX12AG05G  and  NNX14AD33G  (APRA)

Principle

• Conversion of the phase aberrations into intensity variations ‣ Ic=α sin φ + β

• Small aberrations:‣ Closed-loop during high-contrast

observations‣ simplicity of the phase retrieval

algorithm

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A B C

Focal plane mask (FPM)π/2 phase shift

Entrance pupil Camera

Angel 1994, Bloemhof & Wallace 2003, Dohlen 2004, Guyon 2005

Small aberrations: Ic =αφ + β

Zernike phase mask sensor

Properties

3N’Diaye et al. SPIE 2014

• Capture range with ZOPD=λ/4‣ max: 340nm @ λ=680nm‣ -85nm < range < 255nm

• Smaller phase shift:‣ more symmetric range‣ reduced sensitivity

• Larger mask:‣ larger sensitivity ‣ larger variability across the pupil

Trade-offs between linearity, sensitivity, and variability across the pupil for the Zernike sensor design

0"

0.5"

1"

1.5"

2"

2.5"

3"

3.5"

4"

)0.5" )0.4" )0.3" )0.2" )0.1" 0" 0.1" 0.2" 0.3" 0.4" 0.5"

ZELD

A&intensity

&(arbitrary&un

it)&

Wavefront&aberra7on&(lambda)&

0.1"lambda"

0.25"lambda"

0.4"lambda"

-λ/8 3λ/8dynamic range

for λ/4 OPD mask

Mask OPD

ZOPD=0.4λ

ZOPD=λ/4

ZOPD=0.1λ

Large aberrations: Ic=α sin φ + β

Mask diameter d∼1λ/D

Manufacturing aspects

• Mask dimensions: ‣ ZOPD=λ0/4 and d=1.06λ0F‣ λ0=680nm, F=91

• Fabrication on silica substrate by photolithography

• Quality of the components‣ roughness: ~1 nm‣ mask transition width: ~1 µm

4N’Diaye et al. A&A 2010, SPIE 2011

SEM of a phase mask prototype

d=64.4μm

z=0.366μm

Marseille’s prototype

Implementation example: SPHERE

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Deformable mirror

Stellar coronagraph

Image post processing

Image formation of an exoplanetWavefront

sensorReal timecomputer

Extreme adaptive optics (ExAO)

Measurement

Science

Non common path aberrationsTelescope

Speckles ZELDA

Optics

Optics

• Our proposal: ‣ ZELDA a concept based on

phase-contrast technique

• Measurement strategies:‣ VLT/SPHERE: off-line phase diversity‣ GPI: Mach-Zehnder interferometer behind

coronagraph

Sauvage et al. 2007, Wallace et al. 2010

Zernike sensor for Extremely accurate measurements of Low-level Differential Aberrations

ZELDA prototype in SPHERE

• Goal: improve SPHERE performance by implementing high-order NCPA calibration as a future path upgrade

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ZELDA in coronagraph wheel

Installation during SPHERE integration at Paranal in April 2014

Diameter D=70.7μmDepth z=0.815μm

λ0=1.62μm (H-band)F/40 beam

Preliminary tests

• Scan of the internal point source image across the ZELDA mask‣ modification of the reference slopes of the differential tip tilt sensor in front of

the coronagraph wheel‣ tilt ramp with steps of 0.5 to 1 pixel of 12.5mas (0.3λ/D @ 1.6µm) on the sensor

7N’Diaye et al. SPIE 2014

Pixel by pixel analysis for wavefront maps obtained with Zelda in H-band

Experiment before the first SPHERE commissioning run (May 2014)

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-100

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100

200

-6-4-20246

Near-zero tilt 19 mas tilt 31 mas tilt

nm

Preliminary results

• Evolution of the low-order aberrations in the presence of tilt ramp‣ Wavefront map decomposition on the Zernike polynomial basis

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• Linear behavior of the tilt ramp for small aberrations • Parabolic evolution of focus and astigmatism with stationary values

around zero because of the limited ZELDA dynamic range

Tilt

Implementation: general considerations

• Zernike sensor for low-order aberrations‣ behind a reflective coronagraph FPM

(hole):- option adopted at STScI on HiCAT with

Hybrid SP/APLC solutions

‣ in the same plane as the coronagraph FPM- currently developed at JPL (See Fang

Shi)

‣ after a beam splitter upstream the coronagraph FPM

• Zernike sensor for Mid/High order aberrations‣ downstream the FPM using a pupil

imaging arm

‣ in the plane of the coronagraph FPM at a sensing wavelength different from science wavelength

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Closed-loop around the null WFE with ZS

Stroke Minimization for open-loop calibration

Pre-compensation with ZS using its capture range

Example of possible LOWFSC strategy

4D Fizeau interferometer

O1

O2

O5

O6O34

ApodO7

O8FPM

DM1DM2

O9Lyot stop

Pupil camera

Image camera

O10 Space for phase retrieval

Beam launcher

2.4m x 1.2m (8x4 ft) optical table

Pupil mask

4D Fizeau interferometer

Lens

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Apodizer

Focal plane maskLyot stop

Zernike sensors in HiCAT

Zernike LOWFS

Zernike LOWFS

or HOWFS

335μm size

FPM from Lyot project, courtesy of R. Oppenheimer

Combination with Hybrid Shaped Pupil/APLC coronagraphs

• APLC with Shaped pupil apodization to produce broadband star image

• Implementation of this type of solution on HiCAT with reflective FPM

• Use of the FPM transmitted light to feed a Zernike sensor for LOWFS measurements

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Aperture

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10% broadband coronagraphic image

Shaped pupil

Lyot stop Intensity at λ0 before stop

Intensity at λ0 after stoplog

scale

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Angular separation in l0êD

Normalized

intensityinlogscale

mê2= 4.00l0êDri= 3.5l0êD ro= 20.0l0êD

20% central obstruction10% bandwidth

N’Diaye et al. ApJ 2015 and in prep

OAP1

OAP2

PAR34

IrisAOPUP

OAP5OAP6

Apod

TRD7

BOS1

BOS2

TRD8

FPM

SPH9

Lyot

CamP

CamF

Fold1Fold2

BS

LpLfFiber

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HiCAT (STScI High-contrast imaging testbed)- Design with room for static/dynamic Zernike sensors - Static prototype already available- Implementation in the forthcoming year

Room for Zernike LOWFS behind the

reflectiveFPM

DM characterization images, 01/30/2015, credit Mazoyer & N’Diaye

Final delivery (two kilo-DM) in Oct 2014Integration in Spring

Conclusion and future prospects

• ZELDA: ‣ is a Zernike phase mask sensor‣ is a precision WFS for near-coronagraph NCPA calibration ‣ helps to extend the discovery space of directly imaged exoplanets with current and future

high-contrast instruments

• ZELDA in VLT/SPHERE: ‣ Prototype installed in April 2014, ‣ Promising preliminary results with intern source‣ On-sky tests expected in 2015 for potential future upgrade on the instrument‣ Combination with coronagraphic phase diversity (COFFEE, Paul et al. 2013)

• Zernike sensors in HiCAT (STScI high-contrast testbed, N’Diaye et al. 2013, 2014): ‣ Integration of a static and/or dynamic Zernike WFS for low-order aberration measurements‣ Static prototype already available in our laboratory‣ implementation in the forthcoming year

13DM characterization images, 01/30/2015, credit Mazoyer & N’Diaye