Erick Bondoux, Sandra Bosio, Rijuparna Chakraborty, Wassila Dali-Ali, Antoine Labeyrie, Bruno Lacamp, Jerome Maillot, Denis Mourard, Paul Nunez, Jordi Pijoan, Rémi Prudhomme, Pierre Riaud, Martine Roussel, Arun Surya, Bernard Tregon, Thomas Houllier, Thierry Lepine, Patrick Rabou, André Rondi, Yves Bresson, David Vernet
Hypertelescopes are large optical interferometric arrays, employing many small mirrors and a miniature pupildensifier before the focal camera, expected to produce direct images of celestial sources at high resolution. Their peculiar imaging properties, initially explored through analytical derivations, had been verified with simulations before testing a full-size testbed instrument. We describe several architectures and optical design solutions and present recent progress made on the Ubaye hypertelescope experiment. Arecibo-like versions with a fixed spherical primary meta-mirror, or an active aspheric one, have a suspended focal beam combiner equipped for pupil-drift accommodation, with a field-mosaic arrangement for observing multiple sources such as exoplanetary systems, globular clusters or active galactic nuclei. We have developed a cable suspension and drive system with tracking accuracy reaching a millimeter at 100m above ground.
A. Labeyrie, F. Allouche, D. Mourard, F. Bolgar, R. Chakraborty, J. Maillot, N. Palitzyne, J. R. Poletti, J.-P. Rochaix, R. Prud'homme, A. Rondi, M. Roussel, A. Surya
For information-rich direct images at high resolution, hypertelescopes combine light from a sparse array of
many subapertures, using pupil densification. Among the possible architectures, the Arecibo-like spherical class
has fixed mirrors arrayed as elements of a common spherical locus, matching approximately the natural curvature
of a crater or valley. A focal gondola suspended on the focal sphere, is tracking the primary star image, and several
more can be added for independent observations of di.erent sources. Since no delay lines are needed, hundred of
mirrors can be used for reaching the theoretical information gain with respect to fewer apertures. The aperture
size of such instruments may range from 50 to perhaps 1200m at available terrestrial sites. As an example of their
broad science capabilities, we have simulated the resolved and spectro-imaging- of an exoplanet transiting across
the disk of its parent star, achievable with adaptive optics. Faint cosmological sources may also become observable
if a Laser Guide Star can be fitted. We describe the current construction and in situ opto-mechanical testing
of a 57m hypertelescope, later expandable to 200 with 100 or more sub-apertures. The preliminary operating
experience gained in a year, without stellar fringes yet, indicates the likely feasibility of larger versions at suitable
sites. Labeyrie et al., (this conference) discuss an "Extremely Large Hypertelescope" (ELHyT) having 1200m
sparse aperture and, at similar cost, a larger collection area and higher limiting magnitude than a 40m ELT.
The hypertelescope construction initiated in the Southern Alps (Labeyrie et al., this conference) has
provided some preliminary operating experience indicating that larger versions, up to perhaps
1200m, are probably feasible at suitable sites. The Arecibo-like architecture of such instruments
does not require the large mount and dome which dominate the cost of a 40m ELT. For the same
cost, an "Extremely Large Hyper Telescope” ( ELHyT) may therefore have a larger collecting area.
It may thus in principle reach higher limiting magnitudes, both for seeing-limited and, if equipped
with a Laser Guide Star and adaptive phasing, for high-resolution imaging with gain as the size ratio,
i.e. about 30 with respect to a 40m ELT. Like the radio arrays of antennas, such instruments can be
grown progressively. Also, they can be up-graded with several focal gondolas, independently
tracking different sources. Candidate sites have been identified in the Himalaya and the Andes. We
describe several design options and compare the science achievable for both instruments, ELTs and
ELHyTs. The broad science addressed by an ELHyT covers stellar chromospheres, transiting exoplanets
and those requiring a high dynamic range, achieved by array apodization or coronagraphy.
With a Laser Guide Star, it extends to faint compact sources beyond the limits of telescopes having a
smaller collecting area, supernovae, active galactic nuclei, gamma ray bursts. The sparse content of
remote galaxies seen in the Hubble Deep Field appears compatible with the crowding limitations of
an ELHyT having 1000 apertures.
