Julien Dejonghe

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

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.

Imaging capabilities of hypertelescopes with a pair of micro-lens arrays

We verify the imaging performance of hypertelescopes on the sky, using a new scheme for pupil densification. To avoid seeing limitations, we used a miniature version with a 10 cm aperture containing 78 sub-apertures of 1 mm size, arrayed periodically as a square grid. The pupil densification is achieved with a pair of micro-lens arrays, where each pair of facing lenses behaves like a tiny demagnifying telescope. We have tested the direct snapshot performance with laboratory-simulated multiple stars and observed the binary star Castor (α Gem). We measured a separation of 3.8 �� and a magnitude difference of 0.85 which is in agreement with current orbital data. This verified the theoretical expectations for hypertelescopes in terms of field of view and fluxes and qualified the new optical implementation for future arrays at the scale of meters and beyond.

Tests with a Carlina-type diluted telescope - Primary coherencing

Studies are under way to propose a new generation of post-VLTI interferometers. The Carlina concept studied at the Haute- Provence Observatory is one of the proposed solutions. It consists in an optical interferometer configured like a diluted version of the Arecibo radio telescope: above the diluted primary mirror made of fixed cospherical segments, a helium balloon (or cables suspended between two mountains), carries a gondola containing the focal optics. Since 2003, we have been building a technical demonstrator of this diluted telescope. First fringes were obtained in May 2004 with two closely-spaced primary segments and a CCD on the focal gondola. We have been testing the whole optical train with three primary mirrors. The main aim of this article is to describe the metrology that we have conceived, and tested under the helium balloon to align the primary mirrors separate by 5-10 m on the ground with an accuracy of a few microns. The servo loop stabilizes the mirror of metrology under the helium balloon with an accuracy better than 5 mm while it moves horizontally by 30 cm in open loop by 10-20 km/h of wind. We have obtained the white fringes of metrology; i.e., the three mirrors are aligned (cospherized) with an accuracy of {\approx} 1 micron. We show data proving the stability of fringes over 15 minutes, therefore providing evidence that the mechanical parts are stabilized within a few microns. This is an important step that demonstrates the feasibility of building a diluted telescope using cables strained between cliffs or under a balloon. Carlina, like the MMT or LBT, could be one of the first members of a new class of telescopes named diluted telescopes.

The Segmented Pupil Experiment for Exoplanet Detection: 2. design advances and progress overview

The SPEED project - the Segmented Pupil Experiment for Exoplanet Detection - in development at the Lagrange laboratory, aims at gearing up strategies and technologies for high-contrast instrumentation with segmented telescopes. This new instrumental platform offers an ideal environment in which to make progress in the domain of ELTs and/or space-based missions with complex apertures. It combines all the required recipes (phasing optics, wavefront control/shaping, and advanced coronagraphy) to get to very close angular separation imaging. In this paper, we report on the optical design and subsystems advances and we provide a progress overview.

Design, specification, and manufacturing of a PIAACMC for the SPEED testbed

The Phase-Induced Amplitude Apodization Complex Mask Coronagraph (PIAACMC) is a promising corona- graphic device for direct detection of exoplanets with complex segmented telescope apertures. This concept features the bright idea of generating a pupil apodization by reflection on two mirrors whose wavefront maps are specifically optimized, and a complex focal plane mask. In this paper, we report on the design, specifications, and manufacturing of such a coronagraph for the SPEED facility (Segmented Pupil Experiment for Exoplanet Detection) struggled for deep contrast at small angular separation with complex telescope aperture.

Advances in the development of a Mach-Zehnder interferometric Doppler imager for seismology of giant planets

The measurements of radial velocity fields on planets with a Doppler Spectro-Imager allow the study of atmospheric dynamics of giant planets and the detection of their acoustic oscillations. The frequencies of these oscillations lead to the determination of the internal structure by asteroseismology. A new imaging tachometer, based on a Mach-Zehnder interferometer, has been developed to monitor the Doppler shift of solar lines reflected at the surface of the planets. We present the principle of this instrument. A prototype was designed and built, following the specifications of a future space mission. The performance of the prototype, both at the laboratory and on the sky, is presented here.

SPICA, a new 6T visible beam combiner for CHARA: science, design and interfaces

We present the recent developments preparing the construction of a new visible 6T beam combiner for the CHARA Array, called SPICA. This instrument is designed to achieve a large survey of stellar parameters and to image surface of stars. We first detail the science justification and the general idea governing the establishment of the sample of stars and the main guidance for the optimization of the observations. After a description of the concept of the instrument, we focus our attention on the first important aspect: optimizing and stabilizing the injection of light into single mode fibers in the visible under partial adaptive optics correction. Then we present the main requirements and the preliminary design of a 6T-ABCD integrated optics phase sensor in the H-band to achieve long exposures and reach fainter magnitudes in the visible.

The segmented pupil experiment for exoplanet detection: Part 3. Advances and first light with segments cophasing

SPEED (Segmented Pupil Experiment for Exoplanet Detection) is an instrumental testbed designed to offer an ideal cocoon to provide relevant solutions in both cophasing and high-contrast imaging with segmented telescopes. The next generation of observatories will be made of a primary mirror with excessive complexity (mirror segmentation, central obscuration, and spider vanes) undoubtedly known to be unfavorable for the direct detection of exoplanets. Exoplanets detection around late-type stars (M-dwarfs) constitutes an outstanding reservoir of candidates, and SPEED integrates all the recipes to pave the road for this science case (cophasing sensors, multi-DM wavefront control and shaping architecture as well as advanced coronagraphy). In this paper, we provide a progress overview of the project and report on the first light with segments cophasing control and monitoring from a coronagraphic image.

SPEED design optimization via Fresnel propagation analysis

Future extremely large telescopes will open a niche for exoplanet direct imaging at the expense of using a primary segmented mirror which is known to hamper high-contrast imaging capabilities. The focal plane diffraction pattern is dominated by bright structures and the way to reduce them is not straightforward since one has to deal with strong amplitude discontinuities in this kind of unfriendly pupil (segment gaps and secondary support). The SPEED experiment developed at Lagrange laboratory is designed to address this specific topic along with high-contrast at very small separation. The baseline design of SPEED will combine a coronagraph and two deformable mirrors to create dark zones at the focal plane. A first step in this project was to identify under which circumstances the deep contrast at small separation is achievable. In particular, the DMs location is among the critical aspect to consider and is the topic covered by this paper.

An end-to-end Fresnel propagation model for SPEED: PIAACMC implementation and performance

Future extremely large telescopes, equipped with high-contrast instruments targeting very small Inner Working Angle, will provide the requisite resolution for detecting exoplanets in the habitable zone around M-stars. However, the ELT segmented pupil shape is unfavourable to high-contrast imaging. In this context, the SPEED project aims to develop and test solutions for high contrast with unfriendly apertures. SPEED will combine a PIAACMC coronagraph and two deformable mirrors for the wavefront shaping. In this paper, we describe an end-to-end model of SPEED, including the Fresnel wavefront propagation, the PIAACMC implementation and the dark hole algorithm, and present a statistical analysis of the predicted performance.