1. 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
I will present here an optical concept to enable very large apertures in space.
This cobncept yields high resolution and high dynamic range images
With very simple optics (no optics at all?)
I will try to summarize four years of work in 10 minutes.

2. I. Concept
Three parts :
concepts,
tests,
space mission

3. Optical concept: Light focalization
Lens
Plane wavefront
focus
Spherical wavefront
Lens (or miror): focusing by refraction (or reflexion)
Fresnel array: focusing by diffraction …
Binary transmission function g(x)
focus
Order 0 : plane wave
Order 1 : convergent
The wavefront is clipped by the mask segments as beams from individual apertures.
wavefronts segments interfere constructively together.
With a zero phase shift, => order zero: a plane wave in, a plane wave out.
2pi phase shift => also interfere constructively, =>. Focalization
Alll shifts at same time, beam is split. part is focused : maxi 10% of light.

4. Optical concept : Image formation
Can light travel free in vacuum all the way from source to focus?
Image
Aperture
Quasi no stray light except in four spikes.
remove glass support, light travel in vacuum all the way to focus => no aberrations, no spectral limitations
need 2D development, need hold structure. circular, => FZP,
I came up with this "XOR" solution, mechanically connex, Apod Square Aper
EITHER interferometer no beam combiner millions of subapertures : dense array
ortho => HDR, interf and FZP too.
Transmission:
g(x) "xor" g(y)
Transmission:
g(x)
non linear luminosity scale,
to show the spikes.

5. Basic concepts: Fresnel arrays versus solid aperture
Images of a point source by:
300 Fresnel zones
3000 Fresnel zones
Solid square aperture
At large Fresnel zone numbers, the PSF is the same as with a solid aperture the same size.
luminosity scale: Power 1/4 to show spikes

6. Basic concepts: Dynamic range & resolution PSF for 300 zones (720 000 apertures)
Numerical
Fresnel propagation
apodized prolate,
order 0 masked
Log dynamic
Numerical sumulation by Fresnel propagation
prolate Apodisation is achieved by apertures size modulation. It could also be done by "PIAA Guyon" in focal optics
Show central peak, spikes, fHD field.
Detection can go beyond raw dynamic range : only the variance of the noise counts.
1/4 field represented
Position in the field (resels)

7. "Against" Fresnel Arrays:
Chromaticity... But can be canceled
by order -1 chromaticity after focus,
Channel bandpass limitations: Δλ/λ= 15%
f = D2/8zλ D
transmission efficiency to focus:
6% to 10%
Shupmann design : cancel by order -1 at pupil plane
Removes the 2pi phase shifts => COPHASED !
Beam combiner
Cost per photon rather than cost per aperture diameter
1km to 100km focal lengths => Formation flying in space
f = D2/8zλ

8. "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.
A factor of compared to classical optical constraints
Light weight deployable membrane
Requires field telescope 1/6th to 1/10th of the diameter

9. II. Tests
Does it work ?

10. 2x2 cm array I have it here, It's working. live demo
after this session.
For those who
Haven't already
Seen it…
La montrer

11. 8x8 cm array: tests on lab sources (2005-2008)
116 zones, x 8 cm
26680 apertures
"orthocircular" design.
F= 23 m at = 600 nm
Precision: 5m on holes positioning
=> /70 on wavefront.
Achievements:
Diffraction limited
Broad band imaging ( nm)
10-6 dynamic range
Achievements.
improved light efficiency : bars contribute to focalization.
metal foil 100 m thick
Photo T.Raksasataya

12. 20x20 cm array: test on sky sources (2009-2011)
0.8" resolution 1000x1000 field λ0 = 800 nm Δλ = 100 nm
Simulate formation flying in space:
Primary array module;
refractor, 19 meter long tube,
Focalization (click)
Focal instrumentation module
Photo D.Serre

13. 20 cm Metal foil 9.7 105 apertures, Slightly apodized,
696 Fresnel zones.
Close up on primary array module
Close to a million aperture in this APODIZED array
Photo P.Deba

14. 20x20 cm array: first light on stars
ALINEMENT PHASE images:
temporary 116 zones secondary
(instead of 702 zones)
Tip-Tilt not yet implemented
fresh results Made this summer. First time show to public
First alignment tests, on binary stars. NOT representative of the real resol nor Dyn.
Much better expected expected soon
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

15. 20 cm array goals Assess performances on real sky objects:
do better than other 20 cm aperture instruments.
Targets:
High contrast objects
extended sources
dense fields
deep sky
do better than other 20 cm aperture instruments.
Check the behaviour

16. ESA study
ESA call for Feasibility study of a membrane telescope 350 k€
ESA is interested in the concept

17. III. Space Mission
Science ?
Credibility ?

18. 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
Opens the way to very large apertures in space
focal Instrumentation:
Spectro-imagers