The Carlina hypertelescope is a planned sparse aperture 100 m telescope with pupil densification. The telescope
has a spherical primary with segments located in a valley between mountains, and additional optical elements in a
gondola suspended in eight cables some 100 m above the primary mirror. The resolution is about 1.2×10-3 arcsec.
It is imperative that the position and attitude of the gondola be maintained within tight tolerances during
observation and star tracking. The present design has servo-controlled winches on the ground for control of
the gondola via the cables. An integrated model of the system, including optics, cables, gondola, position and
attitude control system, and wind disturbances has been set up. The structural and control models are linear.
Calculations in the frequency domain and simulations in the time domain show that the performance of the
telescope with the present design seems adequate for short exposures. However, for long-exposure operation, the
gondola stability should be improved by about two orders of magnitude. Recommendations are given on possible
approaches for performance improvement.
In a previous paper,1 we discussed an original solution to improve the performances of coronagraphs by adding,
in the optical scheme, an adaptive hologram removing most of the residual speckle starlight.
In our simulations, the detection limit in the flux ratio between a host star and a very near planet (5λ/D)
improves over a factor 1000 (resp. 10000) when equipped with a hologram for cases of wavefront bumpiness
imperfections of λ/20 (resp. λ/100).
We derive, in this paper, the transmission accuracy required on the hologram pixels to achieve such goals. We
show that preliminary tests could be performed on the basis of existing technologies.
For imaging faint and complex sources at high angular resolution, hypertelescopes (direct-imaging many-aperture
interferometers using a densified pupil) gain sensitivity with respect to few-aperture interferometers and to Fizeau
interferometers. Steps are taken to expand the Carlina-Proto technical prototype built at Observatoire de Haute-Provence,
18m in aperture size, and to define a larger (100-200m) Carlina-Science version, incorporating 100 or more small
apertures. Following initial observing by Speckle Interferometry, adaptive co-pistoning is expected to become available,
using "Dispersed Speckle" piston sensing on bright stars, and a modified Laser Guide Star on faint (mv > 25) fields.
"Extremely Large Hypertelescope" versions of such instruments, with aperture size beyond a kilometer, are considered
for deep-field imaging on cosmological sources. These can be interferometrically coupled with ELTs, or arrays of
telescopes, at sites such as the Macon range (Andes) considered by ESO for its E-ELT. Space versions are proposed to
ESA and NASA.
Our work deals with the improvement of "light harvesting" in organic photovoltaic cells by using photonic nanostructures. We have theoretically studied a periodically nanostructured poly
(3-hexylthiophene)(P3HT)/6,6-phenyl C61-butyric acid methyl ester (PCBM) thin film in order to increase its absorption in the near infrared spectral range. We have used a software, based on the FDTD (Finite-Difference Time-Domain) method, to calculate the absorption of light in organics solar cells. We have also considered the nanostructured photoactive layer of solar cells as a photonic crystal
and we have computed band diagrams to study the dispersion curves of this structure. We have first studied a blend (bulk heterojunction) with the same proportions of P3HT and PCBM. This material
provide at this time the best results in terms of photovoltaic efficiency. Nevertheless, in order to improve the
transport of charges to the electrodes, a model with P3HT and PCBM independently nanostructured (ordered
heterostructure) was also used. Moreover, this periodic nanostructuration allows "slow Bloch modes" to be coupled
inside the device with a low group velocity of electromagnetic waves. Thus, the interaction duration between light and
organics materials is improved.
The P3HT/PCBM photonic crystal parameters have been adjusted to maximize the density of Bloch modes and to obtain
flat dispersion curves. We have found that the light matter interaction was strongly enhanced which resulted in a 35.6%
increase of absorption in the 600 nm to 700 nm spectral range. In order to realize nanostructured organic solar cells, we
are also developing an experimental prototype, based on a patented process, which allows to nanostructure several kinds of polymers.
In the way of major new instruments for ground-based optical astronomy, maximizing the science favors a large
hypertelescope. If equipped with adaptive optics and a laser guide star, it can provide direct high-resolution images of
faint extra-galactic and cosmological sources. The signal/(photon noise) ratio is theoretically higher than with
interferometer schemes relying upon aperture synthesis, using a few large apertures to reconstruct images. The crowding
limit on complex objects, the direct-imaging field, and the dynamic range are also improved with many small apertures.
