The Fresnel Imager for Exoplanet Study, Pathways to Habitable Planets BarcelonaSeptember 17, 20091...

The Fresnel Imager for Exoplanet Study, Pathways to Habitable Planets Barcelona September 17, 2009 1

Laurent Koechlin 1, Jean-Pierre Rivet 2, Truswin Raksasataya 1, Paul Deba 1,Denis Serre 3.

1 Université de Toulouse CNRS2 Observatoire de la côte d'Azur CNRS3 Leiden University, the Netherlands

The Fresnel Imager for exoplanet study


I. Concept


Plane wavefront

focus

Order 0 : plane wave

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

Fresnel array: focusing by diffraction …

Optical concept: Light focalization

Order 1 : convergent

focus

Lens

Binary transmission function g(x)

Spherical wavefront


Optical concept : Image formation

non linear luminosity scale,to show the spikes.

Image

Aperture

Quasi no stray light except in four spikes.

Transmission: g(x)Transmission: g(x) "xor" g(y)

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


Basic concepts: Fresnel arrays versus solid aperture

3000 Fresnel zones

Images of a point source by:

300 Fresnel zones Solid square aperture

luminosity scale:Power 1/4 to show spikes

The Fresnel Imager for Exoplanet Study, Pathways to Habitable Planets Barcelona September 17, 2009 6 Position in the field (resels)

1/4 field represented

Log dynamic

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

apodized prolate, order 0 masked

Numerical Fresnel propagation


"Against" Fresnel Arrays:

Chromaticity... But can be canceledby order -1 chromaticity after focus, Channel bandpass limitations: Δλ/λ= 15%

transmission efficiency to focus:6% to 10%

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

f = D2/8zλ

D

f = D2/8zλ

f


"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

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.

High dynamic range: 108 on compact objects, more with coronagraphy & postprocessing.

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


II. TestsDoes it work ?


2x2 cm array

I have it here,It's working.

live demoafter this session.

For those whoHaven't alreadySeen it…


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

116 zones, 8 x 8 cm26680 apertures"orthocircular" design.

F= 23 m at = 600 nm

Precision: 5m on holes positioning=> /70 on wavefront.

Achievements:

Diffraction limitedBroad band imaging (450-850nm)10-6 dynamic range

Photo T.Raksasataya

metal foil 100 m thick


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

Photo D.Serre

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


Metal foil9.7 105 apertures,Slightly apodized,696 Fresnel zones.

20 cm

Photo P.Deba


20x20 cm array: first light on stars

52 Cygni a-bVa= 4.2 vb= 8.7 sep= 6.4"

ALINEMENT PHASE images:

temporary 116 zones secondary(instead of 702 zones)

Tip-Tilt not yet implemented

STT 433 a-bVa=4.36 ; Vb=10.0 Sep= 15"

more results at our workshop in Nice next week


20 cm array goals

Assess performances on real sky objects:do better than other 20 cm aperture instruments.

Targets:High contrast objectsextended sources dense fieldsdeep sky


ESA study

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


III. Space MissionScience ?

Credibility ?


The Fresnel Imager Space Mission

Field optics telescope1/10th to 1/20th the diameterof Primary Array.

Dispersion correction: order -1 diffractionBlazed lens or concave grating,10 to 30 cm diameter

focal Instrumentation:Spectro-imagers

Thin membrane "Primary Array" module:

3m to 100m diameter, or more.


Rationale

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 conceptsλ/50 wavefront, at any λ => High Dynamic range from IR to UVmas angular resolution1000x1000 resel. fieldsSpectral resolution

Suitable for Exoplanetsand other fieds too…


Strategy, sky targets

- Start with UV domain?

- spectral lines in UV for: Photosynthesis break, O3, CH4, CO2 , auroras …

- polarimetry

- limited budget => limited aperture, but high resolution

- High quality wavefront at any wavelength

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


Space Mission: optical scheme

Focal instruments

5 km (for a 4m aperture)to 100 km (for a 30m aperture)

image plane 2 achromatic

pupil plane

Spacecraft 1

Spacecraft 2

image plane 1dispersed

Diffraction order 1: focused, but with chromatic aberration.

