Tests with a carlina-type hypertelescope prototype: I. Demonstration of star tracking and fringe acquisition with a balloon-suspended focal camera

• Published in: Astronomy and astrophysics (Berlin) Vol. 426; no. 2; pp. 721 - 728
• Main Authors: LE COROLLER, H, DEJONGHE, J, ARPESELLA, C, VERNET, D, LABEYRIE, A
• Format: Journal Article
• Language: English
• Published: Les Ulis EDP Sciences 01.11.2004
• Subjects: instrumentation: adaptive optics; Planets; balloons; Adaptive optics; instrumentation: high angular resolution; Spherical envelope; Curvature; instrumentation: interferometers; techniques: interferometric; Multiple image
• ISSN: 0004-6361; 1432-0746
• DOI: 10.1051/0004-6361:20041088

Labeyrie (1996, A&A, 118, 517) established the feasibility of snapshot images with a multi-aperture interferometer having a densified exit pupil. The numerous widely spaced mirrors in these instruments, called hypertelescopes, do not alleviate the usual difficulty of adjusting and phasing interferometers. A simplification is however possible, in the form of the optical and mechanical architecture called Carlina (Labeyrie et al. 2002, Proc. SPIE, 4838). It is configured like a diluted version of the Arecibo radio-telescope. Above the diluted primary mirror, made of fixed co-spherical segments, a helium balloon carries a gondola containing the focal optics and detector. We describe in more detail the Carlina concept, including versions equipped with an equatorial drive and a coude train. The optical design with a clam-shell corrector of spherical aberration is optimized with a ray-tracing code. A two-element prototype of a sparse aperture, multi-element, optical dish has been built using a steerable balloon-suspended secondary optical structure. Following imaging and tracking tests with a single mirror, which give encouraging results, fringes have been obtained on Vega with a pair of closely spaced mirrors. We developed adjustment techniques for co-spherizing the mirrors within one or a few microns, using a light source at the curvature center. The absence of delay lines is a major simplification with respect to conventional interferometers, paving the way towards using hundreds or thousands of sub-apertures for producing direct images with rich information content. These results demonstrate the short-term feasibility of large Carlina hypertelescopes, with effective aperture size possibly reaching 1500 m at suitable terrestrial sites. Such interferometers will provide snapshot images of star surfaces, and of exo-planets if equipped with an adaptive coronagraph. Collecting areas comparable to those of ELTs appear feasible at a lower cost, while providing a higher resolution and similar limiting magnitude.