[show abstract][hide abstract] ABSTRACT: The achromatic phase shifter (APS) is a component of the Bracewell nulling interferometer studied in preparation for future space missions (viz. Darwin/TPF-I) focusing on spectroscopic study of Earth-like exo-planets. Several possible designs of such an optical subsystem exist. Four approaches were selected for further study. Thales Alenia Space developed a dielectric prism APS. A focus crossing APS prototype was developed by the OCA, Nice, France. A field reversal APS prototype was prepared by the MPIA in Heidelberg, Germany. Centre Spatial de Liege develops a concept based on Fresnel's rhombs. This paper presents a progress report on the current work aiming at evaluating these prototypes on the Synapse test bench at the Institut d'Astrophysique Spatiale in Orsay, France.
[show abstract][hide abstract] ABSTRACT: Context. Nulling interferometry has been suggested as the underlying principle for the Darwin and TPF-I exoplanet research missions. Aims. There are constraints both on the mean value of the nulling ratio, and on its stability. Instrument instability noise is most detrimental to the stability of the nulling performance. Methods. We applied a modified version of the classical dithering technique to the optical path difference in the scientific beam. Results. Using only this method, we repeatedly stabilised the dark fringe for several hours. This method alone sufficed to remove the 1/ f component of the noise in our setup for periods of 10 minutes, typically. These results indicate that performance stability may be maintained throughout the long-duration data acquisitions typical of exoplanet spectroscopy. Conclusions. We suggest that further study of possible stabilisation strategies should be an integral part of Darwin/TPF-I research and development
Astronomy and Astrophysics 05/2008; · 5.08 Impact Factor
[show abstract][hide abstract] ABSTRACT: Planets less massive than about 10 MEarth are expected to have no massive H–He atmosphere and a cometary composition (∼50% rocks, 50% water, by mass) provided they formed beyond the snowline of protoplanetary disks. Due to inward migration, such planets could be found at any distance between their formation site and the star. If migration stops within the habitable zone, this may produce a new kind of planets, called ocean-planets. Ocean-planets typically consist in a silicate core, surrounded by a thick ice mantle, itself covered by a 100 km-deep ocean. The possible existence of ocean-planets raises important astrobiological questions: Can life originate on such body, in the absence of continent and ocean–silicate interfaces? What would be the nature of the atmosphere and the geochemical cycles? In this work, we address the fate of hot ocean-planets produced when migration ends at a closer distance. In this case the liquid/gas interface can disappear, and the hot H2O envelope is made of a supercritical fluid. Although we do not expect these bodies to harbor life, their detection and identification as water-rich planets would give us insight as to the abundance of hot and, by extrapolation, cool ocean-planets. The water reservoir of these planets seems to be weakly affected by gravitational escape, provided that they are located beyond some minimum distance, e.g. 0.04 AU for a 5-Earth-mass planet around a Sun-like star. The swelling of their water atmospheres by the high stellar flux is expected not to significantly increase the planets' radii. We have studied the possibility of detecting and characterizing these hot ocean-planets by measuring their mean densities using transit missions in space—CoRoT (CNES) and Kepler (NASA)—in combination with Doppler velocimetry from the ground—HARPS (ESO) and possible future instruments. We have determined the domain in the [stellar magnitude, orbital distance] plane where discrimination between ocean-planets and rocky planets is possible with these instruments. The brightest stars of the mission target lists and the planets closest to their stars are the most favorable cases. Full advantage of high precision photometry by CoRoT, and particularly Kepler, can be obtained only if a new generation of Doppler instruments is built.
