October 10th, 2007Osservatorio Astrofisico di Arcetri1 Application of the pyramid wavefront sensor to the cophasing of large segmented telescopes F. Quirós-Pacheco,

October 10th, 2007 Osservatorio Astrofisico di Arcetri 1

Application of the Application of the pyramid wavefront sensor pyramid wavefront sensor

to the cophasing of to the cophasing of large segmented telescopeslarge segmented telescopes

F. Quirós-Pacheco, E. Pinna, S. Esposito, A. Puglisi, F. Quirós-Pacheco, E. Pinna, S. Esposito, A. Puglisi, P. Stefanini, M. Bonaglia, F. Pieralli P. Stefanini, M. Bonaglia, F. Pieralli


ContentsContents

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


Segmented TelescopesSegmented 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 II

E-ELTTMT


Segmentation: optical effectsSegmentation: 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)

p

N

eNSR 2;

)1(12

Keck precision: <50 nm surf rmsδp/10 @ 1m

SR=68%


Active Phase Experiment (APE)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


The pyramid sensor is sensitive to phase steps:The pyramid sensor is sensitive to phase steps:Analytical Analytical (E. Pinna, Tesi di Laurea, 2004)(E. Pinna, Tesi di Laurea, 2004)

NumericalNumerical

Experimental Experimental (S. Esposito et. al., O.L., 2005)(S. Esposito et. al., O.L., 2005)

Phasing with the pyramidPhasing with the pyramid

segment

segment

segment

Big local WF derivative!Big local WF derivative!


PYPS phasing techniquesPYPS 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, 2007

PYPS Acceptance Test Report, 2007

PYPS Signal


Pyramid Phasing Sensor (PYPS)Pyramid Phasing Sensor (PYPS)


PYPS simulationsPYPS 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: ~ / r0

– Atmospheric turbulence: nf independent turbulence realizations

averaged at each closed-loop iteration.

nf Final WFE ntot=15nf Equivalent time*

5 60 nm 75 0.5 min

10 40 nm 150 1 min

40 20 nm 600 4 min

* Considering that two turbulence realizations become de-correlated after 0.4s


PYPS experiments at ArcetriPYPS 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.


Interaction Matrix AcquisitionInteraction 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)


Mono-Mono-λλ Closed-loop Phasing Closed-loop Phasing

Narrow band filter selected: CW700nm-BW40nmInitial piston error: >100 nm wf rmsA total of 50 MEMS segments controlledFiltering technique applied:

– Low-order removal (LOR).– Removes low-order aberrations mostly due to turbulence.– Implementation: nz Zernikes (starting with piston) removed from to WFS

signals at each closed-loop iteration.


LOR experimental resultsLOR 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).


Synthetic IM CalibrationSynthetic 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).

SX

SY

Four simulated sub-pupils


Conclusions and PerspectivesConclusions 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 2008.Paranal: On-sky tests: End 2008 / Beginning 2009 (TBD).

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October 10th, 2007Osservatorio Astrofisico di Arcetri1 Application of the pyramid wavefront sensor to the cophasing of large segmented telescopes F. Quirós-Pacheco,