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A terrestrial Hypertelescope

On the scale of a mountain

“A telescope outside the norm that reverses the principles of traditional instruments”

schema_techniqueThe principle of the Hypertelescope is simple. Instead of having a large mirror, Professor Labeyrie has imagined the establishment of many small 15 cm widely spaced mirrors placed on  tripods very precisely on the ground in the bowl of a Valley and which together form a virtual giant concave mirror.

Above the Valley 100 metres in height on a cable, a bucket with a novel optical device developed at the College de France collect the Starlight reflected by mirrors on the ground. This basket is positioned very precisely using 6 cable-stayed “dynamic” operated by winches like a giant puppet. The cables are operated from three sites located about 300 m from each other and connected by a local wifi network powered by solar energy. Their winding/unwinding, controlled by a computer, will compensate for the movement of the Earth’s rotation and therefore point the basket in the direction of the star for the duration of observation. The light collected is received by a camera that can be installed on the gondola, or on the ground, returned by the secondary mirror of the basket.

The image produced by a hypertelescope, if it is equipped with an adaptive corrector compensating for atmospheric turbulence, is a direct snapshot and not an image restored after calculations from successive images. With conventional interferometers with a small number of mirrors, it is necessary to build an image over multiple observations spread over time, in order to benefit from the change in geometry caused by the rotation of the Earth. The largest number of mirrors available in a hypertelescope improves the sampling of the light wave and the formation of a more intense interference peak, observations are instead exploitable simultaneously. Direct imaging thus made possible with the hypertelescope allows improved sensitivity and increases the informational content of the observations..

The feasibility of astronomical observations with a focal gondola suspended from cables of several tens of metres over a network of mirrors has been verified with a prototype at the Haute-Provence Observatory.

Testing of the new instrument has been started three years ago (on a more suitable site), in a deep valley in the Southern Alps. Its regular curvature can accommodate a ‘diluted optical opening’ or ‘meta-ouverture’ whose  diameter could reach 200 m.   For this project, it was necessary to find a valley with the ideal curvature on the ground and oriented East – West to allow the longest observation of various celestial objects. Due to the rotation of the Earth, stars rise in the East and  set in the West.

A Valley corresponding to these criteria was found in the Alpes de Haute-Provence in the Ubaye Valley. It’s the Valley of the Moutiere located at 2100 m above sea level in the town of Uvernet-Fours near the charming town of Barcelonnette, offering a light pollution-free sky and reducing the thickness of the atmosphere thus reducing turbulence.

Just like a radio interferometers consisting of multiple dishes, the hypertelescope lends itself to a scalable modular installation. It therefore has the same advantage: to produce scientific results before the completion of the installation. The current prototype will initially have a set of mirrors totalling 57 metres in diameter. The concept is evolving, and in principle will expand to 200 m the diameter of the ‘diluted mirror’, which would give it a resolution reaching 0.5 millisecond of arc or 80 times better than the Hubble space telescope.

Three stages are planned for operation:

  • Without adaptive optics, rebuilding images by the “speckle interferometry” method that was invented and operated at Mount Palomar by A. Labeyrie in the 1970s.
  • With adaptive Optics for direct imaging by correction of atmospheric effects..
  • With a laser guide star, the adaptive optics can be used for very weak sources. If it turns out to be possible to extend to the hypertelescope this technique already used on large conventional telescopes (and until  hypertelescopes are operating in space). It is the way to benefit cosmology from high angular resolution.

The Ubaye Hypertelescope should at stage 2 provide resolved images of nearby stars allowing to see details of individual morphology. This is a way to better understand the functioning of these natural thermonuclear plants. At the same stage, this kind of image would in  principle allow to see the passage of exo-planets in transit in the form of a dark disc crossing the apparent disk gloss of the parent star. This is a way to also better study them spectroscopically.

While the future “Extremely Large Telescope” European (E – ELT) of 39 m in diameter requires the construction of a giant mount and a dome, optical mirrors of an ELHyT network unfolds in a valley or a crater, without need for constant orientation because it is fixed, and open to the sky. For the same cost, an ELHyT could therefore have a greater collecting area that a E – ELT. If the feasibility of using a laser guide star to form the image of low light sources is confirmed, the ELHyT would allow a gain in sensitivity and magnitude limit and a strongly improved resolution. Concerned scientific fields with an ELHyT would be: stellar physics and transit of extrasolar planets – galaxies with active nuclei and distant galaxies. An ELHyT would allow detection of younger galyxies than those accessible with telescopes such as Hubble, VLT or Keck. Some high valleys in the Andes and the Himalayas seem appropriate to host an ELHyT.