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OHP Preprint No. 114 : Kilometric arrays of 27 telescopes

Kilometric arrays of 27 telescopes:
studies and prototyping for elements of 0.2m, 1.5m and 12-25m size

Antoine Labeyrie
Collège de France & Observatoire de Haute Provence (CNRS),
F-04870 Saint Michel l'Observatoire, France
E-mail: labeyrie@obs-hp.fr

À paraître dans SPIE volume 3350, Astronomical interferometry, Kona, 20-24 march 1998


The "densified pupil" imaging mode, now developed for large multi-telescope interferometers 1, 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 beyong 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 test-bed interferometer involving 27 small mobile heliostats. forming a 100 or 300m ring.

Larger versions of the OVLA, employing unit telescopes of 10 to 25 m, are also considered, in connexion with the large telescope study iniated 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.

keywords:optical, interferometry, multi-telescope; imaging, densified pupil



The extraordinary science potential of multi-telescope interferometric arrays has been stressed for two decades, but some ingredients were missing to implement them:

  1. a full understanding of image formation with highly diluted arrays;
  2. efficient phasing algorithms, needed to concentrate the energy in high-resolution images, in the presence of atmospheric and instrumental jitter;
  3. designs for compact and movable telescopes also having a low cost.

All three ingredients are now becoming available, and detailed designs for large arrays with large or small elements are under way: a recent analysis of image formation with densified exit pupils 1, 2, 3, identified the possibility of a significant improvement leading to high-quality snapshot images. In addition, new phasing algorithms 4, tested under computer simulation are found to perform efficiently on resolved and unresolved sources. Prototype elements are being built for active mirror systems and mounts suited to the interferometric requirements 5.

We discuss ways of building instruments with 27 apertures or more. Their size may be modest or large, but has to be homogeneous for efficient imaging according to the densified-pupil principle. The instruments thus considered are: a modest "Micromegas" array involving 27 mirrors of 20 cm size, and spanning a few hundred meters; a more powerful "Optical Very Large Array", previously described 5, 15 but now upgraded to provide densified-pupil imaging; and a more ambitious array of large telescopes, 10 to 25m in size and spanning 10 to 20 kilometers. The latter is capable, in principle, of producing images of extra-solar planets where detail such as their continents would be resolved 1. Space versions of such instruments, spanning as much as 10,000 to 100,000 km when observing neutron stars, are also discussed.

The more modest "Micromegas" array is expected to serve initially as a test-bed for the central recombination system of the OVLA, but should also produce useful snapshot images of bright stars and their surroundings. It also appears applicable to solar imaging with 5 milli-arcsecond resolution.

The OVLA, capable of spectro-imaging with high spatial and spectral resolution, has obvious uses in stellar physics, and should access the brighter extra-galactic sources. If fully equipped with adaptive optics and laser guide stars on each sub-aperture, it is expected to exceed visual magnitude 20.

Arrays of 10-25m adaptive telescopes will address the kind of science considered by Mountain 6. They will provide detailed images of stellar surfaces and their environment, of galactic features, of extra-galactic and cosmological sources with resolution levels eventually reaching 10 micro-arcseconds. Objects such as the remote galaxies appearing in the Hubble Deep Field, although very faint, can in principle be imaged with sub-milliarcsecond resolution, considering the high degree of light concentration achievable with an adaptively-phased densified pupil.

In space, densified-pupil arrays of small or large telescopes will likely have free-flying elements 7. In the absence of residual "seeing", their coronagraphic and dark-hole 8 imaging performance can be markedly improved with respect to the above-mentioned ground-based interferometers. When the parent star is unresolved, dark-speckle imaging 9 will also be applicable to further improve the coronagraphic sensitivity. Operating 10,000 to 100,000 km baselines appears feasible, with remote out-rigger collectors, and these can provide nano-arc-second resolution on neutron stars and other small objects of high intrinsic luminosity.


As explained elsewhere 1, 2, 3 , densified-pupil imaging involves the kind of image recombination originally used by Michelson and Pease at the 20-feet interferometer, but with many rather than just two beams, and a conserved pattern of sub-pupil centers in the exit pupil, where sub-pupils are proportionally larger than in the entrance pupil. With adequate phasing, sub-pupil orientations being preserved for polarisation homogeneity, the image of a point source becomes dominated by a central interference peak. A usable image is also obtained on resolved sources if, and only if, the pattern of sub-pupil centers in the exit pupil is a scaled-down version of the pattern in the entrance pupil.

With a strongly densified pupil, the energy otherwise dispersed in a wide halo of side-lobes becomes concentrated in a smaller halo.

Efficient images, concentrating most of the collected energy, can thus be produced with highly diluted arrays of many sub-apertures. The analysis 1 indicates that snapshot images of extended objects, formed in such conditions, can have NxN resolved elements if N is the number of sub-apertures. These image elements can be contiguous, as is the case when observing the apparent disk of a star, or separated as would be the case for a cluster of unresolved stars. However, more than N2 such stars within the Airy disk of an entrance sub-pupil causes their interference peaks to vanish amidst the side-lobes of their added halos. The limitation already exists in a N-aperture Fizeau interferometer, owing to the S1/2 growth of the halo's speckle noise when S unresolved sources are present in the halo field. For imaging extended objects, the sub-apertures of a Fizeau array should be larger than D/N, if D is the array size. This implies nearly contiguous sub-apertures for a ring-shaped array. With a densified exit pupil, the entrance sub-pupils cannot be contiguous, and no extended object can provide snapshot images unless it fits within the NxN pixel high-resolution field of the recombined image. Multiple exposures or observations with different aperture patterns improves things, although not as much as would a comparable multiplication of N, by smoothing the halo in the resulting image where intensities are summed.

