Tests with a Carlina-type hypertelescope prototype

I. Demonstration of star tracking and fringe acquisition with a balloon-suspended focal camera

¹ Laboratoire d'Interférométrie Stellaire et Exo-planétaire & Collège de France, Observatoire de Haute-Provence, 04870 St-Michel l'Observatoire, France e-mail: coroller@obs-hp.fr
² Collège de France, Observatoire de la Côte d'Azur, Avenue Nicolas Copernic, 06130 Grasse, France

Received: 15 April 2004
Accepted: 28 June 2004

Abstract

Labeyrie ([CITE], 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. [CITE], 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 coudé 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.