first results of the csl piston sensor breadboard and further application géraldine guerri post-doc...

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




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


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


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


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)


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


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


Piston sensor validation breadboard

4 March 2011 10

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

4 March 2011 11


Piston sensor experimental results

• Application of a piston ramp on a sub-aperture :

4 March 2011 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

4 March 2011 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

4 March 2011 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

4 March 2011 15


Application

• Cophasing of 3 silicon bimorph mirors developed at ULB (Rodrigues et al., 2009)

4 March 2011 16


Cophasing demonstrator principle

Piston sensor

Illuminating system

Collimated beam

(Φ=130mm)

3 segments deformable mirror demonstrator

4 March 2011 17


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

4 March 2011 18


3 segment cophasing demonstrator

Illuminating system

• Calibration tests in progress in Liège …

• Validation tests in Bruxelles very soon ..

Piston sensor

4 March 2011 19


Thanks for your attention

4 March 2011 20


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

4 March 2011 21


4 March 2011 22


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

4 March 2011 23


first results of the csl piston sensor breadboard and further application géraldine guerri post-doc...

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