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The Fresnel Imager for exoplanet study. Laurent Koechlin 1 , Jean-Pierre Rivet 2 , Truswin Raksasataya 1 , Paul Deba 1 , Denis Serre 3. 1 Université de Toulouse CNRS 2 Observatoire de la c ôte d'Azur CNRS 3 Leiden University, the Netherlands. I. Concept.

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The fresnel imager for exoplanet study

The Fresnel Imager for exoplanet study

Laurent Koechlin 1,

Jean-Pierre Rivet 2,

Truswin Raksasataya 1,

Paul Deba 1,

Denis Serre 3.

1 Université de Toulouse CNRS

2 Observatoire de la côte d'Azur CNRS

3 Leiden University, the Netherlands

Optical concept light focalization

Lens (or miror): focusing by refraction (or reflexion)


Plane wavefront

Fresnel array: focusing by diffraction …


Optical concept: Light focalization


Spherical wavefront

Binary transmission function g(x)

Order 0 : plane wave

Order 1 : convergent

Optical concept image formation
Optical concept : Image formation

Can light travel free in vacuum all the way from source to focus?



Quasi no stray light except in four spikes.




g(x)"xor" g(y)

non linear luminosity scale,

to show the spikes.

Basic concepts fresnel arrays versus solid aperture
Basic concepts: Fresnel arrays versus solid aperture

Images of a point source by:

300 Fresnel zones

3000 Fresnel zones

Solid square aperture

luminosity scale:Power 1/4 to show spikes

Basic concepts dy namic range resolution psf for 300 zones 720 000 apertures
Basic concepts: Dynamic range & resolution PSF for 300 zones (720 000 apertures)


Fresnel propagation

apodized prolate,

order 0 masked

Log dynamic

1/4 field represented

Position in the field (resels)

Against fresnel arrays
"Against" Fresnel Arrays:


Chromaticity... But can be canceled

by order -1 chromaticityafter focus,

Channel bandpass limitations: Δλ/λ= 15%

f = D2/8zλ


transmission efficiency to focus:

6% to 10%

1km to 100km focal lengths => Formation flying in space

f = D2/8zλ

Pro fresnel arrays
"pro" Fresnel Arrays:

No mirror, no lens : just vacuum and opaque material (except near focal plane). broad spectral domain: λ = 90nm (UV) to (IR) 25μm

High angular resolution: as a solid aperture the size of the array.

High dynamic range: 108 on compact objects,

more with coronagraphy & postprocessing.

Large tolerance in positioning of subapertures:

for λ/50 wavefront quality in the UV on a 30 meters membrane array:

50 μm in the plane of the membrane,

10 mm perp. to membrane,

The tolerance is wavelength independent.

Opens the way to large (up to 100m?) aberration-free apertures.

II. Tests

Does it work ?

2x2 cm array
2x2 cm array

I have it here,

It's working.

live demo

after this session.

For those who

Haven't already

Seen it…

8x8 cm array tests on lab sources 2005 2008
8x8 cm array: tests on lab sources (2005-2008)

116 zones, 8 x 8 cm

26680 apertures

"orthocircular" design.

F= 23 m at = 600 nm

Precision: 5m on holes positioning

=>/70 on wavefront.


Diffraction limited

Broad band imaging (450-850nm)

10-6dynamic range

metal foil 100 m thick

Photo T.Raksasataya

20x20 cm array test on sky sources 2009 2011
20x20 cm array: test on sky sources (2009-2011)

0.8" resolution 1000x1000 field λ0 = 800 nm Δλ = 100 nm

Photo D.Serre

Metal foil

9.7 105 apertures,

Slightly apodized,

696 Fresnel zones.

20 cm

Photo P.Deba

20x20 cm array first light on stars
20x20 cm array: first light on stars


temporary 116 zones secondary

(instead of 702 zones)

Tip-Tilt not yet implemented

52 Cygni a-b

Va= 4.2 vb= 8.7 sep= 6.4"

STT 433 a-b

Va=4.36 ; Vb=10.0 Sep= 15"

more results at our workshop in Nice next week

20 cm array goals
20 cm array goals

Assess performances on real sky objects:

do better than other 20 cm aperture instruments.


High contrast objects

extended sources

dense fields

deep sky

Esa study
ESA study

ESA call for Feasibility study of a membrane telescope 350 k€

Iii space mission
III. Space Mission

Science ?

Credibility ?

The fresnel imager space mission
The Fresnel Imager Space Mission

3m to 100m diameter, or more.

Thin membrane "Primary Array" module:

Field optics telescope

1/10th to 1/20th the diameter

of Primary Array.

Dispersion correction: order -1 diffraction

Blazed lens or concave grating,

10 to 30 cm diameter

focal Instrumentation:



Meet acceptability threshold for a new technology mission

Make it simple, try to keep cost below 300 M€

Scientific return over cost:must be higher than that ofcompeting concepts

λ/50 wavefront, at any λ => High Dynamic range from IR to UV

mas angular resolution

1000x1000 resel. fields

Spectral resolution

Suitable for Exoplanets

and other fieds too…

Strategy sky targets
Strategy, sky targets

- Start with UV domain?

