LISE

LABORATOIRE D'INTERFEROMETRIE STELLAIRE ET EXOPLANETAIRE(LISE)

Laboratory of Stellar and Exoplanetary Interferometry

Antoine Labeyrie's special page

The research group is involved in the study and construction of  «hypertelescopes» , a class of giant diluted telescopes expected to produce much sharper images of stars, their planets,  and other celestial sources emitting at optical wavelengths. Its members work at several sites, including the Haute Provence observatory,  the Observatoire de la Cote d’Azur in Grasse, and the Institut d’Astrophysique in Paris.  Its technical and fabrication facilities are located at the two former sites.

Artist concept of a hypertelescope in space.  A flotilla of mirrors concentrate light from the observed star onto the focal satellite, seen in the foreground at left.

Perspective:

Bigger eyes for the emerging mega-brain of  planet Earth

With its emerging mega-brain, in the form of billions of human brains becoming connected by wires and computers, planet Earth will increasingly observe its cosmic environment with the giant eyes also becoming developed. On stars, their planets, galaxies, and  barely detectable sources at the edge of the known Universe, such eyes will  penetrate much beyond the current limits of astronomical observing. Our understanding of the Universe and its content will greatly improve.

 

Some insects and migratory birds, for millions of years presumably, have navigated at night by observing star positions.  The human forms of astronomical observing also likely began very early, perhaps a million years ago together with the ancient forms of science and technique such as the use of  fire.    Man has long been fascinated by celestial cycles and  events, and their observation may have contributed to both his genetic and cultural  evolution.  Nowadays, the sharpening view of the Universe will increasingly show all kinds of cosmic fireworks and other phenomena, at widely different scales and distances, that will challenge his understanding capabilities. On extra-solar planetary systems, a few hundreds of which were discovered during the last decade,  new instruments currently designed will make it possible to search evidence of life, and perhaps civilization.

 Such endeavours can improve man’s  perception of his position in the Universe, its past  and its future, thus influencing his philosophical views and everyday behaviour.  In addition to professional astronomers, increasingly many amateur astronomers contribute  with their small telescopes, thus relaying the educational and social influence of astronomy,  and science more generally.

Brief history: 

From telescopes to interferometers and now hypertelescopes

The increasing size and performance of telescopes in the centuries following their initial astronomical use  by Galileo in 1610 improved  the observing performance, both regarding the faintness of sources and the finest detail which could be seen.  Both performance aspects, respectively known as luminosity and resolution, indeed improve in theory when the diameter of telescope optics is increased.

In practice however, the resolution becomes limited by the  atmospheric turbulence when the telescope diameter approaches the size of its turbules, typically 10 to 20cm at good mountain sites.  Thus, amateur astronomers using small telescopes with aperture size in this range can rightfully boast of producing images as sharp as those obtained by giant telescopes.  In the recent decades however, the  largest Earth-based telescopes have increasingly been equipped with  devices which remove the atmospheric degradation and recover the full theoretical resolution.  These are  :  1- “Speckle Interferometers”  ( Labeyrie 1970),  which reconstruct full-resolution images of  simple sources;  2- “Adaptive Optics”,  proposed by  Babcock in 1953 and first built by Hardy in 1977, which corrects the atmospheric degradation with a fast deformable mirror;  3- Laser Guide Star systems (Foy & Labeyrie, 1985), which make adaptive optics applicable  to even the faintest observables sources. Such devices are considered essential for the forthcoming generation of “Extremely Large Telescopes”, expected to have mosaic mirrors as large as 42m.

Beyond such dimensions, much larger apertures have already been  achieved in the form of “diluted” giant mirrors, also called interferometers (see Wikipedia) . These combine light captured by separate mirrors and produce interference patterns carrying information on fine structures which may be present on the source.  Such interferometers having as few as two mirror elements, spaced tens or hundreds of meters apart, improve the resolution with respect to monolithic telescopes, and in proportion to their  baseline size relative to the telescope diameter.  Early versions of such “two-aperture interferometers”, of modest size,  were demonstrated in the late 19th century by Stephan  at Marseilles observatory and during the 1920s by Michelson and Pease who operated a 20 feet , and then a 50 feet, orientable structure carrying a pair of small mirrors.    A step toward much larger sizes was the proposal ( Labeyrie,  1972 , see box below), and subsequent demonstration (Labeyrie,  1975),  to combine light captured by separate telescopes, spaced tens or hundreds  of meters apart and aiming the same star.