The adaptive phasing of hypertelescopes, achievable on bright stars with modified wave sensing techniques such as
"dispersed speckle" analysis, is also achievable on very faint sources with a modified version of a laser guide star. This
makes large hypertelescopes capable of observing cosmological deep fields of faint galaxies. Pending space versions,
the size of which can in principle reach hundreds and thousands of kilometers, terrestrial hypertelescopes limited in size
to one or two kilometers can be built at suitable sites and used efficiently from ultra-violet to millimeter wavelengths.
Some sites can allow the coupling of a hypertelescope with an ELT, an alternate option which can also be efficient for
imaging deep fields with a high-resolution.
Smithsonian Astrophysical Observatory (SAO) has set up a program to study coronagraphic techniques. The program consists of the development of new fabrication methods of occulter masks, characterization of the manufactured masks, and application of the masks to study speckle reduction technique. Our occulter mask fabrication development utilizes a focused ion beam system to directly shape mask profiles from absorber material. Initial milling trials show that we can shape nearly Gaussian-shaped mask profiles. Part of this development is the characterization of absorber materials, poly(methyl methacrylate) doped with light-stable chromophores. For the characterization of the masks we have built a mask scanner enabling us to scan the transmission function of occulter masks. The real mask transmission profile is retrieved applying the maximum entropy method to deconvolve the mask transmission function from the beam profile of the test laser. Finally, our test bed for studying coronagraphic techniques is nearing completion. The optical setup is currently configured as a classical coronagraph and can easily be re-configured for studying speckle reduction techniques. The development of the test bed control software is under way. This paper we will give an update of the status of the individual program elements.
SAO has set up a testbed to study coronagraphic techniques, starting with Labeyrie's multi-step speckle reduction technique. This technique expands the general concept of a coronagraph by incorporating a speckle corrector (phase and/or amplitude) in combination with a second occulter for speckle light suppression. Here we are describing the initial testbed configuration. In addition, the testbed will be used to test a new approach of the phase diversity method to retrieve the speckle phase and amplitude. This method requires measurements of the speckle pattern in the focal plane and slightly out-of-focus. Then we will calculate a phase of the wave from which we can derive a correction function for the speckle corrector. Furthermore we report results from a parallel program which studies new manufacturing methods of soft-edge occulter masks. Masks were manufactured using the spherical caps method. Since the results were not satisfying we also investigated the method of ion beam milling of masks. Here we will present the outline of this method. Masks manufactured with both methods will be fully characterized in our mask tester before their use in the testbed.
Laboratory testing is undertaken for exploring the feasibility of "Laser Trapped Mirrors". These involve a soft membrane or two-dimensional array of nano-spheres having sub-micrometer thickness. It is constrained to an accurate optical figure by radiation pressure trapping in polychromatic standing waves formed by a pair of diverging and counter-propagating laser beams. The optical design and resulting properties are discussed as well as the mission aspects for several architectures of telescopes and hypertelescopes at scales from meters to hundreds of kilometers.
The Submillimeter Probe of the Evolution of Cosmic Structure (SPECS) is a space-based imaging and spectral ("double Fourier") interferometer with kilometer maximum baseline lengths for imaging. This NASA "vision mission" will provide spatial resolution in the far-IR and submillimeter spectral range comparable to that of the Hubble Space Telescope, enabling astrophysicists to extend the legacy of current and planned far-IR observatories. The astrophysical information uniquely available with SPECS and its pathfinder mission SPIRIT will be briefly described, but that is more the focus of a companion paper in the Proceedings of the Optical, Infrared, and Millimeter Space Telescopes conference. Here we present an updated design concept for SPECS and for the pathfinder interferometer SPIRIT (Space Infrared Interferometric Telescope) and focus on the engineering and technology requirements for far-IR double Fourier interferometry. We compare the SPECS optical system requirements with those of existing ground-based and other planned space-based interferometers, such as SIM and TPF-I/Darwin.
