THE SUBARU CORONAGRAPHIC EXTREME AO HIGH SENSITIVITY VISIBLE WAVEFRONT SENSORS
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Third AO4ELT Conference - Adaptive Optics for Extremely Large Telescopes
Florence, Italy. May 2013
ISBN: 978-88-908876-0-4
DOI: 10.12839/AO4ELT3.13398
THE SUBARU CORONAGRAPHIC EXTREME AO HIGH
SENSITIVITY VISIBLE WAVEFRONT SENSORS
Christophe Clergeon1a , Olivier Guyon1 , Frantz Martinache1 , Jean-Pierre Veran3 , Eric
Gendron2 , Gerard Rousset2 , Carlos Correia3 , and Vincent Garrel1
1
Subaru Telescope, 650 N A’ohoku place, Hilo Hawaii, USA
2
Observatoire de Paris, 5 place Jules Janssen, Meudon, France
3
HRC-CNRC, 5971 West Saanich Road, Victora BC, Canada
Abstract. A diffraction-limited 30-meters class telescope theoretically provides a 10 mas resolution
limit in the near infrared. Modern coronagraphs offer the means to take full advantage of this angular
resolution allowing to explore at high contrast, the innermost parts of nearby planetary systems to within
a fraction of an astronomical unit: an unprecedented capability that will revolutionize our understanding
of planet formation and evolution across the habitable zone. A precursor of such a system is the Sub-
aru Coronagraphic Extreme AO project. SCExAO [9] uses advanced coronagraphic technique for high
contrast imaging of exoplanets and disks as close as 1 λ/D from the host star. In addition to unusual
optics, achieving high contrast at this small angular separation requires a wavefront sensing and control
architecture which is optimized for exquisite control and calibration of low order aberrations. To com-
plement the current near-IR wavefront control system driving a single MEMS type deformable mirror
mounted on a tip-tilt mount, two high order and high sensitivity visible wavefront sensors have been
integrated to SCEXAO: – a non-modulated Pyramid wavefront sensor (CHEOPS) which is a sensitiv-
ity improvement over modulated Pyramid systems now used in high performance astronomical AO, –
a non-linear wavefront sensor [4] designed in 2012 by Subaru Telescope with the collaboration of the
NRC-CNRC which is expected to improve significantly the achieved sensitivity of low order aberations
measurements. We will present the CHEOPS last results measured in laboratory and during its first light
downstream the Subaru AO188 instrument, and then conclude introducing the primary prototype of the
SCExAO non-linear curvature wavefront sensor which is planned to be tested on sky in 2014.
1 Introduction
One of the biggest challenge for ground-based telescopes, and especially for future Extremely
Large Telescopes (ELTs), is the direct observation of Earth like planets in their host star hab-
itable zone. The high contrast required to directly image an Earth-like planet and the actual
coranagraphic performances reached in laboratories lead the astronomers’ s interest toward the
survey of low flux stars. The direct observation of habitable planets around low flux stars (only
0.08 - 0.12 AU for a M type star at 3 to 5 pc, i.e. 2 to 3 λ /D in H band with a thirty meter
telescope) is a realistic challenge requiring both high telescope resolution and high contrast
imaging. Without an accurate knowledge and control of the low-order aberrations (residual
tip-tilt, defocus etc), high contrast imaging or good resolution of objects will be challenging.
Nowadays, only diffraction limited wavefront sensors [5] are capable of reaching this degree of
accuracy. WFSs such as the pyramid wavefront sensor [10] when non-modulated or the non-
linear curvature wavefront sensor [4] are both promising in the field of planet direct imaging.
a
christophe@naoj.org
Third AO4ELT Conference - Adaptive Optics for Extremely Large Telescopes
We introduce in this paper the first results of the integration of a non-modulated pyramid wave-
front sensor on the Subaru Coronagraphic Extreme AO (SCExAO) project. To conclude, we will
briefly introduce the primary prototype of the SCExAO non-linear wavefront sensor design.