The scheme was demonstrated at Nice observatory in 1974 with a modest “two-telescope interferometer” initally spanning 12 meters, and then with a larger version employing a pair of 1.5-meter telescopes ( Labeyrie et al. , 1986, Mourard et al. , 1989  ). It supported the 1972 suggestion that “interferometer-compatible designs be adopted for.... future large telescopes “, and the idea became considered in the following years by the institutions which built today’s largest existing telescopes, the quartet of 8m telescopes of the European Southern Observatory at  Cerro Paranal ( Chile) and  the pair of 10m Keck telescopes at  Mauna Kea ( Hawaïî).  With the interferometric coupling of its telescopes, at the scale of 100 or 200 meter baselines, both  observatories now obtain high-resolution data providing new insight in the physics of stars and galaxies. Other interferometers use  smaller but fully dedicated telescopes providing even longer baselines .

From reconstructed to direct images at high angular resolution

These interferometers attain the very high resolution achievable in theory with telescopes   if their aperture could be made of comparable size, reaching several hundred meters, but have not yet been able to provide  reconstructed images of complex sources.   Obtaining such images would greatly improve our understanding of phenomena occurring in stars, galaxies and other poorly known objects.  It is possible in theory to reconstruct them from data collected by pairs of apertures, differently arranged on the ground.  Such data painstakingly recorded with repeated observations, employing different aperture patterns for sampling the Fourier components of the object, can be combined, in the computer, a process initially developed by radio-astronomers and called “aperture synthesis” .   Interference fringes from a pair of apertures indeed sample a single Fourier component of the source,  and thus cannot show whether its shape differs from a periodic grid pattern having a sinusoidal intensity profile.

Instead, it became realized in 1996 that direct optical images are efficiently obtainable with a class of interferometers, now called “hypertelescopes”,  which use simultaneously a large number of small apertures, rather than a few large ones.   For efficient image formation the recording camera must be illuminated through a densified image of the dilute aperture  ( Labeyrie 1996). The LISE group has since  further explored the optical properties of hypertelescopes, theoretically and with small-scale testing.  As it became confirmed that they can greatly improve the imaging performance, with respect to conventional interferometers having few apertures, the group has worked toward designing and building increasingly large hypertelescopes.  Space versions have also been proposed to NASA and to the European Space Agency (ESA ). 

Science goals with high-resolution observing

The group’s science heritage was derived from the initial development of «speckle interferometry» and observations at the Palomar 200-inch telescope,  from two-telescope interferometry,  and  from the contribution to the Hubble Space Telescope design regarding its initial exo-planet coronagraph. It influences its current work toward gaining a better view of stars, their planets, and more remote sources such as external galaxies and even those located at the limits of the observable universe.

Hypertelescopes are seen as a promising new approach toward such science. Their improved sensitivity, with respect to existing interferometers, should give access to faint and complex sources, with direct images and their spectral content. With ground-based versions, faint sources will be observable if modified Laser Guide Star systems become available for adaptive piston phasing.

Imaging the details of stars

….and their planets,  possible nests for life

1. -detecting signatures of such exo-life
2. -searching  life in outer space

The most immediate use for hypertelescopes is to produce snapshot  images of stars, with enough angular resolution to see their surface details and better understand their complex machinery . Spectral information is also obtainable on each resolved element or resel.

Imaging the planets of close stars

  Equipped with a coronagraph, requiring adaptive optics, hypertelescopes can also in principle detect associated planets , although the contrast problem remains as difficult in principle as with a conventional telescope of identical collecting area.  This is expected to become much easier in space, where even the details of exo-planets will in principle become observable with large hypertelescopes.  The simulated image of an Earth-like exo-planet shown below has enough detail for searching “green spots”  of photosynthetic life.  Although not necessarily green , such coloured spots may be discriminated from mineral colors by their seasonal variations.

Gravitational lensing effects and the detectability of nearby black holes

In addition to the gravitational lensing effects considered since Zwicky  ( 1935) , and observed since the 1980’s,  the possibility of a stronger focusing effect caused by massive objects located within a parsec from the Earth has been explored by Labeyrie ( 1995 ??).  It has not yet been observed, but may become observable as fast and large photon-limited cameras will become available.  This may provide information on the presence of dark bodies, such as lost planets or black holes, which may be located in our close vicinity.