The coronagraphic techniques serving to reject most light from a star, when trying to image a nearby planet, can be pushed with an adaptive holographic element. Located after the coronagraph, it can in principle remove most of the residual star light by adding a phase-shifted holographic reconstruction of it . The scheme is also usable within each sub-aperture of a diluted hypertelescope array, sufficiently large to resolve details of an exo-Earth. A possible panoramic version of the previously mentioned Exo-Earth Imager is shaped as a virtual bubble of 400 km diameter , consisting of thousands of 3-meter mirrors, free-flying and arranged co-spherically. The half-size focal sphere is explored by beam combiners, one for each exo-Earth observed within tens of parsecs. Each beam-combiner includes a kilometer-sized corrector of spherical aberration at F/2, which is also diluted and consisting of small free-flyers. The instrument is expected to provide direct coronagraphic images of exo-Earths, resolved in 50x50 resels, with enough dynamic range obtained in 30mn exposures to search colored features and their seasonal variations, indicative of photosynthetic life .
The prospect of large astronomical mirrors in space, made of a thin trapped membrane or a mono-layer of sub-micron particles, raises appreciable theoretical and technological problems at the frontiers of physics, chemistry and materials science. We explore various approaches towards testing different materials in the laboratory at temperatures matching those of a sun-shielded structure in interplanetary space (80- 100 K). Trap loading and damping are among the issues considered, as well as the pumping of particles towards a central fringe, using polychromatic light.
The interferometric coupling of an ELT with a large multi-aperture imaging interferometer can open new areas of science on compact objects. Numerical simulations indeed show that the combined image retains respectively the high luminosity and the high angular resolution of both instruments. The Canarian site envisaged for the Euro-50 is adjacent to the large Caldera de Taburiente crater, a favorable site for an optical and dilute form of the Arecibo radio-telescope. Our preliminary study indicates that the effective aperture size can exceed 1600 m if a balloon or kite is used to carry the focal optics, also receiving a coude beam from the Euro-50 if coupled. In spite of inherent limitations regarding field size and crowding, the 50 micro-arcsecond resolution thus achievable in visible snapshot images is of interest for stellar physics, active galactic nuclei and deep cosmological imaging of remote galaxies.
A concept is presented for a 10-meter sparse aperture hypertelescoep to detect extrasolar planets by direct imaging from the ground through the turbulent atmosphere. The telescope achieves high dynamic range with good image quality very close to bright stellar sources using pupil densification techniques and real-time atmospheric correction. Active correction of the perturbed wavefront is greatly simplified by several unique design features of the telescope: 1) use of an array of 19 small subaperture flat mirrors, 2) mounting the flats on a steerable parabolic truss structure, 3) operating in the near-IR, and 4) making the subaperture flats comparable in size to the seeing cells. These features relax the requirements on the wavefront sensing and control system. This paper describes the general concept. The details of design and implementation will be addresed separately.
'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.
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.
Only in the recent years did it become realized that multi-aperture interferometric arrays could provide direct snapshot images and coronagraphic images in a non-Fizeau mode. Whereas homothetic mapping of entrance pupil to exit pupil is useless when the aperture is higly diluted, a "densified-pupil" or "hypertelescope" imaging mode can concentrate most light into a high-resolution Airy peak. In addition to the luminosity gain, there is a contrast gain particularly valuable for stellar coronagraphy and exoplanets finding. The current VLTI is able to combine light from two telescopes coherently. In subsequent phases, a combiner is planned for applying closure phase with up to eight telescopes (UT and AT). The small number of apertures currently considered at the VLTI, does not take full advantage of hypertelescope imaging, but still performs significantly better than other observing modes (+3.8mag gain in comparison with Fizeau mode). We propose some possible optical scheme for a densified-pupil combiner for the VLTI. Beyond its science value, the proposed instrument can serve as a precursor for many-element post-VLTI hypertelescopes.
This paper presents the optical layout of the REGAIN beam combiner including the optical delay line LAROCA with its variable curvature mirror, the field rotator devices, the image and pupil tracking systems and the dedicated visible spectrography. Preliminary studies of foreseen improvements, such as adaptive optics, IR spectrograph and addition of a third telescope, will be discussed.