2 A non-modulated Pyramid Wavefront Sensing for High Performance
Adaptive Optics - Theoretical approach and Simulations
While existing pyramid wavefront sensors use modulation to maintain a linear response over
a wide wave-front aberration range, non-modulated pyramid theoretically offers significantly
higher sensitivity for low order aberrations, achieving nearly optimal conversion of wavefront
phase aberration into intensity signal. We succeed to demonstrate with simulations the ability
of the pyramid to close the loop without modulation first downstream an AO correction and
then on the full atmospheric turbulence. Despite a non-linear response, and in ideal but realistic
observation conditions (perfect deformable mirror, bright source, fast detector), we demonstrate
that selecting the right close loop parameters, the correction of the low order aberrations can
converge and reach the pyramid linear response range after only few iterrations, opening new
perspectives on high contrast astronomical imaging.
2.1 Pyramid Low order modes measurement:
2.1.1 Pyramid sensitivity illustration with a simple slope measurement
Similar to a 2x2 quad cells, the pyramid determines the local slope measuring the position of the
centroid on its apex. From Tyler & Fried (1981)[8] we know that the slope measurement error
σ is directly proportional to the size of the spot on the sensor (here the pyramid) and the signal-
noise ratio of the detector (mostly photon noise for recent detectors). When non-modulated (see
fig. 1 a), the slope measurement is estimated with the maximum precision σdi f f proportional to
the diffraction limited sized spot (λ/D).
Fig. 1. PWFS slope measurement sensitivity without (a) and with modulation (b).
When modulated (see fig. 1 b) and with the same number of photons received by the detector, the
spot size on the pyramid gets wider, increasing the slope measurement error σmod proportional
to the modulation radius ξo .
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Third AO4ELT Conference - Adaptive Optics for Extremely Large Telescopes
In other words, the modulation breaks down the measurement sensitivity that we would observe
with diffraction limited conditions.
2.1.2 Frequency sensitivity analysis
This analysis has been completed by Guyon (2005) [3] , looking at precisely the impact of the
modulation as a function of spatial frequency. Creating speckles in the Fourier plane applying
sine waves on the DM, Guyon generalizes to all spatial frequencies the fact that speckles infor-
mation is only accessible when the four pupils record signal (fringes in the pyramid pupils, see
fig. 2.
Thus, the entire wavefront correction is only accessible when no optical saturation occured (see
fig. 2 modulation instants 1, 3, 5 & 7). Introducing the sensitivity to photon noise parameter,
he highlighted the fact that the pyramid wavefront sensor is more impacted by the photon noise
during modulation: rotating the PSF and the speckles around the apex, the signal is more often
saturated (see fig. 2). The modulation pulls down consequently the signal-noise ratio.
Fig. 2. Speckle signal detection evolution for one PWFS modulation period. The figure describes the
modulation sequence of the PWFS. Two speckles surround the PSF core (sine wave applied to the DM).
When located in two different pyramid quadrants (see Guyon [3]) the speckles interfere with the PSF
rings creating fringes (signal) in the pyramid pupils (positions 0, 2, 4 and 6). When located in the same
quadrant, no fringes, but one of the pyramid pupil saturates (positions 1, 3, 5 and 7).
Figure 3 shows the correlation between spatial frequencies and photon noise sensitivity. When
close to the PSF core (low spatial frequencies), the signal is statistically more often saturated
(as explained previously). For higher spatial frequencies (speckles far from the PSF core), the
sensitivity to photon noise decreases when the separation angle from the PSF core increases
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Third AO4ELT Conference - Adaptive Optics for Extremely Large Telescopes
Fig. 3. Speckle signal detection vs spatial frequency during modulation.
(more signal acquired during the modulation cycle) . When modulated, low spatial frequencies
are measured with poorer sensitivity.
2.2 Simulation of a non-modulated Pyramid loop convergence in low and high
turbulence conditions:
In the previous paragraph we pointed the fact that in our pursuit of low order measurement
sensitivity, a small modulation is still a disadvantage in particular for high contrast imaging.
Developping a high sensitivity visible wavefront sensor for SCExAO, advanced simulations
have been implemented. The goal of this analysis was to identify the conditions required for the
non-modulated pyramid WFS to close the loop downstream an AO correction (low turbulence
usually encountered in ExAO and especially in SCExAO case) and on the full atmosphere
turbulence, understanding the limits of such performance when close to the linearity pyramid
response domain.
Table 1. Simulation Parameters.