Hypertelescopes:  a promising path toward high-resolution imaging

The theory of hypertelescopes is described in   Labeyrie (1996)  and Lardière et al. (2007), Labeyrie Lipson, Nisenson. An easy way of becoming acquainted with the imaging properties of hypertelescopes however consists in playing with simulation software. This is how our students quickly became experts, whereas it originally took us years to visualize crucial notions such as the « Direct Imaging Field », the « Coupled Field », the « Clean Field » and multi-channel imaging ( Lardière et al.,   ).

What is a hypertelescope ?

•  A giant diluted  telescope at the scale of 100 meters to kilometers, and  later perhaps   hundreds of kilometers when flown in space.    Even larger dimensions, approaching a million kilometers  will in principle be operable  for observing bright and compact stars such as neutron stars (  link to Crab Pulsar  etc..)
• …. made of many mirrors, located far apart with respect to their size
• …. and capable of providing  direct "snapshot" images  of stars , their planets, galaxies , etc…with a high angular  resolution showing much finer detail than conventional telescopes, and carrying more information than conventional interferometers.
• .... with full luminosity, by concentrating most light into the image’s central interference peak  from each point source.   Fizeau imaging does not have this property, which results from the pupil densification  achieved in hypertelescopes .

How does it work ?

By making light interfere, as beams from the many apertures are combined into a single image.  Elementary low-resolution images focused by each mirror element are superposed “multi-axially” at a common focal plane .  Interference of light vibrations generates the high-resolution image, using an improved version of the simple “Fizeau  beam combiner”   ( figure  ).  The classical Fizeau arrangement, equivalent to a large mirror carrying a multi-hole mask provides directly usable images if i has enough elements,  but is inefficient in terms of image luminosity,  if the element's spacing is much wider than their size. The idea of the hypertelescope consists in densifying the exit pupil, which intensifies the image by concentrating most light into the interference peak. The optical trick of "pupil densification", “fills in”  the light beams converging toward the camera.

( Figure )

The performance gain with hypertelescope imaging

Previously , high resolution in optics was attained with interferometers using a few apertures, and repeated observations were needed, with different baselines, to reconstruct images via Fourier synthesis.   A comparable collecting area, but built in the form of numerous smaller apertures,  can greatly improve the observing performance in different respects:

• The direct images obtained  contain more information
• The « crowding limit », when too many stars or point sources are present in the field of view, is improved
• The limiting magnitude or signal to noise ra  tio, defined by photon statistics,  is improved
• The « direct imaging field » is enlarged
• In addition to these theoretical gains, some hypertelescope designs can improve the science throughput with multiple imaging channels.

Hypertelescopes can use either  “image plane”  or  “pupil plane” interferometry, as well as work through optical fibers 

The optical notion of pupil and image plane vanishes when using optical fibers. But hypertelescope imaging also works through optical fibers:   a rich direct image is obtainable by combining light waves exiting from a bundle of fibers  ( Labeyrie , Patru et al.,   ) .

Types of ground-based hypertelescope architectures

1. Arrayed telescopes,  with delay lines.

An array of ordinary telescopes, equipped with coudé mirrors for directing their beams toward a common beam-combiner, can operate as a hypertelescope if a small pupil-densifier is installed in the beam-combiner optics. Optionally, optical fibers can replace the coude beams.  In either case, optical delay lines are also needed to synchronize the light waves received from the star. Their complexity and high cost, together with the cost of telescopes, however tends to limits to tens , rather than hundreds or thousands, the practical number of telescopes which can be exploited, and thus restricts the information content of the direct images.

2. Mirrors arrayed in a “spherized” crater or valley: the Carlina  architecture.

An alternate type of design resembles the Arecibo radio-telescope (www. ), but in diluted  form since the cost of optical mirrors is much higher than radio reflectors.  The concepts listed above belong to this class, which favors numerous small apertures rather than a few large ones, which greatly benefits the science yield.

Figure:+ photo of "Carlina acanthifolia" flower

3. Pointable paraboloidal versions.

Unlike Carlinas, which are pointed by moving the focal camera along the focal sphere, versions using a paraboloidal collector array must be accurately pointed.  This is proposed for the initial phase of the Luciola in space, pending its upgrading with numerous mirror segments in the primary flotilla, and several focal spaceships.  It is also considered for « Perce-Neige » a terrestrial version conceived for operating at Dome C in Antarctica.

