October 10th, 2007Osservatorio Astrofisico di Arcetri1 Application of the pyramid wavefront sensor to the cophasing of large segmented telescopes F. Quirós-Pacheco, E. Pinna, S. Esposito, A. Puglisi, P. Stefanini, M. Bonaglia, F. Pieralli

October 10th, 2007Osservatorio Astrofisico di Arcetri2Contents Part I. Introduction – Segmentation: optical effects – The Active Phase Experiment (APE). Part II. Phasing with the Pyramid Phasing Sensor (PYPS) – PYPS simulations: effect of atmospheric turbulence – PYPS interaction matrix calibration issues – PYPS experimental closed-loop results obtained in Arcetri laboratories – Conclusions and Perspectives

October 10th, 2007Osservatorio Astrofisico di Arcetri3 Segmented Telescopes 10-m class telescopes: – Keck I, II and Gran Telescopio Canarias (GTC) – 36 hexagonal segments (1.80m-diameter each) Extremely Large Telescopes (ELTs): – Thirty Meter Telescope (TMT): 738 hexagonal segments (1.2m-diameter each) – European ELT (E-ELT): 42m-diameter primary mirror. >900 hexagonal segments (~1.45m-diameter each) KECK IIE-ELTTMT

October 10th, 2007Osservatorio Astrofisico di Arcetri4 Segmented Telescopes 10-m class telescopes: – Keck I and II – Gran Telescopio Canarias (GTC) Optical design: – Ritchey-Chrétien (f/1.75) – Primary mirror: 36 hexagonal segments. – Segment diameter: 1.80 m KECK II GTC

October 10th, 2007Osservatorio Astrofisico di Arcetri5 Segmentation: optical effects Effects of segment misalignments on the Point Spread Function (PSF): – Appearance of additional diffraction patterns. Random piston errors → speckle distribution. Random tip/tilt errors → Regular structure of diffraction peaks. – Loss of the intensity in the central peak → loss of Strehl Ratio. For random piston errors δp (wf rms) (Chanan, Ap. Opt. 1999): ELT case → tighter tolerances. For high-contrast applications (e.g. exoplanet search) errors should be less than /40 rms. New phasing techniques are required. Simulated PSF for a 61-segmented mirror with a distribution of piston errors. (N. Yaitskova et. al., JOSAA 2003) Keck precision: <50 nm surf rms δp  1  m S R =68%

October 10th, 2007Osservatorio Astrofisico di Arcetri6 Active Alignment System Reconstruction Matrix (M) Positioning Actuators (Piston, Tip and Tilt) Measurement vector from position sensors Reference position vector + - S ref S e = S - S ref d = Me Actuator Commands (d) © Keck

October 10th, 2007Osservatorio Astrofisico di Arcetri7 Optical Phasing and Active Alignment Control Initial Optical Phasing: – After segments integration/replacement. – From big errors (>100  m) down to few nm. – Initial reference position is recorded. Active Alignment Control: – Closed-loop system to keep segments aligned during scientific observations. – Position sensors measure variations from reference position (e.g. due to gravity, thermal gradients, or small wind buffeting). – Sampling frequency: 2 or 3 Hz. Periodical Optical Phasing: – Required to follow-up drifts in position sensors. – Keck: two/three times per month. – ELTs: may be required at the beginning of each night. Optical closed-loop Phasing: – Under study for E-ELT – Optical phasing running during the scientific observation. – Small sampling frequency: ~0.03 Hz – A suitable star should be located within the FoV. Positioning Actuators (Piston, Tip and Tilt) © Keck

October 10th, 2007Osservatorio Astrofisico di Arcetri8 Active Phase Experiment (APE) APE is a technical instrument for the VLT. Part of ELT Design Study funded by FP6. Goals of the APE experiment: – Test new phasing sensors and their related phasing control algorithms for the ELT case. – Study the integration of phasing control into a global scheme of segmented-telescope active control (i.e. Active Optics, Field Stabilization, etc). Phasing sensors: – DIPSI (Curvature) – PYPS (Pyramid) – ZEUS (Phase contrast interferometer) – SHAPS (Shack-Hartmann) Active Segmented Mirror (ASM): – 61 hexagonal segments (4 rings). – Piston, tip and tilt control. – Precision: <2 nm piston; – Max stroke: >15  m. N. Yaitskova, et. al., SPIE Vol. 6267, 2006

October 10th, 2007Osservatorio Astrofisico di Arcetri9 The pyramid sensor is sensitive to phase steps: Analytical (E. Pinna, Tesi di Laurea, 2004) Numerical Experimental (S. Esposito et. al., O.L., 2005) Phasing with the pyramid segment Big local WF derivative!