All this indicates that densified pupils are suitable for star clusters and compact resolved objects of moderate complexity. For large arrays in the kilometric or multi-kilometric size range, densified pupil imaging can intensify the interference peak 104 to 106 times with respect to Fizeau imaging.

To best suit the type of object observed, the trade-off between luminosity and field can be made adjustable, using zoom lenses in each sub-pupil.

In the absence of adaptive phasing, the interference function is a speckle pattern, which may be exploited according to the principles of speckle interferometry, and can similarly provide images if the oject is simple.

Figure 1 Ring arrays are mainly considered here since they provide efficient spread functions and can be operated without delay lines, if the collecting telescopes or
heliostats are movable, but the discussion also applies to multi-ring or square-grid aperture geometries, which can provide more compact exit pupils once densified.

The interference function of a ring array resembles the Airy pattern of the giant filled aperture, with a narrow central peak surrounded by a few rings, becoming broken into a speckle pattern of side-lobes at increasing distances from the peak (figure 1).

The image can be exploited directly, and can also be deconvolved post-detection with deconvolution codes such as CLEAN or WIPE 10. Coronagraphic techniques involving a phase mask 11 and dark-speckle analysis 12, 9 are also potentially applicable for detecting faint stellar companions, or exo-planets. The sensitivity thus achievable, on Earth and in space, should be compared with that expected for other schemes such as Bracewell's "nulling interferometer" 13 and Gay's Achromatic Interference Coronagraph 14.


The arrays considered here, whether using large or small elements, consist of many (27 to perhaps 81 or 243) telescopes which are movable and concentrate light at a common coudé focus where all images are superposed. The motion capability serves for changing the array size and geometry, and also for compensating the optical path difference variations caused by Earth rotation. Telescopes arranged along an elliptical ring thus require no delay lines if the ring shape is made to change during the observation in such a way as to maintain equal optical paths. The coarse mechanical motion is supplemented with a fast adaptive-optics correction of coherence and phase errors among the telescopes.

On bright and moderately resolved stars, the level of positioning accuracy needed to acquire the fringes does not have to be much better than a millimeter. The spectro-imaging camera, with the set of partial images involving 3, 9, etc. sub-pupils (figures 2, 4) can indeed have enough spectral resolution to find fringes with optical path differences reaching a millimeter.

A more accurate definition of the array's geometry is obviously possible with metrological laser systems. A system where each telescope receives 3 laser beams had been considered and tested 16, but found difficult to operate. The principle is also part of NASA's initial design for the Space Interferometry Mission, in which a "metrology boom" carries coherent laser sources, at least three of them illuminating each collecting element.

Railway tracks, oriented radially, are used at the GI2T interferometer, and can certainly be adopted for larger multi-telescope systems. More flexible solutions, also lower in cost, involve hexapod translators, resembling in some respects the hydraulic "walking" translators which have been used for moving nuclear submarines.


Figure 7 In space, the concept of free-flyers arrays proposed in the 1980's 7 , is now studied in more detail by ESA and NASA. With many elements and densified-pupil imaging, instruments of considerable power can be foreseen. Baselines reaching some 100 km should prove operable on ordinary stellar and galactic sources, while the few known pulsars and isolated neutron stars, with their extraordinary surface luminosity, should allow baselines as long as 100,000 km if no surrounding nebulosity hides the central source, and this should allow resolved images to be formed.

NASA has announced plans to operate an array of free-flying 8m telescopes in space after the initial NGST. With the kind of pupil densification shown in figure 7), coronagraphic imaging can be avchieved, using Lyot masks or a phase-mask 11 complemented by dark-speckle techniques. The prospect of exo-planet images showing resolved detail, continents for example, is in principle obtainable, and equivalents of Earth's chlorophyll absorbtion bands can be evidenced if photosynthetic life occurs. The complementarity of these techniques with those utilizing beam splitters for light nulling 13 should be investigated.

For implementing the 10,000 to 100,000 km baselines needed for resolving neutron stars, a free-flyers array can serve as the core instrument, with "out-rigger" mirrors arranged as shown in figure 7). The metrology requirements for such giant baselines, although difficult, can probably be met with the help of forthcoming astrometric catalogues, the position of the outrigger mirrors being monitored by boresight alignment with respect to background stars seen from the central telescopes.


With adaptive optics and "densified pupil" imaging, interferometric arrays of many elements will provide extensive spectro-imaging data on bright and faint objects. Whether modest arrays of small mirrors, ambitious arrays of very large telescopes spread kilometers apart, or intermediate systems such as the OVLA, most of the standing problems which heretofore prevented such projects from emerging are now being solved, and the time has come for practical attempts. Pending major projects such as a multi-25m ARGUS, the moderate cost of an OVLA allows its construction in the near future. Many groups in different countries have now acquired the skills needed for such projects. As done successfully by the radio-astronomy community, it is now time to join efforts towards designing, modelling and building truly powerful instruments. Possible sites, suitably flat at the scale of 10 kilometers, are difficult to find and systematic surveys should be undertaken shortly.


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