| The Carlina Hypertelescope concept | Carlina-2: a larger-scale project |
In 2002, we started the construction of a first prototype, to experiment different technical solutions, and to validate the concept.
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A 12 m long and 4 m diameter helium balloon was designed and manufactured:
This was the biggest we could have in our hangar!
A carbon gondola was built to carry the camera:
We studied the Kevlar-cable made equatorial mount:
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Principle of the equatorial mount: Three cables attached under the balloon form a "cable tripod". While the balloon is moving in the wind, this tripod remains immobile. The summit of the tripod coincide with the curvature center (C) of the segmented primary mirror. A is the intersection between the ground and the Earth rotation axis passing by C. As the gondola is attached to A and C, it rotates around the Earth rotation axis by pulling on the guiding winch. |
A special high-precision winch was constructed by OHP machine shop, as well as the mirror supports.
D. Vernet polished three 70 m curvature segments for the diluted primary.
Once all these elements were implemented on the OHP site, we began the first tests:
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| Overall views of the installation | The balloon and the carbon gondola | View of the primary segment, with the gondola and the balloon above | Night view |
Firstly, we made first light by tracking a star (Psi Ursae Majoris) with only one primary segment, in the beginning of 2004. The main point to verify was the stability of the gondola: with a very low wind, we could track our star with only a few millimeters oscillations.
Pointing method:
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One of the main difficulties is to point the Hypertelescope. We have to bring the gondola at the exact position where the image of the star is formed by the primary mirror M1. For this, a small telescope is placed near M1, and tracks the star. A retroreflector is placed at the entrance of this telescope. Four lights (in red) are deposed around the CCD, so their light reflect on M1 and in the retroreflector, to go in the telescope. An operator, looking in the telescope, sees the image of the star and of the four lights, and have to bring the four lights around the star by moving the gondola: the hypertelescope is pointed! | 
Pointing the Carlina... |
Then, in May 2004, encouraged by this good result, we made the first fringes on Vega, with two juxtaposed mirrors (Coroller et al. 2004, A&A 426, 721):
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| The two juxtaposed mirrors: above, a plane mirror reflects the curvature center at ground level. A light source placed at the curvature center allows to adjust tip/tilt and piston of one mirror with regard to the other. | First
fringes on Vega in May 2004
The two mirrors were 50 mm stopped down, and about 400 mm spaced. |
First fringes video (18s) (797 Ko) |
The following step was to add two spaced out primary segments, and a Mertz corrector, to correct spherical aberration which becomes important if the primary mirrors are distant.
The Mertz corrector: As the diluted primary mirror is spherical, the resulting spherical aberration must be compensed by two high-aspheric mirrors:
The two high-aspheric mirrors were manufactured by SAVIMEX (Grasse, 06), by high-precision diament machining.
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| The Mertz corrector, during lab tests | The Mertz corrector implemented in the gondola |
Three spherical mirrors were disposed on the giant virtual sphere constitiung the primary diluted mirror of Carlina (up to 10.5 m baseline):
At last, in February 2006, we observed Tania Australis with the three-mirrors array and the Mertz corrector, in a non-coherenced mode (without fringes):
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| Video:
First light with Mertz corrector
(27s) (6323Ko)
At the end of the sequence, a small defocus shows the three spots |
After a small adjustment of the primary segments inclination, we were able to track the star more than 40 minutes. The three spots remained well superposed, only by doing some focus corrections. This excellent result shows that the Mertz corrector is functionning very well.
Next step: obtaining fringes with the three spaced out mirrors
To complete the demonstration of the Carlina concept validity, we plan to obtain fringes on the sky mith the three-mirrors array. The pistons of the primary segments must be adjusted with an optical method. A high-sensitive CCD will be implemented, to be able to make very short exposures in order to "freeze" the seeing and the gondola movements.
Carlina-1: main specifications
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Balloon altitude: 120 m |
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Balloon Lift: 70 Kg |
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Primary segments: Zerodur, 250 mm diameter ,spherical surface, 70 m curvature radius |
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Focal length: 35 m |
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Gondola mass: 12 Kg |
| The Carlina Hypertelescope concept | Carlina-2: a larger-scale project |