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First results of the CSL piston
sensor breadboard and further
application
Géraldine Guerri
Post-doc ARC @ CSL
Liège Space Center, Angleur
4 March 2011
Framework : Extremely
Large Telescopes (ELT)
• On the ground :
E-ELT
(Europe)
42 m diameter
1000 segments
GMT
(USA)
25 m diameter
7 segments
TMT
(Europe)
30 m diameter
492 segments
• In space :
– JWST : 18 segments, 6.5m aperture, 25 kg/m² density
– Increasing demand for larger apertures : 20m
diameter, 3 kg/m² density
First results of CSL piston sensor breadboard
Géraldine Guerri
4 March 2011
2
Large lightweight space
telescope
• Technological need :
• Critical issues :
 large diameter
 deployable
 lightweight
 cheap
space mirrors
» 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
First results of CSL piston sensor breadboard
Géraldine Guerri
4 March 2011
3
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)
2 phasing regimes :
– Coarse phasing in open loop
Tip/ Tilt : Rotation of the
sub-pupil perpendicular to
the optical axis (rad or
arsec)
First results of CSL piston sensor breadboard
Géraldine Guerri
– Fine phasing in closed loop :
error < λ/2
4 March 2011
4
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
Tip/tilt measurement
Range:
± 1 mm
Range:
100 µrad
Accuracy:
50 nm
Accuracy:
0.5 µrad
First results of CSL piston sensor breadboard
Géraldine Guerri
4 March 2011
5
Cophasing sensor
architecture
PISTON
TIP-TILT
COARSE
Cf JF Simar presentation
PHASING
Shack-Hartmann Sensor
FINE
Phase retrieval real-time
algorithm
or
COPHASING
Error < λ/2
First results of CSL piston sensor breadboard
(Baron et al., 2008)
Géraldine Guerri
Phase diversity real-time
algorithm
(Mocoeur et al., 2008)
4 March 2011
6
Why phase retrieval
technique ?
• Phase error extracted from one focal image
Focal
plane
image
Object
Phase
retrieval
algorithm
?
• Baron et al., 2008 : For fine cophasing (error <
λ/2), analytical and real-time solutions exists
Only ONE FFT computation needed
First results of CSL piston sensor breadboard
Géraldine Guerri
4 March 2011
7
Phase retrieval algorithm
Differential Piston errors can
be determined from the
intensity of peaks of the
phase of the OTF
Without
Piston
error
PSF
OTF
Modulus
OTF
Phase
3 aperture pupil
With
Piston
error
First results of CSL piston sensor breadboard
Géraldine Guerri
4 March 2011
8
Piston sensor validation
breadboard
First results of CSL piston sensor breadboard
Géraldine Guerri
4 March 2011
9
Piston sensor breadboard
optical setup
P
Laser He-Ne
λ= 633 nm
Creation of
collimated beam
P1
O1
These mirors
reflect only 2 subpupils over 3
3 sub-apertures
Pupil mask
MP
M1
M2
M4
M5
M3
L1
Delay line to
compensate the
OPDs between
the 2 paths
O2
M6
S1
PZT
Miror + PZT :
Introduction of a
piston error on 1
sub-aperture
First results of CSL piston sensor breadboard
Beamsplitter : reformation of the pupil
with 3 sub-apertures
Géraldine Guerri
CCD
Piston sensor
components
4 March 2011
10
Piston sensor
experimental results
• Application of a piston ramp on a sub-aperture :
First results of CSL piston sensor breadboard
Géraldine Guerri
4 March 2011
11
Piston sensor
experimental results
• Metrological standards obtained from
measurements :
[ -λ/2 , λ/2 ]
Measurement range
Linearity
>0.92 (best 0.96)
Resolution
< 20 nm
Deviation around zero point
< 10 nm
Absolute error
±25 nm
• Results presented at SPIE conference
« Astronomical Telescopes and Instrumentation
2010 »
First results of CSL piston sensor breadboard
Géraldine Guerri
4 March 2011
12
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
First results of CSL piston sensor breadboard
Géraldine Guerri
4 March 2011
13
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
First results of CSL piston sensor breadboard
Géraldine Guerri
4 March 2011
14
Application
• Cophasing of 3 silicon bimorph mirors developed at
ULB (Rodrigues et al., 2009)
First results of CSL piston sensor breadboard
Géraldine Guerri
4 March 2011
15
Cophasing demonstrator
principle
3 segments deformable
mirror demonstrator
Collimated
beam
(Φ=130mm)
Piston
sensor
First results of CSL piston sensor breadboard
Illuminating system
Géraldine Guerri
4 March 2011
16
Optical simulation of the
cophasing demonstrator
• Development of an end-to-end simulation
(Matlab, ASAP)
Illuminating system
Piston sensor
Illuminating
system
• 1 pupil mask with
3 sub-apertures
• 1 beamsplitter
(90/10)
• 1 He-Ne Laser
(λ=633 nm)
• 1 pupil imaging
camera
• 1 Microscope
objective
• 1 imaging lens
• 1 Pinhole
(Φ=15 µm)
• 1 Off-axis
parabola
First results of CSL piston sensor breadboard
Piston
sensor
Géraldine Guerri
• 1 focal image
camera
• 1 computer
4 March 2011
17
3 segment cophasing
demonstrator
• Calibration tests in progress in Liège …
Illuminating
system
Piston sensor
• Validation tests in Bruxelles very soon ..
First results of CSL piston sensor breadboard
Géraldine Guerri
4 March 2011
18
Thanks for your attention
First results of CSL piston sensor breadboard
Géraldine Guerri
4 March 2011
19
3 segment cophasing
demonstrator
3 segments
mirror
Illuminating
system
• 1 He-Ne Laser
(λ=633 nm)
• 1 Microscope
objective
• 1 Pinhole
• 1 Off-axis
parabola
Piston sensor
• Calibration tests in
progress in Liège…
• Validation tests in
Bruxelles very soon
• 1 pupil mask with
3 sub-apertures
• 1 beamsplitter
(90/10)
• 1 pupil imaging
camera
• 1 imaging lens
• 1 focal image
camera
• 1 computer
First results of CSL piston sensor breadboard
Géraldine Guerri
4 March 2011
20
First results of CSL piston sensor breadboard
Géraldine Guerri
4 March 2011
21
Work plan
Survey of state of the art of cophasing sensor
Sensor techniques selection
Validation by numerical simulations
Experimental validation
Feasibility demonstrator of the cophasing of 3
sub-apertures with standard optical
components
Study and Design of a space-compatible
breadboard
First results of CSL piston sensor breadboard
Géraldine Guerri
4 March 2011
22
First results of CSL piston sensor breadboard
Géraldine Guerri
4 March 2011
23
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