The CARLINA Hypertelescope Project

The Carlina Hypertelescope concept

First step: Carlina-1, a 10 m prototype built at Observatoire de Haute-Provence (OHP)

Carlina-2: a larger-scale project

Staff:

Hervé Le Coroller (PI)

Cristina Arpesella (Mertz corrector)

Alain Blazit (CCD)

Virginie Borkowski (Wavefront sensor)

Julien Dejonghe (Mechanical and optical studies)

Antoine Labeyrie (Hypertelescope concept)

Olivier Lardière (Simulations)

David Vernet (Optics)

Partners:

Observatoire de Haute-Provence (OHP)

Observatoire de la Côte d'Azur (OCA)

The Carlina Hypertelescope concept: a new type of interferometer, providing high resolution direct images

Principle of the Carlina hypertelescope. In green, many small mirrors are distributed on a spherical surface. In blue, a helium balloon carries a gondola that contains focal optics. The red arrows show the light rays.

Its design is analogous to the Arecibo radio telescope’s, except that it uses a diluted aperture, and that it works in the visible and near-infrared wavelengths.

The giant primary mirror is made of numerous small sparse spherical segments.

The focal optic is carried by a captive helium balloon. Tracking is done by moving the gondola, pulling on the cables with winches.

Overall objectives:

We propose to study and realize a prototype of a new type of astronomical instrument called Hypertelescope. We want to open the way towards high-resolution direct-imaging interferometry. This prototype, called Carlina will be the first interferometer with a high imaging capability, in visible and near-infrared wavelengths (to have better resolution, and to access to the H_a line), and will provide rich science like stars surface imaging and exoplanets imaging in a short time.

We plan to demonstrate the Hyper telescopes interest and feasibility on the one hand, an on the other hand we will be able to start a real scientific program based on speckle mode observations in a first time (without adaptive correction of the atmospheric turbulence), and on adaptive optic mode observations in a second time, when the adaptive optic system will be implemented.

Main technical characteristics of the Carlina prototype:

The primary mirror is constituted with several small spherical segments, dispersed along a virtual giant sphere, all directly anchored in the ground. This wide diluted aperture forms an interferometric image of the sky on the half-radius sphere. A focal optic is placed on this half-radius sphere to catch the image of the chosen star. A carbon gondola connected with a captive helium balloon carries this assembly. High elasticity module Kevlar cables links the balloon to the ground and to the gondola. Two ground-based winches pulling the gondola provide star tracking.

The focal optic is mainly constituted by a spherical aberration corrector, called Mertz corrector, and a pupil densifier. As the giant diluted primary mirror is spherical, the aberrant formed image is corrected by the Mertz corrector. The pupil densifier concentrates the energy in the central peak of the diffraction figure, otherwise this energy would be dispersed in many secondary peaks in the Fizeau mode recombination (Labeyrie 1996). A high-sensitive CCD camera is then placed at the densified focus. The adaptive optic system takes place between the Mertz and the densifier.

Decisive advantages of the Carlina concept:

	No delay-lines: In actual interferometers, as it is necessary to keep optical paths equal one from each others during observation, each coude-beam passes trough a delay-line, which is servo-controlled at very high precision (lambda/4). If there are N apertures, there are N-1 delay-lines. The cost of these high sophisticated systems is very expensive, and this is a crucial limitation for the construction of high-resolution multi-element interferometers in the future. In the Carlina concept, no delay-lines are required, because the spherical shape of the primary naturally keeps optical paths equal.
	Ground stability: The primary segments are directly anchored in the rock. Once they are all adjusted in tip-tilt and piston, the maximum expected movements are in the order of one hundred microns, and will be very slow (in a first test we measured about one micron per night). The required correction will be executed at long time intervals (one day or more).
	Internal metrology: At the center of curvature of the primary sphere, an internal metrology is installed. Using several laser beams at different wavelengths, it allows measuring the tip-tilt and piston of each sub-aperture and to co-spherise all primary segments without using starlight. Once this co-spherisation is established, the hyper telescope performances are only limited by atmospheric turbulence. Consequently, once the mirrors are in coherence, a very limited dispersion of the starlight is required, so the limit magnitude of Carlina is pushed back with regards to classical interferometers.

First step: Carlina-1, a 10 m prototype built at Observatoire de Haute-Provence (OHP)

Carlina-2: a larger-scale project

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