First results of the CSL piston sensor breadboard and further application Géraldine Guerri Post-doc ARC @ CSL Liège Space Center, Angleur
Jan 23, 2016
First results of the CSL piston sensor breadboard and further
application
Géraldine Guerri Post-doc ARC @ CSL
Liège Space Center, Angleur 4 March 2011
4 March 2011 2Géraldine GuerriFirst results of CSL piston sensor breadboard
Framework : Extremely Large Telescopes (ELT)
• On the ground :
• In space :– JWST : 18 segments, 6.5m aperture, 25 kg/m² density– Increasing demand for larger apertures : 20m diameter,
3 kg/m² density
E-ELT(Europe)
GMT(USA)
TMT(Europe)
42 m diameter1000 segments
25 m diameter7 segments
30 m diameter492 segments
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Large lightweight space telescope
• Technological need :
• Critical issues :» manufacturing» wavefront error control» sub-aperture coherent alignement
• CSL concern :– Developpement of demonstrator breadboard of a
cophasing sensor for space segmented mirrors made with 3 or 7 segments
space mirrors
large diameter deployable lightweight cheap
4 March 2011 4Géraldine GuerriFirst results of CSL piston sensor breadboard
What is a cophasing sensor ?
• Measure the relative positioning of each subaperture : determination of piston and tip-tilt errors
Piston : Translation along the optical axis (λ or nm)
Tip/ Tilt : Rotation of the sub-pupil perpendicular to the optical axis (rad or arsec)
2 phasing regimes :
– Coarse phasing in open loop – Fine phasing in closed loop : error < λ/2
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Sensor requirements
• Cophasing of 3 to 7 sub-apertures• Separate measurement of piston and tip/tilt• Low weight and Compacity• Real-time correction • Reduced hardware complexity• Linearity, High range and accuracy
Piston measurement
50 nmAccuracy:
± 1 mm Range:
Tip/tilt measurement
Range: 100 µrad
Accuracy: 0.5 µrad
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Cophasing sensor architecture
PISTON TIP-TILT
COARSE
PHASINGCf JF Simar presentation
FINE
COPHASING
Error < λ/2
Phase retrieval real-timealgorithm
(Baron et al., 2008)
Shack-Hartmann Sensor
or
Phase diversity real-time algorithm
(Mocoeur et al., 2008)
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Why phase retrieval technique ?
• Phase error extracted from one focal image
• Baron et al., 2008 : For fine cophasing (error < λ/2), analytical and real-time solutions exists
Only ONE FFT computation needed
Phase retrievalalgorithm ?
Focal plane image
Object
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Phase retrieval algorithm
3 aperture pupil
PSFOTF
Modulus OTF
Phase
Without Piston error
With Piston error
Differential Piston errors can be determined from the intensity of peaks of the phase of the OTF
4 March 2011 9Géraldine GuerriFirst results of CSL piston sensor breadboard
Piston sensor validation breadboard
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Géraldine GuerriFirst results of CSL piston sensor breadboard
Piston sensor breadboard
optical setupCreation of
collimated beam
These mirors reflect only 2 sub-
pupils over 3
Delay line to compensate the OPDs between
the 2 paths
Miror + PZT :Introduction of a piston error on 1
sub-aperture
Laser He-Neλ= 633 nm
O2L1
O1
P
P1
M5M4
M3M2
MPM1
S1
CCDPZT
M6
Piston sensor components
Beamsplitter : re-formation of the pupil with 3 sub-apertures
3 sub-apertures Pupil mask
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Géraldine GuerriFirst results of CSL piston sensor breadboard
Piston sensor experimental results
• Application of a piston ramp on a sub-aperture :
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Géraldine GuerriFirst results of CSL piston sensor breadboard
Piston sensor experimental results
• Metrological standards obtained from measurements :
• Results presented at SPIE conference « Astronomical Telescopes and Instrumentation 2010 »
Measurement range [ -λ/2 , λ/2 ]
Linearity >0.92 (best 0.96)
Resolution < 20 nm
Deviation around zero point < 10 nm
Absolute error ±25 nm
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Géraldine GuerriFirst results of CSL piston sensor breadboard
Feedbacks from experimental tests
• Dependance of the phase measurements accuracy on :– the wavefront error of each beam until the common
path (<λ/10 rms)– the set-up stability (vibration and drift during the
measurement)– the PSF pattern (‘‘fringe’’) contrast– The beam coherence– The image quality of the imaging lens
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Géraldine GuerriFirst results of CSL piston sensor breadboard
Future prospects
• Experimental feasibility tests of the Phase Retrieval technique with a 7 sub-apertures system
• Study and design of a system to introduce various and precise piston values
• Design and implementation of the coarse piston sensor (cf JF Simar PhD)
• Design and implementation of the tip-tilt measurement
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Géraldine GuerriFirst results of CSL piston sensor breadboard
Application
• Cophasing of 3 silicon bimorph mirors developed at ULB (Rodrigues et al., 2009)
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Géraldine GuerriFirst results of CSL piston sensor breadboard
Cophasing demonstrator principle
Piston sensor
Illuminating system
Collimated beam
(Φ=130mm)
3 segments deformable mirror demonstrator
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Géraldine GuerriFirst results of CSL piston sensor breadboard
Optical simulation of the cophasing demonstrator
• Development of an end-to-end simulation (Matlab, ASAP)
Illuminating system
Piston sensor
Illuminating system
• 1 He-Ne Laser (λ=633 nm)
• 1 Microscope objective
• 1 Pinhole (Φ=15 µm)
• 1 Off-axis parabola
Piston sensor
• 1 pupil mask with 3 sub-apertures
• 1 beamsplitter (90/10)
• 1 pupil imaging camera
• 1 imaging lens
• 1 focal image camera
• 1 computer
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Géraldine GuerriFirst results of CSL piston sensor breadboard
3 segment cophasing demonstrator
Illuminating system
• Calibration tests in progress in Liège …
• Validation tests in Bruxelles very soon ..
Piston sensor
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Géraldine GuerriFirst results of CSL piston sensor breadboard
Thanks for your attention
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Géraldine GuerriFirst results of CSL piston sensor breadboard
3 segment cophasing demonstrator
3 segments mirror
Illuminating system
Piston sensor
• 1 He-Ne Laser (λ=633 nm)
• 1 Microscope objective
• 1 Pinhole
• 1 Off-axis parabola
• 1 pupil mask with 3 sub-apertures
• 1 beamsplitter (90/10)
• 1 pupil imaging camera
• 1 imaging lens
• 1 focal image camera
• 1 computer
• Calibration tests in progress in Liège…
• Validation tests in Bruxelles very soon
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Géraldine GuerriFirst results of CSL piston sensor breadboard
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Géraldine GuerriFirst results of CSL piston sensor breadboard
Work plan
Survey of state of the art of cophasing sensor
Sensor techniques selection
Validation by numerical simulations
Experimental validation
Study and Design of a space-compatible breadboard
Feasibility demonstrator of the cophasing of 3 sub-apertures with standard optical
components
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Géraldine GuerriFirst results of CSL piston sensor breadboard