Current projects

The prototyping work undertaken in 1990 toward the construction of an Optical Very Large Array ( OVLA) was halted in 2000 when  the possibility of more efficient, and simplified,  hypertelescope versions using numerous small mirrors emerged. Telescope arrays are used ( CHARA ) and still constructed  elsewhere ( Magdalena Ridge Observatory ), following the VLTI array of four 8m plus four 1.8m telescopes now exploited in Chile.  Although not designed for hypertelescope imaging, they should be upgradable for such observing with a pupil densifier.  Such upgrading was proposed by Lardière (200?) for the VLTI and by Mourard (   ) for the CHARA.

Hypertelescopes on Earth

Pending larger space versions, such as those proposed to the space agencies (   ), hypertelescopes can be built on Earth with aperture sizes beyond one kilometer.  A challenging design concept, called “Carlina” ( the name of  a large ground-hugging thistle flower),  exploits a natural depression as a “mirror cell” carrying many small mirrors arrayed in a co-spherical  geometry. This is an optical and dilute form of the Arecibo radio-telescope. 

A testing arrangement, built at the Observatoire de Haute-Provence by Julien Dejonghe  and Hervé Le Coroller,  has been demonstrated in 200?  .   A camera suspended from a tethered balloon  could record the combined images focused by a pair of small mirrors,  attached at fixed positions on the ground  (www.   ).

Upgrading the Carlina test to a  Carlina-tech prototype

The test arrangement is now being upgraded to a more complete technical prototype, initially equipped with three mirrors.  It will serve to experiment design solutions intended for much larger versions. The aperture size is initially defined as 18meters, but may become increased to 36m  with a new focal corrector of the spherical aberration.

Steps toward a “Carlina-science”  with 100-200m aperture

The detailed design of a larger hypertelescope version, expected to produce images of many stars, with enough resolution to show many details of their individual surfaces,  is also under way .  A critical part of the instrument is the mountain site where it will be built.  In addition to the traditional qualities sought for telescope sites, particularly in terms of climate, winds and air turbulence, the near spherical topography needed for Carlina designs is quite restrictive. Candidate sites have been prospected with Google Earth, and some of them then visited.A few candidate glacial valleys, deep enough for installing a traversing cable from which focal optics can be suspended,  have a topography which can greatly simplify the construction of a powerful hypertelescope.

Coupling a hypertelescope to an  “Extremely Large Telescope” (ELT)

The European Extremely Large Telescope ( E-ELT)  currently studied by the European Southern Observatories is expected to have a 42m  mosaic mirror.  One of the sites considered, located in the sierra  Macon of the Argentinian Andes, has a mountain pass which may be suitable for also installing a Carlina hypertelescope.  If located closely nearby, the E-ELT would gain, in terms of science, in achieving observations with both instruments interferometrically coupled.   The feasibility is highly dependant on site topography and remains to be explored (   ).

Proposed   hypertelescopes in space

A concept for a space hypertelescope, in the form of a flotilla spanning hundreds of meters,  has been proposed to NASA and ESA as a candidate “Exo-Earth Discoverer version of the Terrestrial Planet Finder. It has been shown to have improved sensitivity for detecting planets, with respect to the DARWIN-TPF concepts.

A precursor version of a space hypertelescope was also proposed to ESA in 200...

The Luciola hypertelescope proposal to the European Space Agency (2007)

In response to ESA’s “Cosmic Vision 2015-2025” call for proposals, we submitted , together with colleagues from NASA  and other laboratories a more ambitious proposal for a one-kilometer hypertelescope flotilla called  Luciola (the original Luciola proposal to ESA for Cosmic Vision 2015-2025 can be downloaded from :http://www.oamp.fr/infoglueDeliverLive/).     A "Stellar Imager" interferometer proposed to NASA  by Carpenter et al.  ( NASA/GSFC) also involves a formation flight of mirrors capable of direct imaging.  A convergence of both instruments is considered, with a Stellar Imager  beam-combiner spacecraft becoming part of a Luciola.

A 100-kilometer “Exo-Earth Imager” hypertelescope in space for detecting life beyond the solar system. 

  This spherical version provides panoramic sky coverage by moving a number of focal cameras within a static «bubble» of mirrors

As the ability to launch, deploy, and  control formation-flying flotillas of satellites will become developed, the size of interferometric arrays may increase beyond the kilometric range, already approached by Earth-based interferometers.  Simulations of imaging with a 100km “Exo-Earth Imager” hypertelescope, featuring 150 mirrors of 3m, have shown that an Earth-like planet at 10 light-years can be imaged in rich detail  ( figure).  Seasonal variations of colored patches will be searchable as indicators of photosynthetic life. 

