1. Géraldine Guerri Post-doc ARC @ CSL
4 March 2011
First results of the CSL piston sensor breadboard and further application
Géraldine Guerri
Post-doc CSL
Liège Space Center, Angleur March 2011

2. 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)
42 m diameter
1000 segments
GMT
(USA)
25 m diameter
7 segments
TMT
(Europe)
30 m diameter
492 segments
Géraldine Guerri
4 March

3. 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
large diameter
deployable
lightweight
cheap
space mirrors
Géraldine Guerri
4 March

4. 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
Fine phasing in closed loop : error < λ/2
Tip/ Tilt : Rotation of the sub-pupil perpendicular to the optical axis (rad or arsec)
Géraldine Guerri
4 March

5. 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:
0.5 µrad
Accuracy:
50 nm
Géraldine Guerri
4 March

6. Cophasing sensor architecture
PISTON
TIP-TILT
COARSE
PHASING
Cf JF Simar presentation
FINE
COPHASING
Error < λ/2
Phase retrieval real-time
algorithm
(Baron et al., 2008)
Shack-Hartmann Sensor or
Phase diversity real-time algorithm
(Mocoeur et al., 2008)
Géraldine Guerri
4 March

7. 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
Object
Focal plane image
Phase retrieval
algorithm ?
Géraldine Guerri
4 March

8. Phase retrieval algorithm
Without Piston error
Differential Piston errors can be determined from the intensity of peaks of the phase of the OTF
OTF Modulus
OTF Phase
PSF
3 aperture pupil
With Piston error
Géraldine Guerri
4 March

9. Piston sensor validation breadboard
Géraldine Guerri
4 March

10. Piston sensor breadboard optical setup
Laser He-Ne
λ= 633 nm P1
Creation of collimated beam O1
3 sub-apertures Pupil mask
These mirors reflect only 2 sub-pupils over 3 MP M4 M5
Delay line to compensate the OPDs between the 2 paths M1 M2 M3 L1 O2 M6
CCD
PZT S1
Miror + PZT :
Introduction of a piston error on 1 sub-aperture
Beamsplitter : re-formation of the pupil with 3 sub-apertures
Piston sensor components
Géraldine Guerri
4 March

11. Piston sensor experimental results
Application of a piston ramp on a sub-aperture :
Géraldine Guerri
4 March

12. 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
Géraldine Guerri
4 March

13. 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
Géraldine Guerri
4 March

14. 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
Géraldine Guerri
4 March

15. Application
Cophasing of 3 silicon bimorph mirors developed at ULB (Rodrigues et al., 2009)
Géraldine Guerri
4 March

16. Cophasing demonstrator principle
3 segments deformable mirror demonstrator
Collimated beam (Φ=130mm)
Piston sensor
Illuminating system
Géraldine Guerri
4 March

17. 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 pupil imaging camera
1 imaging lens
1 focal image camera
1 computer
1 He-Ne Laser (λ=633 nm)
1 Microscope objective
1 Pinhole (Φ=15 µm)
1 Off-axis parabola
Piston sensor
Géraldine Guerri
4 March

18. 3 segment cophasing demonstrator
Calibration tests in progress in Liège …
Validation tests in Bruxelles very soon ..
Illuminating system
Piston sensor
Géraldine Guerri
4 March

19. Thanks for your attention
Géraldine Guerri
4 March

20. 3 segment cophasing demonstrator
3 segments mirror
Illuminating system
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
1 He-Ne Laser (λ=633 nm)
1 Microscope objective
1 Pinhole
1 Off-axis parabola
Calibration tests in progress in Liège…
Validation tests in Bruxelles very soon
Géraldine Guerri
4 March

21. Géraldine Guerri
4 March

22. Sensor techniques selection
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
Géraldine Guerri
4 March

23. Géraldine Guerri
4 March