THE SUBARU CORONAGRAPHIC EXTREME AO HIGH SENSITIVITY VISIBLE WAVEFRONT SENSORS

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1 Florence, Italy. May 2013 ISBN: DOI: /AO4ELT THE SUBARU CORONAGRAPHIC EXTREME AO HIGH SENSITIVITY VISIBLE WAVEFRONT SENSORS Christophe Clergeon 1a, Olivier Guyon 1, Frantz Martinache 1, Jean-Pierre Veran 3, Eric Gendron 2, Gerard Rousset 2, Carlos Correia 3, and Vincent Garrel 1 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 Subaru 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 complement 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 sensitivity 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 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 habitable 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 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 nonlinear curvature wavefront sensor [4] are both promising in the field of planet direct imaging. a christophe@naoj.org

2 We introduce in this paper the first results of the integration of a non-modulated pyramid wavefront 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: 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 signalnoise 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 σ dif 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 σ mod proportional to the modulation radius ξ o. 2

3 In other words, the modulation breaks down the measurement sensitivity that we would observe with diffraction limited conditions 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 information 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 3

4 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. Input Wavefront Error Turbulence Profile Table 1. Simulation Parameters. Kolmogorov (seeing 0.5 zenith angle: 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 % ExAO WFS (SCExAO) WFS DM Pupil diameter Particularity Non-modulated pyramid WFS MEMS 32x32 actuators 28 actuators 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 4

5 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 conventional linear AO control scheme based on a modal (Zernike and sine waves) reconstruction. Hyp 1 Hyp 2 Hyp 3 Table 2. Simulation hypothesis Perfect deformable mirror Very bright source (high SNR) Detector and loop faster than the turbulence coherence time 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 performances 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 absence 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) corrected 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 until 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 pyramid correction. Full lines, the associated residual errors after the non-modulated pyramid WFS correction. 5

6 Fig. 5. Non-modulated Pyramid WFS correction - input: AO188 dynamic wavefront error 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 simulation 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 generated 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) 6

7 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 observation 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 F ratio on the pyramid Pyramid type Detector Pyramid Pupils nm f / 35 Microlens array (SUSS Micro Optic), Pitch 500, focal length: 15mm Andor, Zyla (Pix 6.5, Speed 1.7KHz (ROI 120x120px), RN 1e50 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. 7

8 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 spatial 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 milestone 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 aberration 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 integration 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 curvature 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 camera). The first prototype has been machined and will be implemented in parallel of the nonmodulated PWFS in Fig. 8. First SCExAO Non-Linear Curvature WFS. The non-linear curvature wavefront sensor is a conventional 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 collaboration of the NRC-CNRC (Marc Andre Boucher-INO) will be integrated on SCExAO in

9 References 1. Clergeon, C., Guyon, Verran, Martinache, Gendron, Rousset, Non Modulated Pyramid Wavefront Sensing For High Performance Adaptive Optics: I - Close Loop Convergence, (expected publication winter 2013) 2. Guyon, O. et al, SPIE, Vol (2010) 3. Guyon, O., Limits of adaptive Optics for High contrast Imaging, ApJ, 629, (2005) 4. Guyon, PASP, High Sensitivity Wavefront Sensing with a non-linear Curvature Wavefront Sensor, 122, (2010) 5. Guyon, O., High Contrast Imaging: New Techniques and Scientific Perspectives for ELTs, Proc. of AO4ELTs3, Paper (2013) 6. Guyon, O., Putting the non-linear Curvature Wavefront Sensor on the 6.5m MMT telescope, Proc. of AO4ELTs3, Paper (2013) 7. Carlos Correia, Jean-Pierre Veran, Olivier Guyon, Christophe Clergeon, Wave-front reconstruction for the non-linear curvature wave-front sensor,proc. of AO4ELTs3, Paper (2013) 8. G.A.Tyler, D.L.Fried. Image-position error associated with a quadrant detector, J.Opt.Soc.Al.,Vol. 72, No.6 (1981). 9. Jovanovic, N., Guyon, O., Martinache, Clergeon, C., Singh, G., Vievard, S., Kudo, T., Garrel, V., Norris, B., Tuthill, P., Stewart, P., Huby, E., Perrin, G., Lacour, S., Proc. of AO4ELTs3, Paper (2013) 10. Roberto Ragazzoni, Pupil plane wavefront sensing with an oscillating prism, Journal of Modern Optics, Vol (1996). 9