In the frame of ESA's DARWIN mission technological studies, Alcatel Space, supported by ESA, is designing and manufacturing a nulling interferometer whose aim is to demonstrate in laboratory the extinction of a point source with a stable nulling ratio of 10-6. The results of the first integration phase of this breadboard is presented in this paper.

Nulling cancels out the light of a source by destructive interference between several beams, while coronagraphy is a much larger family of high dynamical range imaging techniques designed for single pupil telescopes. In this paper, nulling techniques and three different classes of coronagraphs are compared using criteria relevant to the detection of Earth-sized planets around nearby stars.

We present a beam combining technique capable of directinstantaneous imaging. The particularity is that the image (or PSID, point source intensity distribution) is formed in apupil plane. We present the concept on a theoretical basis,comment on the PSID for on- and off-axis sources and finallycompare it to Fizeau imaging. Future experiments and reflectionsabout this concept are finally considered.

We describe in this short note the interest of a multiple-stageprolate apodized Lyot coronagraph (PALC). The principle wasproposed by Aime, Soummer and Ferrari (A&A 2002, 389,334-344).Several PALCs are used in a cascading mode. The first stage is aPALC with all its elements. The other stages use only Lyot masksand Lyot stops, without any further aperture apodization. If theextinction factor of the first PALC is Q, then the resultingextinction factor of N such cascading coronagraphs is QN.The total throughput remains that of the first PALC, for perfectoptics.

We describe a prototype instrument built to validate the principleof the dispersed speckle method (Borkowski et al. 2002;Borkowski et al. 2004). This prototype, still under test, is aspectrograph where the multiple pupil, highly reduced in size,replaces the slit. An array of optical wedges is added near thegrating to separate the spectra from each speckle. Thefabrication and properties will be explained.

We describe here the principle of a new type of coronagraph, based on the incident flux division with pupil reversal and phase shift on one beam, then recombination with destructive interferences at the center orf the field. This concept of nulling has already been used in the Interferometrical Achromatic Interfero-Coronagraph (AIC, Gay & Rabbia 1996), which lies on a Michelson interferometer interferometry which does not allow an easy insertion in the focal facility of a telescope. The variant under consideration has a completely coaxial design with an original and very compact optical combination. It is based upon two coaxial thick lenses in the same medium, stuck one to each other with a very narrow gap in between and a proper coating of the interfaces. The very geometry of the device ensures moreover the permanent and rigorous cophasing of the interferometer. The optical combination which fulfills this problem is unique and presents a range of properties which ease its insertion in the focal instrumentation of existing telescopes or next generation ones.

In a few years, the VLTI will be able to combine light from 4 to 8telescopes equipped with adaptive optics. In order to exploit thefull VLTI infrastructure, a second generation instrument, based onthe densified pupil concept (VIDA), has been proposed for veryhigh resolution direct imaging and coronagraphy. This paperpresents some possible coronagraphic devices providing a totalstar extinction on the VLTI when there are no phase errors.Lastly, the expected performances considering cophasing andadaptive optics residual errors are also presented.

A possible method of pupil apodization using Zernike's phase contrastis presented. I recall the phase contrast principle and show through numericalsimulations that it may be used to generate an apodization using very simpleoptics in the monochromatic case. I also demonstrate that the generalformalism for Lyot and Roddier & Roddier coronagraphy can be extended to theZernike case. This may improve the quality of the simulations undertakento demonstrate the technique's performances.

We present results obtained with different adaptive optics systems(ADONIS, PUEO, NAOS), in either imaging or coronagraphic modes, in theframe of several surveys aimed at searching for very low mass starsand substellar objects around nearby M dwarfs as well as in youngcloseby associations. For the M dwarfs of the solar neighborhood, wehave obtained a very accurate mass-luminosity relation that we cancompare to the prediction of recent stellar structure models. We alsopresent multiplicity statistics of M dwarfs (binary fraction, periodand mass ratio distribution), which are crucial constraints forstellar formation scenarii. We also discuss results obtained on twoyoung closeby associations, MBM12 and Tucana-Horologium.

This paper presents the current situation of the direct search for planets andfuture prospects in this field. The performance of the next generation ofadaptive optics instruments, aiming at high angular resolution and highdynamics imaging, is briefly shown and discussed in terms of detectable planetaryparameters. In summary, young jovian planets and evolved massive bodies at the borderbetween giant planets and brown dwarfs will be directly detected within 10years. These studies nicely complement the radial velocity and transit surveysin the search for planetary systems, whereas additionnal potentialscientific cases for this class of instruments concern debris disks, star formation,massive stars ejections...