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1European Southern
Observatory
Non-adaptive Wavefront Control
(Presented by L. Noethe)
OWL Phase A Review - Garching - 2nd to 4th Nov 2005
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2European Southern
Observatory
Specific problems in ELTs and OWL
Concentrate on problems which are specific for ELTs and, in particular, for OWLAlignment and shape correction of optical surfaces
Large number of segments in segmented mirrorsSix optical surfacesTwo segmented mirrorsRequires mainly further development of already existing active optics techniques
Operation in open airAdvantages: thermal equilibrium and predictable wind loadsDisadvantage: larger wind loadsFeasible with extensive use of fast control loops
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3European Southern
Observatory
Overview : Wavefront Control
Pre-alignmentonce
Optical elements and segments
Active optics0.01 - 1 Hz M1 - M6
Gravity, Temperature
AdaptiveOptics
a few 100 HzM5 and M6
Atmosphere
Segments Phasing :inner loop
10 HzSegments of M1 and M2
Wind
Segments Phasing : opticalBeginning of the night
Segments of M1 and M2
Field Stabilisation100 Hz
M6Atmosphere
Main axes control1 Hz
Azimuth/altitude drivesWind
Field Stabilisation10 Hz
M6-supportWind
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4European Southern
Observatory
Control Architecture
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5European Southern
Observatory
Pre-alignment
Alignment of segmentsUse of a spherometer to align a new segment relative to its neighboursStacking of the images produced by individual segments
Alignment of optical elementsUse of a fibre extensiometer to be developed within the FP6 ELT study
Residual errors after pre-alignmentPositions M1 – M6 : ~1 mmTilts M1 - M6 : ~ 1 arcsec
Piston errors of segments : ~ 2 µm
Deformations M3 and M4 : ~ 30 µm
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6European Southern
Observatory
Correction strategy
Complete correction in one stepMeasurements from several Shack-Hartmann sensors and one phasing sensor
Inversion of the matrix relating the actuator degrees of freedom to the measured parameters by singular value decomposition
Calculation of the actuator commands from the measured signals with the inverted matrix
Disadvantage : requires a very large matrix
Alternative approach : split the correction into several stepsMore than one possible strategy
One sequence of correction stepsCorrection of slope errors with the segments
Phasing of the segments
Correction of misalignments and deformations of M1 to M6
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7European Southern
Observatory
Active optics corrections with one Shack-Hartmann and one phasing wavefront sensors
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8European Southern
Observatory
Full correction with wavefront sensors in severalfield positions
Aberrations generated byMisalignments of the mirrorsDeformations of the meniscus mirrorsCharacteristic patterns of additional field aberrationsCorrection with an in-pupil mirror only possible for one field angle
Required wavefront sensors1 Shack-Hartmann sensor with 19 lensets per M1 segmentAt most 6 Shack-Hartmann sensors with 20 by 20 subapertures covering M11 (baseline) or 2 (optional) phasing sensors
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9European Southern
Observatory
Phasing wavefront sensors
Multi-wavelengths techniques Reduce the wavefront piston steps of 2 µm to less than 100 nm
Narrowband techniquesShack-Hartmann: lensletscovering subapertures centeredon segment bordersInformation contained in the position of the maximum and in the shape of the diffraction patternApplied in the Keck Telescope extracting the shape informationProblem : exact positioning of a large lenslet array in the re-imaged pupil
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10European Southern
Observatory
Phasing wavefront sensors: Mach-Zehnder
Spatial filtering in a focal plane
Problem : Alignment of the optics
Telescope focus
Pinhole
Interferogram
Interferogram
Reference channel
Beamsplitter
Beamsplitter
-100 -80 -60 -40 -20 0 20 40 60 80 100
-1.0
-0.5
0.0
0.5
1.0
∆x
Diff
rent
ial s
igna
l, S
Position, mm
∆ϕ=π/2, a=2" ∆ϕ=π/4, a=2" ∆ϕ=π/2, a=3"
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11European Southern
Observatory
Phasing wavefront sensors – phase filtering(LAM/IAC)
Adding of a phase delay in the center of the image
Easier to align than the Mach-Zehnder sensor
D=seeing
λ/9
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12European Southern
Observatory
Phasing wavefront sensors – defocusing (IAC) and pyramid sensor (Arcetri)
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13European Southern
Observatory
Phasing wavefront sensors – identification of borders
Possible algorithm : contrast enhancement and Hough transform
Promising for large piston stepsTo be validated for small piston steps
Imaging of the gaps10% reduction of the intensity for pixels covering gaps
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14European Southern
Observatory
Phasing of petals
The M2/corrector support structure may optically divide M1 into six petals.Struts thin enough to allow optical phasing of the full segmented mirrorBackup solution : additional special wavefront sensor for the phasing of the petals relative to each other
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15European Southern
Observatory
Phasing – disentangling M1/M2 segmentation
Segmentation patterns originating from M1 and M2 are superimposed in the detection planes of the wavefront sensorsDisentangling could be done by
Spatial filtering in the Fourier spaceUse of two or three phasing wavefront sensors in the field
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16European Southern
Observatory
Active Phasing Experiment
Comparison of different phasing wavefront sensors
Test of simultaneous correction of wavefront errors generated by segmented and flexible meniscus mirrors
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17European Southern
Observatory
Full scale pressure measurements
Jodrell Bank Radio TelescopeDiameter : 76 m
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18European Southern
Observatory
Jodrell Bank – Location of pressure sensors
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19European Southern
Observatory
Jodrell Bank – Power spectrum
Wind speed: 10 m/sec, Sampling rate : 8 kHz, Integration time : 78 min
von Karman spectrum
Slope: -7/3Corner frequency: 0.03 Hz
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20European Southern
Observatory
Main axes control - wind spectra and altitudetransfer function
Von Karman wind spectraWind speed 10 – 14 m/sec
Turbulence intensity 0.15
Corner frequency 0.02 Hz
Altitude axis transfer functions
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21European Southern
Observatory
Main axes control - Residual tracking errors
Goal: Residual tracking errors be within the correction range of the fast tip-tilt corrections with the M6-support
Closed-loop bandwidth: 1 Hz
Robust design with sufficient modulus margins
Residual RMS errors: 0.19 arcsec for 0 deg 0.32 arcsec for 60 deg
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22European Southern
Observatory
Rolling friction effects of Bogies
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23European Southern
Observatory
Correction of tracking errors by M6-support
Power spectral density of the tilt error with and without closed-loop corrections by the M6-support (closed-loop bandwidth: 3.5 Hz)
Residual tilt error as a function of the closed-loop bandwidthWill be corrected by adaptive M6
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24European Southern
Observatory
Control of segment position
Control of the segment position based on signals from the edge sensors
Wind speed : 10 m/sec, corner frequency: 0.07 Hz, turbulence intensity: 0.15
Lowest segment piston frequency : 60 Hz
Residual closed-loop error with a 10 Hz bandwidth : ~ 7 nm
Further reduction may be possible with acceleration feedback control
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25European Southern
Observatory
Wind Evaluation Breadboard
Test edge sensors, position actuators and control algorithms under observatory conditions
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26European Southern
Observatory
Conclusions
Further development required for phasing wavefront sensors
Disentangling of overlapping segmentation patterns
Detection of the segmentation pattern
Optionally continuous wavefront sensing
Questions are addressed by the APE experiment
Operation of the telescope seems feasible in open airSegmented mirrors controlled by fast feedback loops
Large meniscus mirrors shielded from the wind
No fundamental problems with the non-adaptive wavefront control