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 - 10km terrestrial platform, can provide snapshot images having 10-4 to 10-5 arc-second resolution. On visible objects as faint as mv=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 10m 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.
Keywords : telescope array, stellar interferometer, hign-resolution observing, optical interferometric array, spherical telescope
The time has come for optical arrays of many telescopes, spanning up to 10km on Earth and, at a later stage, possibly 100 km in space. Since 1974 1, pairs of telescopes have been operated interferometrically to obtain high-resolution information. Baselines now approach 100 meters, allowing 10-3 arc-second resolution 2,3,4,5. Dedicated telescopes of 1.5 meter are used 6,7. A triplet of small apertures has also been used successfully to reconstruct images by synthetic aperture techniques 8. On the theoretical side, recent results establish the possibility of obtaining high-resolution images directly at the recombined focus of a sparse multi-telescope array 9. This powerful observing mode requires adaptive phasing techniques, both intra and inter-aperture, similar to those currently developped for single large telescopes.
2. PRINCIPLE OF LARGE TELESCOPE ARRAYS
Figures 1 and 2 illustrate the principle of recombining images produced by many telescopes of arbitrary size. Since a kilometric-size system cannot be rotated like a telescope to compensate for the Earth's rotation, although each component telescope does rotate, phasing the system requires translating at least one mirror, or the telescopes themselves, during the observation. In terms of image and pupil structure, the system can be made optically equivalent to a single giant telescope carrying an aperture mask with holes. When configured in this mode, often called the Fizeau mode, the recombining optics superposes the sub-images, produced by the individual telescopes, and a narrow central interference peak appears, within the broader Airy peak from the sub-apertures, when phasing is achieved on a point source.
Some clarification of the algorithms usable for phasing multiple telescopes on extended sources is still required. In addition to methods previously discussed9, one should explore the performance of "hierarchical triplet phasing". The method consists in steps of forming fringes from triplets of sub-apertures, then triplets of triplets, etc...until the phasing parameters are determined for the full aperture. At each step, the energy concentration can be used as the parameter to be optimized, through the adjustment of two phases. Starting with triplets formed with the smallest baselines, for which the object is least resolved, the algorithm proceeds towards the longest baselines. The whole sequence has to be repeated at intervals shorter than the life-time of "seeing". The fast splitting of the image into triplets of sub-images is achievable with the tilt acuators on each sub-aperture.
3. CURRENT GROUND-BASED PROJECTS
Fore-runner interferometers with two or three elements and baselines in the 100-meter range have began to produce significant science, despite their modest image-forming capacity . With the experience gained in building and operating these instruments, it becomes possible to build multi-telescope arrays with vastly improved scientific impact. Projects such as the Very Large Telescope of the European Southern Observatories, or the interferometric Keck array, are intended to exploit the light gathering power of large apertures, few in number but complemented by several smaller telescopes. Although these instruments were not specifically optimized for interferometric uses, they should provide useful high-resolution images, in the arc-millisecond range, through Earth-rotation synthesis techniques. Although far less efficient than the snapshot imaging achievable with a few dozen telescopes, the relatively slow Earth-rotation synthesis is of interest for long-lived objects.
A first step towards multi-telescope systems specifically designed for full interferometric efficiency is the Optical Very Large Array (OVLA) project7. With 27 telescopes of 1.5 meter, forming a "dotted ring" aperture of 600m, the system is expected to provide snapshot images. The telescopes will be moving on hexapods during the observation, for full flexibility of array configuration, as well as for avoiding the use of optical delay lines. A prototype telescope, being built at the observatory of Haute Provence by Luc Arnold, Claude Cazalé, Julien Dejonghe and the technical group, is to be completed within two years. It has unusual characteristics: a spherical mount, a thin primary mirror (25mm) made of ordinary glass, carried by 30 active supports, and a simple coudé train with a single flat mirror. The telescope will join the Grand Interféromètre à 2 Télescopes (GI2T) to provide a third element. The construction of the full OVLA may then proceed, unless funding becomes available for a full-size instrument such as the IGT.
A second step will be the construction of the IGT, a multi-telescope array with larger elements, 8-meter for example. It requires the design and qualification of large telescopes especially adapted to interferometric uses. Generally speaking, all designers of new large telescopes should be aware of their potential usability for large arrays, and may want to introduce the corresponding requirements in their design concept. Any successful large telescope is likely to become duplicated, triplicated, and built in series at some stage. The additional science achievable with arrays is so wide-ranging that single large telescopes will appear as unnecessarily restricted. The IGT under study at Haute Provence involves 27 telescopes of 8 to 10 meters, to be arranged along a "dotted ring" of several kilometers11. Figure 4 shows a preliminary design concept for the unit telescope, where the main mirror is an active mosaic of thin hexagonal mirrors similar to the OVLA's. These telescopes have a low weight, of the order of 20 tons for an 8-meter mirror, and they will have an hexapod translator.
4. CURRENT STUDIES OF SPACE ARRAYS
Although adaptive optics and adaptive phasing can go a long way towards smoothing the phase corrugations created by the atmosphere on incoming optical waves, space offers perfect seeing and isoplanatism, in addition to the ultra-violet access. With free-flying elements, it also favors very long baselines, which may reach a hundred kilometers at some stage. Projects for interferometric arrays in space have been proposed and considered by the space agencies. Among them are relatively modest systems involving a boom structure to carry more or less rigidly the interferometer elements. Projects of this kind, involving structures of 3 to 20 meters are currently studied in Europe and the United States, mostly for astrometric uses. Considered as precursors of more ambitous systems, their study is actively supported by both NASA and ESA.
The more ambitious interfermeter projects involve separate free-flyers as the elements of the collecting optics. Concepts have also been proposed for use on the Moon. A recent study by the European Space Agency concludes in the feasibility of a free-flyers array with long baselines. The lunar option, also discussed in the report, appears more costly. To compete fully with forthcoming ground-based arrays, those in space will need comparable baselines, sub-aperture sizes, and telescope count. The study of foldable space telescopes, seen as successors to the Hubble Space Telescope, is of considerable interest in this respect.
Current developments in long-baseline interferometry, with component telescopes of moderate to large sizes, announce a major breakthrough for optical astronomy at visible and infra-red wavelengths: imaging arrays providing high-resolution "snapshot" images and access to previously unattainable science. Following the precursor instruments, currently being built with 1.5m telescopes, much larger telescopes will become used in such configurations. Their special requirements for this use shoud not be overlooked in current design studies.
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