The Fresnel Diffractive imager is an innovative concept of distributed space telescope, for high resolution (milli arc-seconds) spectro-imaging in the IR, visible and UV domains. This paper presents its optical principle and the science that can be done on potential astrophysical targets.
The novelty lies in the primary optics: a binary Fresnel array, akin to a binary Fresnel zone plate. The main interest of this approach is the relaxed manufacturing and positioning constraints. While having the resolution and imaging capabilities of lens or mirrors of equivalent size, no optical material is involved in the focusing process: just vacuum. A Fresnel array consists of millions void subapertures punched into a large and thin opaque membrane, that focus light by diffraction into a compact and highly contrasted image. The positioning law of the aperture edges drives the image quality and contrast.
This optical concept allows larger and lighter apertures than solid state optics, aiming to high angular resolution and high dynamic range imaging, in particular for UV applications. Diffraction focusing implies very long focal distances, up to dozens of kilometers, which requires at least a two-vessel formation flying in space.
The first spacecraft, “the Fresnel Array spacecraft”, holds the large punched foil: the Fresnel Array. The second, the “Receiver spacecraft” holds the field optics and focal instrumentation. A chromatism correction feature enables moderately large (20%) relative wavebands, and fields of a few to a dozen arc seconds.
This Fresnel imager is adapted to high contrast stellar environments: dust disks, close companions and (we hope) exoplanets. Specific to the particular grid-like pattern of the primary focusing zone plate, is the very high dynamic range achieved in the images, in the case of compact objects.
Large stellar photospheres may also be mapped with Fresnel arrays of a few meters opertaing in the UV. Larger and more complex fields can be imaged with a lesser dynamic range: galactic or extragalactic, or at the opposite distance scale: small solar system bodies. This paper will briefly address the optical principle, and in more detail the astrophysical missions and targets proposed for a 4-meter class demonstrator:
– Exoplanet imaging, Exoplanet spectroscopic analysis in the visible and UV,
– Stellar environments, young stellar systems, disks,
The Fresnel Diffractive Imager is based on a new optical concept for space telescopes, developed at Institut de Recherche en Astrophysique et Planétologie (IRAP) in Toulouse, France. We propose it for space missions dedicated to science cases in the Ultra-Violet with aperture ranges from 6 to 30 meters. Instead of a classical mirror to focus light, this concept uses very light-weight diffractive optics : the Fresnel array. Our project has already proved its performances in terms of resolution and high dynamic range in the laboratory, in the visible and near IR. It has been tested successfully on real astrophysical sources from the ground.
At present, the project has reached the stage where a probatory mission is needed to validate its operation in space. In collaboration with institutes in Spain and Russia, we will propose a mission to the Russian space agency Roscosmos, to board a small prototype Fresnel imager on the International Space Station (ISS) for a UV astronomy program.
We have improved the Fresnel array design to get a better Point Spread Function (PSF), 2 different ways. Numerical simulations have first allowed us to confirm these optical improvements, before manufacturing the diffractive optics and using them for new lab tests.
In our previous setups, the opaque Fresnel zones in the primary Fresnel array (playing the role of the telescope objective) were maintained with an orthogonal bars mesh, following the pseudo-period of the Fresnel zones. We show that the PSF improves when these bars are regularly spaced. Furthermore, the optical system is apodized to get a better peaked PSF, and increase its high contrast performances.
In our case, to apodize a binary mask the solution is to modulate the Fresnel zones in relative thickness ratio (opaque versus void), thus driving the local light transmission ratio. In earlier implementations, our Fresnel arrays were apodized with a circularly symmetric law, but an orthogonal apodization law is more efficient. That is why we are developing this particular type of apodized square aperture Fresnel arrays.
We propose a new concept of diffractive optics: Fresnel arrays, for a 4 m aperture space telescope in the UV
Fresnel arrays focus light by diffraction through a very thin binary mask. They form images optically and
deliver very high quality wavefronts, specially in the UV. Up to 8% of the incident light is focussed, providing
high angular resolution and high contrast images of compact objects.
Due to their focal lengths of a few kilometers in the UV, large Fresnel arrays will require two spacecraft
in formation flying, but with relatively tolerant positioning. Diffraction focusing is also very chromatic; this
chromatism is corrected, allowing relatively broad (30 to 100 nm) spectral channels in the 120-350 nm range.
A 4 m aperture Fresnel imager providing 7 to 10 milli arc seconds resolution is very competitive for imaging
compact and high contrast objects such as protoplanetary disks and young planetary systems, AGNs, and deep
We have developed prototypes to validate the optical concept and related technologies : first a laboratory
setup, then a 20 cm aperture ground-based prototype, which provides high contrast and diffraction limited images
of sky objects in the visible and close IR. A new laboratory prototype is also being prepared for validation in
the 250 - 350 nm wavelength range.
This paper presents the results of a Fresnel Interferometric Array testbed. This new concept of imager involves
diffraction focussing by a thin foil, in which many thousands of punched subapertures form a pattern related
to a Fresnel zone plate. This kind of array is intended for use in space, as a way to realizing lightweight large
apertures for high angular resolution and high dynamic range observations. The chromaticity due to diffraction
focussing is corrected by a small diffractive achromatizer placed close to the focal plane of the array.