Diffraction order 0: unfocussedwill be focused by field optics, then blocked.

Field optics telescope

Chromatic correction:

Blazed Fresnel grating

Large "Primary Fresnel Array:Thin foil,4 to 30 m diameter, or more.

Order zero

blocked

Solar Baffle, to protect from sunlight

Focal instrum.


Targets: S/N on exoplanets in UV Signal / noise as a function of

1 Jupiter diameter planet 0.5 Jupiter diameter

Images & spectra of exoplanets:1 UA from solar type star, 10 Pc away

Signal / noise < 3Signal / noise > 3

Signal / noise > 30

Signal / noise < 3

Signal / noise > 3

4m aperture, 10h integration

spectral res. = 50 dynamic range of raw image: 2 10^-8

uv uv


Conclusion

Science cases: Exoplanets, and also

stellar physicscompact objectsreflection nebulaeextragalacticsolar system objectsobservation of the earth

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

Build up a proposal for a 2020 / 2025 launch

Web site: search with key words "fresnel imager Nice"http://www.ast.obs-mip.fr/users/lkoechli/w3/space_borne_page/page_congres.html

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)



Bonus slides


Circular Fresnel Zone Plate => PSF with isotropic rings

Image

Optical concept : Image formation

non linear luminosity scaleto show the rings.

Aperture

Isotropic rings


Géométrie "orthogonale pure" (2005)

4 % de la lumière focalisée

expansion 2D Cartésienne

Réseau de Fresnel ou

Interféromètre à forte densité d’ouvertures ?

1740 motifs individuels

Géométrie "ortho-circulaire" (2008)



The field vs spectral bandpas tradeoff

Chromatically aberrated beam at prime focus

Field delimited by field mirror

The chromatic correctordoes a good job,but it corrects only what it collects.


Fresnel Imager specifications

UV spectro-Imaging at High Dynamic Range4m aperture3 spectral bands Δλ/λ=20%: 2 in the UV, 1 in the visible λ/50 wavefrontspectral resolution λ/δλ=50angular resolution 7 to 25 mas depending on λfield 1000x1000 => 7 to 25 arc secondsraw dynamic range >108

- with 10 hours integrations time:jovian exoplanets with apertures of 1 m or moretelluric Exoplanets: only with 10 meters apertures or larger

- Other fields of astrophysics


Gen III prototype: Primary array

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 : 5mi.e. /30 on wavefront

R&T financed by CNES & STAE


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


eclipticque

sun

petite Lissajou

periode : 6 mois

1 eclipse en 6 ans, évitable

sun

éclipticqueTerre

Lune

L2

20

0 0

00

km

8°14°

Fresnelbaffle

Lingne de visée

Antenne RA fixe et GS fixe possible à partir de 100 000km de la Terre

• 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

Orbites et pointages

Moon

40°

SUN

Trajectoire typique montrant qu'au -delà de 100 000 km de la Terre,la MGA pointée comme le GS voit la Terre avec ses 40° d'ouvertur e

Moon

40°

SUN

Trajectoire typique montrant qu'au -delà de 100 000 km de la Terre,la MGA pointée comme le GS voit la Terre avec ses 40° d'ouvertur e


Résultats qualitatifs : dynamique mesure optique et simulation numérique

8 cm 116 zones

image Optique

8 cm 116 zonespropagation de Fresnel numerique

par tous les éléments optiques

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

Luminosité amplifiée x1000

propagation numerique de Fresnel développée pour tester de grands réseaux

Luminosité amplifiée x1000


Géométrie "orthogonale pure" (2005)


expansion 2D Cartésienne

Réseau de Fresnel ou

Interféromètre à forte densité d’ouvertures ?

1740 motifs individuels

Géométrie "ortho-circulaire" (2008)



Fields obtained with 2 exposures rotated 45°


scenarios

Date post:	27-Mar-2015
Category:	Documents
Upload:	robert-grady
View:	214 times
Download:	1 times

The Fresnel Imager for Exoplanet Study, Pathways to Habitable Planets BarcelonaSeptember 17, 20091...

Documents