The 'densified pupil' imaging mode, now developed for large multi-telescope interferometers, will provide images and spectro-images of compact objects, directly at the recombined focus. It requires telescopes of identical sizes and allows trading field for luminosity. The principle is applicable to dilute arrays of small, medium or large telescopes, 0.2m to beyond 25m in size, using similar recombination systems and cophasing methods. Design solutions are discussed for each case, and particularly for the medium-scale Optical Very Large Array of 27 telescopes, spanning one kilometer, studied at Haute Provence. We build a prototype 1.5m OVLA element. Solutions for the beam recombiner are discussed, and will be assessed with a testbed interferometer involving 27 small mobile heliostats forming a 100 or 300m ring. Larger versions of the OVLA, employing unit telescopes of 10 to 25m, are also considered, in connection with the large telescope study initiated by the Lund group. In space, arrays of free-flying telescopes can in principle resolve continental detail of exo-planets. Equipped with additional out-rigger mirrors providing baselines of 10,000 to 100,000 kilometers, such space arrays can in principle provide images of pulsars and naked neutron stars.
A prototype telescope for the optical very large array (OVLA) project is under construction at the Observatoire de Haute-Provence (OHP), France. The OVLA will b a long- baseline optical interferometer of 27 mobile 1.5m- telescopes. In 2000, the functioning of the OVLA prototype will be tested outdoors alongsite the two other telescopes of the GI2T to form a 3-telescope interferometer. Firstly, we briefly present the design of this telescope highlighting its unusual characteristics, which include a spherical mount and a thin active primary mirror. We had to study a specific control system for driving mount and for the active mirror cell. Hardware and software design of these two systems are also presented, as well as some test results. Lastly, we propose a complete electronic architecture for the fully equipped OVLA prototype telescope. The telescope system is partitioned into elementary distinct subsystems each controlled by a small embedded calculator linked to each other by an addressable serial bus. With this kind of architecture, the telescope is fully autonomous. Thus the future installation of the OVLA prototype telescope at the GI2T site should be easier, as well as the installation of a large interferometer such as OVLA where 27 telescopes are expected.
Interferometric arrays of many large telescopes will follow the current precursor interferometers. A few dozen telescopes, equipped with adaptive optics for intra and inter-aperture phasing, mobile on a 1 - 10 km terrestrial platform, can provide snapshot images having 10-4 to 10-5 arc-second resolution. On visible objects as faint as mv equals 25, blind phasing is achievable with the help of laser guide stars on each telescope. The corresponding science is particularly rich and relevant to current issues in stellar physics and cosmology. Following the completion and test of a prototype 1.5 meter telescope, specifically designed for a 27- element interferometric array, larger component telescopes of 8 to 10 m may become buildable at a sufficiently low cost for affordable arrays. A preliminary design concept is presented. In space, arrays of free-flying telescopes currently studied by NASA and ESA, can in principle provide a better limiting magnitude and longer baselines, reaching perhaps 100 km. The current pace of space projects however makes it likely that large ground-based interferometers will be in use before space equivalents.
We describe a concept for an interferometric space mission dedicated to global (wide-angle) astrometry. The GAIA satellite contains two small (baseline APEQ 3 m) optical interferometers of the Fizeau type, mechanically set at a large and fixed angle to each other. Each interferometer has a field of view of about one degree. Continuous rotation of the whole satellite provides angular connections between the stars passing through the two fields of view. Positions, absolute parallaxes and annual proper motions can be determined with accuracies on the 20 micro-arcsec level. The observing programme may consist of all objects to a limiting magnitude around V = 15-16, including 50 million stars. The GAIA concept, which has been proposed for a Cornerstone Mission within the European Space Agency's long-term science programme, is based on the same general principles as the very successful ESA Hipparcos mission, but takes advantage of the much higher resolution and efficiency permitted by interferometry and modern detector techniques.
The operation of the GI2T two-telescope inteferometer, a precursor to the Optical Very Large Array (OVLA), is described. The GI2T, consisting of two 1.5-meter telescopes, is beginning to provide high-resolution data on stellar envelopes. It is also used to test components for the 27-telescope OVLA. Prototypes of the metrology system, the compact telescopes, and a walking robot are under construction. These developments are preparatory to the construction of the OVLA and its moon-based version with three to 33 telescopes. Following tests of tracking and vibration performance, the prototype 1.5-m telescope will operate with the GI2T.
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