- limited budget => limited aperture, but high resolution

- High quality wavefront at any wavelength

- angular resolution : 7 mas with a 4m Fresnel array

- spectral lines in UV for:

Photosynthesis break, O3, CH4, CO2 , auroras …

- polarimetry

Space mission optical scheme
Space Mission: optical scheme

Spacecraft 1

Solar Baffle, to protect from sunlight

Large "Primary Fresnel Array:

Thin foil,

4 to 30 m diameter, or more.

Field optics telescope

Order zero blocked

Focal instrum.

Diffraction order 1: focused,

but with chromatic aberration.

Spacecraft 2

pupil plane

Diffraction order 0: unfocussed

will be focused by field optics, then blocked.

Focal instruments

Chromatic correction:


Fresnel grating

5 km (for a 4m aperture)

to 100 km (for a 30m aperture)

image plane 1dispersed

image plane 2 achromatic

Targets s n on exoplanets in uv
Targets: S/N on exoplanets in UV

Signal / noise as a function of 



Signal / noise > 30

0.5 Jupiter diameter

1 Jupiter diameter planet

Signal / noise > 3

Signal / noise < 3

Signal / noise > 3

Signal / noise < 3

Images & spectra of exoplanets:

1 UA from solar type star, 10Pc away

4m aperture, 10h integration

spectral res.  /= 50

dynamic range of raw image: 2 10^-8


Build up a proposal for a 2020 / 2025 launch

Science cases: Exoplanets,

and also

stellar physics

compact objects

reflection nebulae


solar system objects

observation of the earth

21/2 days workshop in Nice, Sept. 23-25 (next week)

Free registration

Web site:search with key words "fresnel imager Nice"

We are starting a "Fresnel Imager Astro Applications" group.

Thank you for your attention!

You are welcome to join us.

Workshop: sept 23-25 (next week)

Optical concept image formation1
Optical concept : Image formation

Circular Fresnel Zone Plate => PSF with isotropic rings



Isotropic rings

non linear luminosity scale

to show the rings.

Expansion 2d cart sienne
expansion 2D Cartésienne

Réseau de Fresnel


Interféromètre à forte densité d’ouvertures ?

Géométrie "orthogonale pure" (2005)

4 % de la lumière focalisée

1740 motifs individuels

Géométrie "ortho-circulaire" (2008)

6 % de la lumière focalisée

The field vs spectral bandpas tradeoff

Chromatically aberrated beam at prime focus

The field vs spectral bandpas tradeoff

Field delimited by field mirror

The chromatic corrector

does a good job,

but it corrects only what it collects.

Fresnel imager specifications
Fresnel Imager specifications

UV spectro-Imaging at High Dynamic Range

4m aperture

3 spectral bands Δλ/λ=20%: 2 in the UV, 1 in the visible

λ/50 wavefront

spectral resolution λ/δλ=50

angular resolution 7 to 25 mas depending on λ

field 1000x1000 => 7 to 25 arc seconds

raw dynamic range >108

- with 10 hours integrations time:

jovian exoplanets with apertures of 1 m or more

telluric Exoplanets: only with 10 meters apertures or larger

- Other fields of astrophysics

Gen iii prototype primary array
Gen III prototype: Primary array

R&T financed by CNES & STAE

Size : 8 cm, square

240 Fresnel zones

(110 000 apertures)

metal sheet 100 m thick, laser carved

Operates in the UV (250-350 nm)

Focal length: 26.6 m for = 250 nm

Precision on array : 5m

i.e./30 on wavefront

Gen iii prototype critical point concave blazed mirror in focal module
Gen III prototype Critical point: concave blazed mirror infocal module

Orbites et pointages
Orbites et pointages

petite Lissajou

periode : 6 mois1 eclipse en 6 ans, évitable

Lingne de visée






200 000 km






  • Seul, Lagrange L2 répond à tous les besoins:

    • pas de gradient de gravité sur une grande base

    • masquage du soleil et de la Terre dans un angle réduit

    • bonne liberté de pointage

  • Pour la très haute dynamique: nécessité de masquer toute lumière parasite avec taille pare soleil réduit et possibilité de dépointage acceptable:

  • 35% de la voûte à tout instant, 100% en 4 mois.

  •  implique petite ou moyenne orbite de Lissajou

Antenne RA fixe et GS fixe possible à partir de

100 000km de la Terre

R sultats qualitatifs dynamique mesure optique et simulation num rique
Résultats qualitatifs : dynamiquemesure optiqueet simulation numérique

Dans ces images d'un point, saturées, le fond moyen est à 2 *10 -6

Luminosité amplifiée x1000

Luminosité amplifiée x1000

8 cm 116 zones

image Optique

8 cm 116 zones

propagation de Fresnel numerique

par tous les éléments optiques

propagation numerique de Fresnel développée pour tester de grands réseaux

Expansion 2d cart sienne1
expansion 2D Cartésienne

Réseau de Fresnel


Interféromètre à forte densité d’ouvertures ?

Géométrie "orthogonale pure" (2005)

4 % de la lumière focalisée

1740 motifs individuels

Géométrie "ortho-circulaire" (2008)

6 % de la lumière focalisée