Genie

The GENIE project (Ground-based European Nulling Interferometer Experiment) is in line with preparations for the DARWIN mission (described later). GENIE, initially being considered for use on the VLT-I had several aims:

• to validate, by means of a 'life-size' project, the concept of dark-fringe interferometry;

• to train the European community in the techniques for nulling interferometry;

• to achieve part of the preparatory scientific work for DARWIN (in particular, describing the zodiacal environment of DARWIN's targets).

Preliminary studies carried out both by industrial consortia and by ESA have come to the following conclusions:

• the choice of spectral band L (3.8 |m): this is a compromise between band N (10 |m), where atmospheric turbulence has less effect than at shorter wavelengths, but where the thermal background predominates, and band K (2.2 |m), where the sky background is less significant, but turbulence is more critical.

• Inadequacies between the current structure of the VLT-I and dark-fringe interferometry. Before reaching the recombination laboratory, the interferometer beam has to undergo more than 10 reflections by mirrors, which limits the propagation efficiency, and increases the relative proportion of emission from the instrument.

As a result, the final results are mediocre, and the project of installing GENIE on the VLT-I has been abandoned. The concept might, however, be considered at a better site, in particular, in Antarctica.

ALADDIN and Antarctic Projects

The basic principles that have led to the ALADDIN concept are identical to those for GENIE. Scientifically, it aims at describing the zodiacal environment around DARWIN's targets, to a sensitivity about 20 times better than our Solar System's

Fig. 8.24 Artist's impression of the ALADDIN instrument. The two dark-fringe recombination telescopes are mobile along a rail, the orientation of which may also be changed (Alcatel Alenia Space)

integrated zodiacal emission. Because the site and infrastructure of the VLT-I would not enable us to achieve the desired results, it is a question of finding a site, and a design that are optimum for the purpose.

The Antarctic, and more particularly the location of Dome C, offers some major assets:

• The humidity if low, allowing excellent observations in the infrared.

• Because of the low temperature, the first turbulent layer appears to lie within 30 metres of the surface. Equipment installed above this height would therefore avoid the effect of this layer.

• The coherence time set by the turbulence is significantly longer than at Paranal, for example. This particular characteristic means that the passband of the adaptive systems may be reduced, gaining sensitivity or more accurate correction.

• There is an infrastructure at Dome C (the Franco-Italian Concordia base), which would allow new equipment to be located at this site.

The ALADDIN instrument (Antarctic L-band Astrophysics Discovery Demonstrator for Interferometric Nulling), makes use of the scientific specification for GENIE, without having to include the VLT-I's specifications and technical constraints. The resulting concept has been put forward by an industrial and scientific consortium. It involves two 1-m-class telescopes, mounted on a base that allows the spacing to be altered (from a few metres to 40 m), and operating as a Bracewell interferometer. Rotation of the base would enable the transmission map of the instrument to be altered (Fig. 8.24).

Again, spectral band L is a good compromise between the sky background and atmospheric turbulence.

It is estimated that ALADDIN could observe 30 per cent of DARWIN's potential targets. A direct-recombination mode also means that it is equally suited to the more classical type of observations in stellar physics (measurement of stellar diameters, study of environments close to stars, etc.).

The site is currently being tested to confirm the points mentioned above (Agabi et al., 2003). The nature of the environment (polar night, impossibility of resupply during the southern winter, logistical difficulties, etc.) means that any project at Dome C has additional difficulties which mean that it somewhat resembles spaceborne projects. The site is equally coveted for other projects, most of which are also of interest for the discovery and study of extrasolar planets: searching for planets by the transit method (A-STEP), asteroseismology projects (SIAMOIS), and extreme adaptive-optics projects.

In the more distant future, the possibility of installing an Extremely Large telescope or a kilometric interferometer array (the KEOPS project) has also been suggested.

The results from the site-testing campaigns currently under way should enable rational decisions about the use of the site to be taken.

8.2.2.2 Space Observatories

DARWIN and TPF-I

The DARWIN space mission (Fig. 8.25) and the interferometer version of its American counterpart TPF-I (Terrestrial Planet Finder) are based on the Bracewell interferometer principle (Fridlund et al., 2000, Beichman et al., 1999) described earlier in Chap. 2.