The laboratory test results presented here are obtained with an 8 centimeter side orthogonal array, yielding
a 23 meter focal length at 600 nm wavelength. The primary array and the focal optics have been designed and
assembled in our lab. This system forms an achromatic image. Test targets of various shapes, sizes, dynamic
ranges and intensities have been imaged. We present the first images, the achieved dynamic range, and the
This paper presents progress made regarding the field to resolution ratio for aperture synthesis interferometers. In order to overcome a limit established for the field to resolution ratio of interferometric arrays, we propose an interferometer configuration which allows a better coverage of the spatial frequency plane. This setup requires large sub-apertures, which can be built more easily with a diffractive Fresnel plates than with large mirrors. We compare a dense array of 9 Fresnel sub-apertures, which gives a snapshot field-resolution ratio of 400, versus a sparse array of 150 small apertures, which yields a field-resolution ratio of 150.
'Densified-pupil multi-aperture imaging arrays', also called hypertelescopes, provide a path towards rich images obtained directly at the focal plane. They typically involve a large Fizeau arrangement, with a small attached 'pupil densifier' serving to gain luminosity at the expense of field. At scales ranging from kilometers to perhaps a million kilometers, such architectures appear of interest for stellar physics, galaxies, cosmology, and neutron star imaging with the larger sizes. Ground testing is initiated and space versions are proposed, particularly to NASA for its Terrestrial Planet Finder. The coronagraphic imaging achievable with this space version is expected to improve the detection sensitivity to attenuating the sky background contribution. Subsequent laser versions can in principle resolve the 'green spots' on an Earth seen at several parsecs. Current design work for a precursor array of 'flying mirrors' driven by solar sails in geostationary orbit will be presented.
This paper presents a theoretical limit to the field/resolution ratio for the imaging mode of aperture synthesis interferometers. This limit is a function of both the number of apertures in the interferometric array and the dynamic range of the visibility and phase data. It does not depend on the optical setup of the instrument. This work is based on the theory of information.
The 8-m class telescopes are now in full operation, while 100-m
baseline interferometers (VLTI, KeckI) are starting routine
operation too. A working group from the French high angular
resolution community tried to identify what could be our
post-VLT/VLTI instruments after 2010. Possible future instruments,
ground or space-based, can be split into three main categories:
Extremely large filled aperture telescopes, diluted
interferometric arrays for direct imaging, and diluted
interferometric arrays for aperture synthesis imaging. These
concepts are compared in terms of observing capabilities and
performances (spatial resolution, field of view, imaging
capability, sensitivity, photometric dynamical range, etc.),
technological issues (adaptive optics, phasing, instrument mount,
etc.) and R&D priorities.
We present the general architecture of the GI2T/REGAIN control system. Based on a Graphical User Interface and different client-server communications, the system has to control both telescopes, the delay line, the beam-combiner, the data acquisition system and the real-time processing used as fringe tracker. We also describe in details the implementation of a real-time fringe tracker based on 4 monochromatic images and which used the fractional excess algorithm. Numerical simulations are shown. The control system is also dedicated to the acquisition of all the relevant data for the visibility calibration. We will also describe in details the data reduction package that provides the corrected visibilities. This architecture is very general and robust and has been developed having in mind that GI2T/REGAIN should be used by a wide community of astronomers.
Our goal is to improve fringe tracking in ground-based Michelson interferometry in order to reach fainter limiting magnitudes, and lower fringe visibility thresholds. The classical technique is the Fourier analysis of dispersed fringes (peak detection). It can be regarded as a maximum likelihood estimator. Although such an estimator is optimal by complying with Cramér-Rao bound rule, it does not use a priori information about the optical path difference (OPD) to be measured. We introduce a new signal analysis procedure based on the OPD drifts measured at the GI2T interferometer: if the signal is autocorrelated, then it would be possible to use a linear estimator giving a likelihood function from previous OPD values, reducing the noise in fringe Fourier analysis. Keywords: fringe tracking, photon counting, Fourier analysis, autoregressive modeling
We plan to build a photon counting camera able to yield photo-event coordinates (x,y, and t) at maximum rates superior to a million per second with a high temporal resolution (2.6 μs) and a 512 X 592 field. Ground-based interferometric techniques (single or multi-aperture) require detectors providing short frames, to deal with atmospheric turbulence. In order to reach the high signal-to-noise ratios required for imaging capabilities with aperture synthesis, such photon counting detectors are required.
The analysis of the imaging properties of an interferometric device essentially depends on the parameters that govern the Fourier synthesis operation: extension and density of the experimental frequency coverage (resolution, field, robustness), size and nature of the input errors (input data for the study of the error propagation). Among these parameters, the Field- to-Resolution Ratio (FRR) proves to play a key role, in particular, for array configuration optimization: the number of contiguous elements that can be resolved in the synthesized field is bounded by (FRR). The robustness of the corresponding reconstruction process is of course involved in the precise definition of this concept. The corresponding analysis is based on a new method of Fourier synthesis (WIPE), in which all these aspects are clearly specified (WIPE reminds of CLEAN, a well-known deconvolution technique in astronomy).
The Very Large Telescope Interferometer (VLTI) is one of the operating modes of the VLT. In addition to consisting of the four stationary 8-meter-diameter telescopes, it includes a number of movable Auxiliary Telescopes which both complement the (u,v) plane coverage of the large telescopes and provide a powerful interferometric facility by itself (available 100 percent of the time). The current plans for the implementation of the VLTI are described. These plans will be finalized after the choice of the VLT site in 1990.