October 10th, 2007Osservatorio Astrofisico di Arcetri10 PYPS phasing techniques Initial Phasing – Wavelength sweeping technique Huge capture range (>50  m) but low precision. – Segment sweeping technique For high-precision reference position. Periodical phasing – Mono-wavelength / Multi-wavelength techniques – Capture range in closed-loop operation: ±λ/2 – Filtering techniques developed to get rid of the atmospheric- turbulence disturbance in an efficient way. Calibration requirements – Interaction Matrix Acquisition Acquired on the sky (i.e. affected by atmospheric turbulence). Synthetically generated (i.e. based on a simulation tool). M. Bonaglia, tesi di Laurea, 2007PYPS Acceptance Test Report, 2007 PYPS Signal

October 10th, 2007Osservatorio Astrofisico di Arcetri11 Pyramid Phasing Sensor (PYPS)

October 10th, 2007Osservatorio Astrofisico di Arcetri12 PYPS simulations Evaluate the effect of atmospheric turb. Simulation characteristics: – Closed-loop control (piston, tip and tilt) of ASM’s segments. – PYPS end-to-end model. Sampling: 6 subapertures/side. Modulation radius: ~ / r 0 – Atmospheric turbulence: n f independent turbulence realizations averaged at each closed-loop iteration. nfnf Final WFEn tot =15n f Equivalent time * 560 nm750.5 min 1040 nm1501 min 4020 nm6004 min * Considering that two turbulence realizations become de-correlated after 0.4s

October 10th, 2007Osservatorio Astrofisico di Arcetri13 PYPS experiments at Arcetri Experimental Setup: – Reflecting phase screen: s=0.6’’ for 8-m telescope. Wind speed equivalent to 15 m/s – MEMS (Boston  SLM140): 12x12 squared segments. Pitch: 300  m. Piston: 20 nm resolution, max stroke 2  m (in wavefront). – Optical design: 3 mm system pupil. 10 segments across pupil.

October 10th, 2007Osservatorio Astrofisico di Arcetri14 Interaction Matrix Acquisition Interaction Matrix Masking (IMM): – Removes turbulence signal wide-spread over the whole pupil. – Allows to perform a parallel interaction matrix acquisition. Interaction Matrix acquired on the sky (i.e. affected by turbulence)

October 10th, 2007Osservatorio Astrofisico di Arcetri15 Mono-λ Closed-loop Phasing Narrow band filter selected: CW700nm-BW40nm Initial piston error: >100 nm wf rms A total of 50 MEMS segments controlled Filtering technique applied: – Low-order removal (LOR). – Removes low-order aberrations mostly due to turbulence. – Implementation: n z Zernikes (starting with piston) removed from to WFS signals at each closed-loop iteration.

October 10th, 2007Osservatorio Astrofisico di Arcetri16 LOR experimental results Lab conclusions: LOR filtering technique allows to reduce integration time by factor ~10. Expected on the sky: factor ~100. E. Pinna, F. Quirós-Pacheco, S. Esposito, A. Puglisi, P. Stefanini, Signal spatial filtering for co-phasing in seeing-limited conditions, Optics Letters (Accepted).

October 10th, 2007Osservatorio Astrofisico di Arcetri17 Synthetic IM Calibration Synthetic Calibration: – Interaction Matrix generated ‘synthetically’ using a calibrated end-to- end simulation tool. – Critical model parameters: Pupil registration (pupil radius and center coordinates) Sampling factor (number of subapertures). Experimental Results: – 36 MEMS actuators controlled in piston. – Integrator gain needs fine-tuning. – Synthetic and measured IMs provide a comparable final precision (<10 nm wf rms). SXSX SYSY Four simulated sub-pupils

October 10th, 2007Osservatorio Astrofisico di Arcetri18 Conclusions and Perspectives Phasing control algorithms for the Pyramid Phasing Sensor (PYPS) validated in the laboratory. Atmospheric-turbulence filtering techniques that improve final phasing precision and converge time were developed and tested. Interaction Matrix Calibration: Both ‘on-sky’ and synthetic acquisition demonstrated. PYPS passed its Acceptance Test on April 2007 with a visiting committee from ESO. PYPS will be integrated on the APE bench (held at ESO Garching) by the end of the year. Garching: Experiments to compare different phasing sensors in first- half of Paranal: On-sky tests: End 2008 / Beginning 2009 (TBD).