[show abstract][hide abstract] ABSTRACT: We revisit the nulling interferometer performances that are needed for direct detection and the spectroscopic analysis of exoplanets, e.g., with the DARWIN [European Space Agency-SCI 12 (2000)] or TPF-I [JPL Publ. 05-5, (2005)] missions. Two types of requirement are found, one concerning the mean value of the instrumental nulling function (nl(lambda)) and another regarding its stability. The stress is usually put on the former. It is stringent at short wavelengths but somewhat relaxed at longer wavelengths. The latter, which we call the variability noise condition, does not usually receive enough attention. It is required regardless of telescope size and stellar distance. The results from three nulling experiments performed in laboratories around the world are reported and compared with the requirements. All three exhibit 1/f noise that is incompatible with the performances required by the mission. As pointed out by Lay [Appl. Opt. 43, 6100-6123 (2004)], this stability problem is not fully solved by modulation techniques. Adequate solutions must be found that are likely to include servo systems using the stellar signal itself as a reference and internal metrology with high stability.
[show abstract][hide abstract] ABSTRACT: Nulling interferometry is one of the promising techniques for the study of extra terrestrial planets. This technique will be applied in the future space missions Darwin and TPF-I, and from the ground with GENIE. The nulling interferometry techniques require high symmetry of the interfering beams, to obtain the required contrast (typically 10^6 to detect terrestrial exo-planets in the thermal infrared). In this paper we consider the polarization symmetry issue, such as polarization rotation and polarization phase shifts occurring on slightly misaligned optics. We study the consequences of these symmetry requirements on a nulling interferometer design. We find the relation between the misalignment tolerances and the achievable nulling, and we show that this tolerance is highly dependent on the interferometer configuration (the way beams turn right, left, up or down in the interferometer arms). It is typically of the order of the arcminute (not the arcsecond) for a 10^6 contrast. We present a analytical and numerical analyses.
Proceedings of the International Astronomical Union 01/2006; 1.
[show abstract][hide abstract] ABSTRACT: The Darwin/TPF mission aims at detecting directly extra solar planets.
It is based on the nulling interferometry, concept proposed by Bracewell
in 1978, and developed since 1995 in several European and American
laboratories. One of the key optical devices for this technique is the
achromatic phase shifter (APS). This optical component is designed to
produce a π phase shift over the whole Darwin spectral range (i.e.
6-18 μm), and will be experimentally tested on the NULLTIMATE
consortium nulling test bench (Labèque et al). Three different
concepts of APS are being simulated: dispersive plates focus crossing
and field reversal. In this paper, we show how thermal, mechanical and
optical models are merged into a single robust model, allowing a global
numerical simulation of the optical component performances. We show how
these simulations help us to optimizing the design and present results
of the numerical model.
Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series; 10/2004
[show abstract][hide abstract] ABSTRACT: In the context of the Darwin mission, aiming to detect terrestrial extrasolar planets, European Space Administration (ESA) has an R&D program trying to solve the crucial problems, like flotilla spacecraft control, optical spatial filtering, etc... One of the key optical devices of this mission will be Achromatic Phase Shifter (APS) able to accurately provide a 180° phase shift in the IR 6 -18 microns range. The Institut d'Astrophysique Spatiale (IAS) is leading, in the frame of an ESA granted contract, an European consortium of 9 universities and companies, named Nulltimate, aiming to develop and test three different APS. IAS itself is in charge of the cryogenic test bench facility which is presented here.
[show abstract][hide abstract] ABSTRACT: A new family of planets is considered which is in between the rocky terrestrial planets and the gaseous giants, “Ocean-Planets.” We present the possible formation, composition and internal structure of these putative planets. We consider their oceans, as well as their possible Exobiology interest. These exoplanets should be detectable by Space missions such as Eddington, Kepler, and possibly COROT (launch scheduled in 2006). They have a density lower than that of rocky planets. Their rather large radius would make them attractive targets for exoplanet spectroscopic missions such as Darwin/TPF, all the more because a robust biosignature appears to exist.