Input Wavefront Error
Turbulence Profile Kolmogorov (seeing 0.5” zenith angle: 0.785 rad, alt.: 4200m).
First AO correction parameters
Instrument Subaru Adaptive Optics (AO188)
WFS Curvature with 188 elements
DM Bimorph 188 actuators
Correction speed 2 KHz
Output wavelength 750nm
Expected performances SR 40-60 %
ExAO WFS (SCExAO)
WFS Non-modulated pyramid WFS
DM MEMS 32x32 actuators
Pupil diameter 28 actuators
Particularity Full PWFS signal calibrated
The algorithm loads a Kolmogorov based model phasemap partially corrected (or not depending
on the study case) with a first conventional AO system (simulations parameters defined in tables
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Third AO4ELT Conference - Adaptive Optics for Extremely Large Telescopes
1 and 2). The pyramid apex plane image is computed applying a Fourier transform (FT) to the
wavefront. The Fourier plane image is divided in four parts. Four pyramid pupil images are then
determined applying the inverse FT for each PSF quadrants. The control loop follows a con-
ventional linear AO control scheme based on a modal (Zernike and sine waves) reconstruction.
Table 2. Simulation hypothesis
Hyp 1 Perfect deformable mirror
Hyp 2 Very bright source (high SNR)
Hyp 3 Detector and loop faster than the turbulence coherence time
2.2.1 Post ao wavefront correction (SCExAO case)
The next figures present the close loop results (simulations) observed on a dynamic wavefront
error with a non-modulated Pyramid WFS. Figures 4 and 5 (right) show the expected perfor-
mances in good AO correction conditions (AO188 SR=60 %). 500 Fourier modes (sine waves)
are corrected with an ideal simulated DM. The red curve (see fig. 5) represents the evolution
of the first level AO residual error received at SCExAO input. The blue curve represents the
expected residual after the non-modulated PWFS correction. The aberrations correction (in ab-
sence of noise) succeed to converge until the linearity limit (1 rad) after few iterrations (c.f. blue
curve, 0.5 rad rms residual error at 60% SR). The PSF is centered and the bright speckles cloud
diminished until forming a diffraction-limited PSF core within the DM correction limits ( see
figure 4, DM correction at ± 12 λ/D).
Fig. 4. Post ao wavefront correction: (left) SCExAO input: Kolmogorov phasemap error corrected with
the AO188 (188 elements curvature WFS), (center) simulated PSF after AO188 correction, (right) cor-
rected PSF after SCExAO non-modulated PWFS correction (DM correction at ± 12 λ/D ).
For low AO188 performances conditions (see figure 5 left, 40% SR), the loop is closed un-
til reaching the pyramid linear domain in less than 20 iterations (100 Zernike modes sensed
and corrected in this simulation). The dashed lines represent, the general, the first100 Zernike
modes, and 6th first Zernike contributions in the wavefront error fluctuations, without pyra-
mid correction. Full lines, the associated residual errors after the non-modulated pyramid WFS
correction.
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Third AO4ELT Conference - Adaptive Optics for Extremely Large Telescopes
Fig. 5. Non-modulated Pyramid WFS correction - input: AO188 dynamic wavefront error
2.2.2 Full atmospheric turbulence correction
Results described in the previous section demonstrated the ability of the non-modulated pyramid
wavefront sensor to close the loop after a first level AO correction. Thus, pointing that despite a
non-linear response, the pyramid correction converges to the linear domain after few iterations,
especially for the low order aberrations. Taking the optimal tested loop configuration (Number
of sensed and corrected Zernike modes=100, gain=1, WFS wavelength=750nm), a last simula-
tion has been tested on a full atmospheric turbulence error to verify the non-modulated pyramid
limits during general astronomical observations. The phasemap used in this test has been gen-
erated with a Kolmogorov profile simulator.
Fig. 6. Non-modulated Pyramid WFS correction - input: Full atmosphere and dynamic turbulence.
Figures above show the full atmospheric wavefront correction expected in ideal conditions with
a non-modulated pyramid WFS. On the left, the residual error converges and stays stable at 1.47
rad rms (out of pyramid linear range at 750nm) in less than 40 iterations, in dynamic conditions.