19. Rationale Make it simple, try to keep cost below 300 M€
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 of competing 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…
Wavefront is lambda intependant

20. 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
One visible channel too
- spectral lines in UV for:
Photosynthesis break, O3, CH4, CO2 , auroras …
- polarimetry

21. 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
From phase Zero study
I'll not go into it
Chromatic correction:
Blazed
Fresnel grating
5 km (for a 4m aperture)
to 100 km (for a 30m aperture)
image plane 1 dispersed
image plane 2 achromatic

22. Targets: S/N on exoplanets in UV
Signal / noise as a function of  uv uv
Signal / noise > 30
0.5 Jupiter diameter
1 Jupiter diameter planet
Signal / noise > 3
Signal / noise < 3
Signal / noise > 3
Signal / noise < 3
Exposure time calculators on the web soon by Denis Serre and Truswin Raksasataya.
You enter your object and spectral res,
it replies with the S/N vs integration time chart
Check them, stay tuned
Images & spectra of exoplanets:
1 UA from solar type star, 10 Pc away
4m aperture, 10h integration
spectral res.  / = 50
dynamic range of raw image: 2 10^-8

23. Conclusion Build up a proposal for a 2020 / 2025 launch
Science cases: Exoplanets,
and also
stellar physics
compact objects
reflection nebulae
extragalactic
solar system objects
observation of the earth
21/2 days workshop in Nice, Sept (next week)
Free registration
This concept allows for exoplanet study
But can also be competitive in many other fields of astrophysics
We have some funding and
We are looking foe collaborations with specialists of different domains:
Stellar, galactic, solar system,
To find the mission proposal that would serve the best.
Web site: search with key words "fresnel imager Nice"
We are starting a "Fresnel Imager Astro Applications" group.

24. Thank you for your attention!
You are welcome to join us.
Have plans for next week?
Don't forget the demo after the session
Workshop: sept (next week)

26. Bonus slides

27. Optical concept : Image formation
Circular Fresnel Zone Plate => PSF with isotropic rings
Image
Aperture
Isotropic rings
Iregular FZP. deas for using it in space have been proposed by Chesnokov 1993, Hyde 1999, Massonnet 2003
This has also been proposed for EUV and X by Baez 1960 with radial beams to support the rings.
In the case of an FZP, the stray light is not confined into spikes, but spread isotropically into rings => lower dynamic range.
non linear luminosity scale
to show the rings.

28. expansion 2D Cartésienne
Réseau de Fresnel ou
Interféromètre à forte densité d’ouvertures ?
Géométrie "orthogonale pure" (2005)
4 % de la lumière focalisée
1740 motifs individuels
As the light is focused by diffraction, each hole in this grid behaves like an individual aperture in an interferometric array
Géométrie "ortho-circulaire" (2008)
6 % de la lumière focalisée 28

29. The field vs spectral bandpas tradeoff
Field delimited by field mirror
Chromatically aberrated beam at prime focus
The chromatic corrector
does a good job,
but it corrects only what it collects.

30. 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

31. Gen III prototype: Primary array
R&T financed by CNES & STAE
Size : 8 cm, square
240 Fresnel zones
( apertures)
metal sheet 100 m thick, laser carved
Operates in the UV ( nm)
Focal length: 26.6 m for = 250 nm
Precision on array : 5m
i.e. /30 on wavefront

32. Gen III prototype Critical point: concave blazed mirror in focal module

33. Orbites et pointages 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
eclipticque
sun
petite Lissajou
periode : 6 mois 1 eclipse en 6 ans, évitable
éclipticque
Terre
Lune L2 km 8°
14°
Fresnel
baffle
Lingne de visée
Antenne RA fixe et GS fixe possible à partir de
km de la Terre

34. Résultats qualitatifs : dynamique mesure optique et simulation numérique
Dans ces images d'un point, saturées, le fond moyen est à 2 *10 -6
Dynamique: DEFINIR : valeur moyenne dans le champ propre) / (maximum du pic central
Simulation numérique par Denis Serre.
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 34

35. expansion 2D Cartésienne
Réseau de Fresnel ou
Interféromètre à forte densité d’ouvertures ?
Géométrie "orthogonale pure" (2005)
4 % de la lumière focalisée
1740 motifs individuels
As the light is focused by diffraction, each hole in this grid behaves like an individual aperture in an interferometric array
Géométrie "ortho-circulaire" (2008)
6 % de la lumière focalisée 35

36. Fields obtained with 2 exposures rotated 45°
Fields form A&A paper Koechlin Serre Duchon 2005

37. scenarios