[show abstract][hide abstract] ABSTRACT: The Darwin mission is a project of the European Space Agency that should
allow around 2015 the search for extrasolar planets and a spectral
analysis of their potential atmospheres in order to detect gases and
particularly tracers of life. The basic concept of the instrument is a
Bracewell nulling interferometer. It allows high angular resolution and
high dynamic range. However, this concept, proposed 25 years ago, is
very difficult to implement with high rejection factor and has to be
demonstrated in laboratory before being applied in space. Theoretical
and numerical approaches of the question highlight strong difficulties:
- The need for very clean and homogeneous wavefronts, in terms of
intensity, phase and polarisation distribution; - The need for
achromatic optical devices working in a wide spectral range (typically 6
to 18 microns for the space mission). A solution to the first point is
the optical filtering which has been successfully experimentally
demonstrated at 10 microns using a single mode laser. We focus now on
the second point and operate a test bench working in the near infrared,
where the background constraints are reduced. We present this test bench
and the first encouraging results in the 2-4 microns spectral range. We
particularly focus on recent optical developments concerning achromatic
component, and particularly the beam combiners and the phase shifter,
which are key-points of the nulling interferometry principle. Finally,
we present the future of this experimental demonstration, in the thermal
infrared, covering the real and whole spectral range of Darwin.
[show abstract][hide abstract] ABSTRACT: A new family of planets is considered which is in between the rocky terrestrial planets and the gaseous giants, "Ocean-Planets". We present the possible formation, composition and internal structure of these putative planets. We consider their oceans, as well as their possible exobiology interest. These exoplanets should be detectable by Space missions such as Eddington, Kepler, and possibly COROT (launch in 2006). They have a density lower than that of rocky planets. Their rather large radius would make them attractive targets for exoplanet spectroscopic missions such as Darwin / TPF, all the more because a robust biosignature appears to exist.
[show abstract][hide abstract] ABSTRACT: The validation of nulling interferometry principle is a crucial step on
the way to space missions DARWIN/TPF concepts that aim at detecting and
analysing extrasolar planets. In this context, different laboratory
techniques of recombination are currently under development. We present
in this poster the new polychromatic test bench at IAS and its very brst
results in the 2 to 2.5 μm band.
[show abstract][hide abstract] ABSTRACT: Several concept of space missions dedicated to the direct detection and analysis of extrasolar planets are based on nulling interferometry principle. This principle, which is theoretically very promising requires the capability of propagating and combining beams with very high accuracy in term of amplitude phase and polarization. In order to validate the principle of nulling interferometry, it is necessary to develop laboratory techniques of recombination. In this paper, we present a new test bench that should allow measuring rejection rate up to 105 in a large spectral band between 2 and 4 microns.
[show abstract][hide abstract] ABSTRACT: A new family of planets is considered which is in between rocky terrestrial planets and gaseous giant ones, Ocean-Planets. We envision that these planets would have formed in an ice-rich environment and migrated closer to their star, in the Habitable Zone. We present hereafter preliminary internal models of these putative planets. We assume that their mass is between 1 and 8 times that of the Earth, and that they are made of equal amounts of metals + rocks and water ice. We show that in the likely case where other light gases (N2/NH3, CO/CH4, H2, He, Ne) are not present in major amount, either due to formation processes and/or to subsequent escape, the planets can possess a surface water ocean, whose depth should not exceed ~100 km. These planets should be detectable by COROT, and would be ideal targets for future missions such as DARWIN/TPF. Finally, the simultaneous presence of O3, H2O and CO2 is pointed out as a possible biosignature, which is not the case of O2 , H2O and CO2 . If they are rather abundant and as resistant to evaporation and atmospheric escape as models predict, the COROT mission (launch scheduled in 2006) should detect several of them.
[show abstract][hide abstract] ABSTRACT: Nulling interferometry has been suggested as the underlying principle for an instrument which could provide direct detection and spectroscopy of Earth-like exo-planets, including searches for potential bio-signatures. This paper documents the potential of optical path difference (OPD) stabilisation with dithering methods for improving the mean nulling ratio and its stability. The basic dithering algorithm, its refined versions and parameter tuning, are reviewed. This paper takes up the recently presented results1 and provides an update on OPD-stabilisation at significantly higher levels of nulling performance.