zelda: a zernike phase mask sensor for coronagraphic ......phase-contrast technique • measurement...
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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)
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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
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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
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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
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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
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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
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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)
-200
-100
0
100
200
-6-4-20246
Near-zero tilt 19 mas tilt 31 mas tilt
nm
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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
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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
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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
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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
-12
-10
-8
-6
-4
-2
-6-4-20246
10% broadband coronagraphic image
Shaped pupil
Lyot stop Intensity at λ0 before stop
Intensity at λ0 after stoplog
scale
0 10 20 30 40 50-12
-10
-8
-6
-4
-2
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
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OAP1
OAP2
PAR34
IrisAOPUP
OAP5OAP6
Apod
TRD7
BOS1
BOS2
TRD8
FPM
SPH9
Lyot
CamP
CamF
Fold1Fold2
BS
LpLfFiber
12
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
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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
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