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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
October 10th, 2007 Osservatorio Astrofisico di Arcetri 2
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
October 10th, 2007 Osservatorio Astrofisico di Arcetri 3
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
October 10th, 2007 Osservatorio Astrofisico di Arcetri 5
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%
October 10th, 2007 Osservatorio Astrofisico di Arcetri 8
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
October 10th, 2007 Osservatorio Astrofisico di Arcetri 9
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!
October 10th, 2007 Osservatorio Astrofisico di Arcetri 10
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
October 10th, 2007 Osservatorio Astrofisico di Arcetri 11
Pyramid Phasing Sensor (PYPS)Pyramid Phasing Sensor (PYPS)
October 10th, 2007 Osservatorio Astrofisico di Arcetri 12
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
October 10th, 2007 Osservatorio Astrofisico di Arcetri 13
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.
October 10th, 2007 Osservatorio Astrofisico di Arcetri 14
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)
October 10th, 2007 Osservatorio Astrofisico di Arcetri 15
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.
October 10th, 2007 Osservatorio Astrofisico di Arcetri 16
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).
October 10th, 2007 Osservatorio Astrofisico di Arcetri 17
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
October 10th, 2007 Osservatorio Astrofisico di Arcetri 18
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).