A Zernike mode decomposition (scalar projection, right figure) completed before (blue curve)
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Third AO4ELT Conference - Adaptive Optics for Extremely Large Telescopes
and after (red curve) closing the loop confirm the good correction of the lower order aberrations
and the contribution of the non-corrected modes on the residual.
3 CHEOPS, the SCExAO visible wavefront sensor first Lab & Sky
results
The goal of the presented analyses was to identify the conditions required for the non-modulated
pyramid WFS to close the loop. Despite a non-linear response and in ideal but realistic obser-
vation conditions we demonstrate that selecting the right close loop parameters, the correction
of the low order aberations can converge and reach the pyramid linear response range after few
iterrations. Starting from this encouraging results and taking into account the high speed and
low noise new detector generation, we made the choice to develop a non-modulated pyramid
wavefront sensor on the SCExAO visible channel (see figure 7). The table below (3) gathers the
main specifications of our new high sensitivity visible wavefront sensor.
Table 3. Optical Design Specifications
WFS wavelength 725 - 850 nm
F ratio on the pyramid f / 35
Pyramid type Microlens array (SUSS Micro Optic), Pitch 500�, focal length: 15mm
Detector Andor, Zyla (Pix 6.5�, Speed 1.7KHz (ROI 120x120px), RN 1e-
Pyramid Pupils 50 px/pupil diameter (0.16” on sky per microlens)
Fig. 7. CHEOPS first loop closure in static abberations conditions: (Left) CHEOPS optical design on
SCExAO visible channel – (Center) Top: PWFS pupils image, 4 sine waves applied on the 1024 actuators
MEMS (Boston micromachines) between 1 and 12 λ/D (0.5 rad amplitude at 750 nm) . Gain = 0.01 (
May 2013), bottom: associated PSF in the image plan – (Right) Pyramid image and associated PSF after
closing the loop.
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Third AO4ELT Conference - Adaptive Optics for Extremely Large Telescopes
After closing the loop in static conditions on 10 Zernike modes in January 2013, we recently
implemented a modal correction algorithm with sine waves to take advantages of the full spa-
tial resolution of our deformable mirror (MEMS type mirror). A successfull test in May 2013
permited to close the loop on 10 Fourier modes at 200 Hz in static aberation conditions. In
September 2013, the SCExAO non-modulated pyramid wavefront sensor reached a new mile-
stone closing the loop in laboratory on 260 Fourier modes on a static AO188-type wavefront
error. Our next challenge will be to close the loop on a dynamic AO188-type wavefront aberra-
tion before the SCExAO PWFS engineering nights in december 2013 on Sky.
4 Conclusion
In this analysis, we tried to demonstrate the necessity of non-modulation to take full advantages
of the sensitivity offered by the pyramid wavefront sensor. Convinced with the simulations
results that it is possible to close the loop after a first level AO correction, we started the inte-
gration of the non-modulated PWFS on SCExAO visible channel. Since May 2013, we succeed
to achieve encouraging results in laboratory correcting static wavefront errors, and especially
AO188-type phasemap errors. The next crucial tests to be continued in Fall 2013, before our
engineering night on Sky in December 2013, will be the correction of a dynamic wavefront with
the non-modulated PWFS.
In parallel we developed with the collaboration of the NRC-CNRC, a prototype of the actual
non-linear curvature wavefront sensor tested in MMT [6] for SCExAO. The non-linear cur-
vature wavefront sensor [4] is a conventional curvature wavefront sensor (Roddier) using a
non-linear reconstruction algorithm to retrieve the phase error ( Phase diversity or Gerchberg-
Saxton, [7]). From four quasi-monochromatic beams, four pupils plans conjugated with four
different altitudes are reimaged at the same time on the same detector (visible high speed cam-
era). The first prototype has been machined and will be implemented in parallel of the non-
modulated PWFS in 2014.
Fig. 8. First SCExAO Non-Linear Curvature WFS. The non-linear curvature wavefront sensor is a con-
ventional curvature wavefront sensor (Roddier) using a non-linear reconstruction algorithm to retrieve
the phase error ( Phase diversity or Gerchberg-Saxton, [7]). The first prototype designed with the collab-
oration of the NRC-CNRC (Marc Andre Boucher-INO) will be integrated on SCExAO in 2014.
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