According to the spherical architecture for hypertelescopes ( also called Carlina)  the giant diluted mirror can be made  in the form of a static spherical bubble, with beam-combiner spaceships  exploiting the half-size focal sphere.  Correctors of spherical aberration can themselves be built in the form of a flotilla of small mirrors. 

Laser trapped mirrors for a hypertelescope flotilla in space :   feasibility of a “laser-trapped hypertelescope

In accordance with the theory of hypertelescopes, showing that many small mirrors are better than fewer large ones , at given collecting area,  the option of laser-trapping a large number of inch-sized mirrors is under study ( Labeyrie et al., Experimental Astronomy 2008). (Labeyrie et al.,   Experimental Astronomy: Volume 23, Issue1 (2009), Page 463, downloadable for a fee at the publisher's site (http://dx.doi.org/10.1007/s10686-008-9123-8 )  )  Laboratory testing is under way .

Feasibility of a   100,000-kilometer Neutron Star Imager

Even larger sizes, approaching the million kilometers needed for resolving the details of a neutron star such as the Crab Pulsar, can in principle be operated.   The collector mirrors however must be large enough, typically 8-meters, to limit the diffractive divergence of the reflected beams, so as to keep the remote focal optics within a manageable size . 

Related technical developments

• Piston sensing with «dispersed speckles»
• Modified Laser Guide Star for hypertelescopes
• Extreme coronagraphy with holographic adaptive nulling
• Laser Trapped Mirrors for space hypertelescopes

Extreme coronagraphy  using a hologram

Since proposed in the early 1970’s for the Hubble Space Telescope (Bonneau, Josse & Labeyrie,  197?) the use of coronagraphic optics for detecting exo-planets has been considerably refined  ( Trauger,   ).  It will be part of Hubble’s successor, the JWST  ( Labeyrie ,   ...).   Much of the starlight contaminating the faint image  of a nearby planet can be  rejected, especially if adaptive optics removes its faint residues.  Such “extreme coronagraphy” is studied by some of us, who explore a holographic form of starlight nulling ( Ricci, Le Coroller & Labeyrie, 2008).

In addition to the following list of current group members, many colleagues have contributed to its work for different projects and debates.

A.LABEYRIE Working in high-resolution astronomy since 1970, AL has been involved in instrument theory, design, building, observing, and the resulting astrophysical science production.  He proposed the concept of “ direct-imaging multi-aperture densified-pupil interferometers” , which he called hypertelescopes, explored their properties, and concluded in the possibility of major progress with them.  , the  new-generation  instruments developed in the group for use on Earth and in space. 

JULIEN DEJONGHE Originally trained in mechanical engineering, J.D. became an expert designer and builder for both the mechanical and optical systems developed in the group, using respectively CATIA and Zemax code.

HERVE LE COROLLER Originally an expert in shock-wave phenomena in stellar atmospheres, HLC built with JD the prototype “Carlina-tech” hypertelescope (http://www.obs-hp.fr/~hlecorol/) and contributes to upgrading it and designing the forthcoming larger versions. He also contributed to the analysis of an “extreme” stellar coronagraphy scheme which uses a hologram for removing residual starlight down to the level needed for detecting planets.

DAVID VERNET is an expert in the subtle art of mirror fabrication for telescopes and hypertelescopes. He directed the construction of polishing machines, which he operates for figuring mirrors up to 1.5m in size. He polished more than 80? Mirrors, which he also tested, and mounted in telescopes, some of which were built by himself. He has developed super-polishing techniques for coronagraphic applications, and techniques for thin mirrors, such as the 1.5m 25mm thick paraboloid , apertured at F/2 which he figured for the prototype compact telescope intended for the OVLA. He figured several mirrors for the “Carlina-tech” hypertelescope prototype and studies techniques for their production in hundreds. DV also uses ray-tracing codes for optical design, and thus contributed to designing the solar sail of the nano-satellites for the Luciola hypertelescope flotilla.

RALPH KRIKORIAN pursues theoretical work on optical aspects of General Relativity at the Institut d’Astrophysique in Paris.

VALERIE GARCIA expert in public relations and communication, is administrative assistant in the group. She designed its web site, which she also maintains.

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