Download L. Koechlin Optical principles of diffraction focusing

Preparing the way
To space borne
Observatoire de Nice
September 23-25
2009
Fresnel Imagers
Optical principles of diffraction focussing,
Preparing the way to space borne Fresnel imagers
Nice
September 23-25, 2009
This workshop is organized and financed by:
Optical principles of diffraction focussing,
Preparing the way to space borne Fresnel imagers
Nice
September 23-25, 2009
Plan of the workshop
Session A
Session B
Session C
Session D
Session E
Session F
The basics of Fresnel imaging
Validation of concepts & performance assessment
The baselines of a Fresnel Imager space mission
Science cases in general
Exoplanets in particular
Strategies for a successful space mission proposal
Optical principles of diffraction focussing,
Preparing the way to space borne Fresnel imagers
Nice
September 23-25, 2009
Preparing the way
To space borne
Observatoire de Nice
September 23-25
2009
Fresnel Imagers
Optical principles of diffraction focussing,
Preparing the way to space borne Fresnel imagers
Nice
September 23-25, 2009
Optical principles of diffraction
focusing
Laurent Koechlin
Laboratoire d'Astrophysique de Toulouse-Tarbes
Université de Toulouse, CNRS
Optical principles of diffraction focussing,
Preparing the way to space borne Fresnel imagers
Nice
September 23-25, 2009
Optical concepts: Light focalization
Lens
focus
Plane wavefront
Spherical wavefront
Lens (or miror): focusing by refraction (or reflexion)
Fresnel array: focusing by diffraction …
Binary transmission function g(x)
Order 0 :
plane wave
Order 1 :
convergent
focus
Optical principles of diffraction focussing,
Preparing the way to space borne Fresnel imagers
Nice
September 23-25, 2009
Light focalization
x
focus
f
Binary transmission function g(x)
Optical principles of diffraction focussing,
Preparing the way to space borne Fresnel imagers
Nice
September 23-25, 2009
Image formation
Circular Fresnel Zone Plate => PSF with isotropic rings
Image
Aperture
Binary transmission function g(r)
Isotropic
rings
non linear luminosity scale
to show the rings.
Optical principles of diffraction focussing,
Preparing the way to space borne Fresnel imagers
Nice
September 23-25, 2009
Image formation
Rings radii:
For zmax Fresnel zones
Binary transmission function g(r)
f
Optical principles of diffraction focussing,
Preparing the way to space borne Fresnel imagers
Nice
September 23-25, 2009
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.
Transmission:
g(x) "xor" g(y)
Optical principles of diffraction focussing,
non linear luminosity scale,
to show the spikes.
Preparing the way to space borne Fresnel imagers
Nice
September 23-25, 2009
Image formation
Second source in the field:
Optical principles of diffraction focussing,
Preparing the way to space borne Fresnel imagers
Nice
September 23-25, 2009
Image formation
Second source in the field:
Optical principles of diffraction focussing,
Preparing the way to space borne Fresnel imagers
Nice
September 23-25, 2009
Light focalization: transmission function
x
focus
f
Binary transmission function g(x)
Optical principles of diffraction focussing,
Preparing the way to space borne Fresnel imagers
Nice
September 23-25, 2009
Light focalization: efficiency
Order -1
4 to 8%
Image
Order 0
40%
Aperture
Incident light
100%
Order 1
4 to 8%
Transmission:
Transmission:
g(x)g(x)
"xor" g(y)
or "orthocircular"
Optical principles of diffraction focussing,
Preparing the way to space borne Fresnel imagers
Nice
September 23-25, 2009
Light focalization: optimizing
efficiency
Fresnel zone plate
or
High density interferometer ?
Pure orthogonale geometry (2005)
4 % of incident light focused
1740 motifs individuels
ortho-circular Geometry (2008)
6 % of incident light focused
Optical principles of diffraction focussing,
Preparing the way to space borne Fresnel imagers
Nice
September 23-25, 2009
Light focalization: optimizing
efficiency
4%
6 to 8%
The bars contribute to focalization
Phase shift
within one aperture:
2π OPD / λ
Optical principles of diffraction focussing,
Preparing the way to space borne Fresnel imagers
Nice
September 23-25, 2009
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
Optical principles of diffraction focussing,
Preparing the way to space borne Fresnel imagers
Nice
September 23-25, 2009
Dynamic range & resolution
Example for 300 zones (720 000 apertures)
Will be presented in
more detail by
Denis Serre
apodized prolate,
order 0 masked
Log dynamic
Numerical
Fresnel propagation
Code & results
By Denis Serre
Position in the field (resels)
Optical principles of diffraction focussing,
1/4 field represented
Preparing the way to space borne Fresnel imagers
Nice
September 23-25, 2009
Examples of fields obtained by combining 2 exposures rotated 45°
4.5 λ/D
<6 10-6
8 λ/D
<2 10-6
Optical principles of diffraction focussing,
Preparing the way to space borne Fresnel imagers
Nice
September 23-25, 2009
"Against" Fresnel Arrays:
f
Chromaticity... But can be canceled
by order -1 chromaticity after focus,
Channel bandpass limitations: Δλ/λ= 15%
C
Square aperture:
f = C2/8 zmax λ
transmission efficiency to focus:
6% to 10%
1km to 100km focal lengths => Formation flying in space
f = D2/8zλ
Optical principles of diffraction focussing,
Preparing the way to space borne Fresnel imagers
Nice
September 23-25, 2009
"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.
Optical principles of diffraction focussing,
Preparing the way to space borne Fresnel imagers
Nice
September 23-25, 2009
Optical scheme of dispersion correction in Fresnel
Arrays
Primary Fresnel array
e.g. 20 m
Diffractive lens
at order -1
e.g. 10 cm
Field Optics
10 to 100 km e.g. 2m
Converging lens
e.g. 10 cm
mask
Order 0 rays
Focal
Instrumentation
image plane 1
dispersed
Img. plane 2:
achromatic
pupil plane
Order 1 rays, focused
by primary array
Spacecraft 1
holding primary Fresnel array
Optical principles of diffraction focussing,
Spacecraft 2 holding focal instrumentation
Preparing the way to space borne Fresnel imagers
Nice
September 23-25, 2009
The field vs spectral bandpass 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.
Optical principles of diffraction focussing,
Preparing the way to space borne Fresnel imagers
Nice
September 23-25, 2009
The field vs spectral bandpass tradeoff
Will be presented in
more detail by
Paul Deba
Illustration and formulas by Paul Deba
Optical principles of diffraction focussing,
Preparing the way to space borne Fresnel imagers
Nice
September 23-25, 2009
Conclusion
We have a concept that opens the way to very large apertures in space
It can be validated on ground based facilities only for small apertures
Large apertures need to be tested in space,
ut it's hard to get a large mission approved if ti's based on a new technology
Build up a proposal for a 2020 / 2025 launch
Science cases:
Exoplanets
stellar physics
compact objects
reflection nebulae
extragalactic
solar system objects
observation of the earth
Optical principles of diffraction focussing,
Preparing the way to space borne Fresnel imagers
Nice
September 23-25, 2009
Preparing the way
To space borne
Observatoire de Nice
September 23-25
2009
Fresnel Imagers
Optical principles of diffraction focussing,
Preparing the way to space borne Fresnel imagers
Nice
September 23-25, 2009