The object of this mission is to detect exoplanets directly (both giant and terrestrial-type) around nearby stars, with the aim of carrying out spectroscopic analysis of the composition of any likely atmospheres. The primary idea is to carry out a comparative study of different systems, and equally to try to detect signs of biological activity.

DARWIN is an interferometer, that is, the beams of light from several physically separated telescopes are recombined to give information with a spatial resolution that is equivalent to that given by a monolithic telescope of comparable size to the distance between the telescopes in the array.

Several designs have been advanced or are in the process of being evaluated in minimal versions with just three telescopes, or in more complex versions with five or six telescopes. The number of telescopes is a compromise between:

• the complexity of recombination;

• the performance of the central extinction: DARWIN's special feature is that it works in a high-angular-resolution, conjugate-recombination mode with a great dynamic range, known as 'dark-fringe interferometry', the aim of which is to extinguish the central source of light to examine a nearby faint object;

• the capacity to extract a planetary signal from its surroundings (the zodiacal light in the exosystem, for example). We may recall that in the Solar System, Earth's

Fig. 8.25 Artist's impression of the DARWIN mission, where six telescopes are arranged in a circle. The central satellite is the recombination laboratory. An eighth satellite, not shown here, handles transmission of information to Earth and monitors the spacing of the overall array (image credit: courtesy ESA)

own emission is one-300th part of the integrated emission from the zodiacal light over the whole celestial sphere.

In any scheme, each telescope, as well as the recombination laboratory and the communications equipment, is carried by an independent, free-flying satellite, automatically manoeuvred by small thrusters, and whose position is controlled by a measurement system capable of ensuring a constant (but alterable) distance between the telescopes, to an accuracy of a few millimetres. The array's phase is controlled by delay lines, with a path length of a few centimetres. Configurations with more than two telescopes allow sub-interferometers to be isolated within the array, and these may be recombined by applying a rapid phase modulation between each sub-interferometer. This technique of internal modulation enables the planetary signal to be modulated relative to that from the star, to achieve what is the equivalent of synchronous detection, without turning the whole array, as was initially proposed by Bracewell (Mennesson et al., 2005).

DARWIN/TPF has been proposed to ESA's new Cosmic Vision programme for the period 2015-2025. It would possibly be launched in 2025 and placed in orbit around the L2 Lagrangian point. This point is particularly suitable for orbiting several satellites in formation, because it has weak gravity gradients. It is equally suitable in terms of the thermal environment. The principal instrument on DAR-WIN/TPF will be a dark-fringe interferometer recombiner, the scientific results of which will be maps of the planetary systems, derived from analysis of the chromatic, modulated signals from sub-interferometers, as well as spectra of the different components of the planetary systems (Fig. 8.26).

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Fig. 8.26 Numerical simulation of the scientific results from DARWIN's dark-fringe interferometer. Left, a map of the Solar System such as might have been observed by DARWIN on 1 January 2001, 10 pc above the north pole of the Sun (centre in the image). Three objects are visible and these are Venus, Earth, and Mars. Right, the spectrum of the Earth as it would be observed by DARWIN. The histogram represents the spectrum, reconstructed after simulation of the observation. The continuous line shows the Earth's actual spectrum, sampled at the instrument's resolution, and a dashed line shows the spectrum of a blackbody at 300 K. This diagram clearly shows that DARWIN/TPF will be able to detect the presence of certain components of biological interest in the atmospheres of terrestrial-type planets, even at a low spectral resolution (20 in this image). H2O, CO2, and O3, in particular, are clearly visible relative to the blackbody at 300 K (After Mennesson and Mariotti, 1997)

Wavelength (micrometres)

Fig. 8.26 Numerical simulation of the scientific results from DARWIN's dark-fringe interferometer. Left, a map of the Solar System such as might have been observed by DARWIN on 1 January 2001, 10 pc above the north pole of the Sun (centre in the image). Three objects are visible and these are Venus, Earth, and Mars. Right, the spectrum of the Earth as it would be observed by DARWIN. The histogram represents the spectrum, reconstructed after simulation of the observation. The continuous line shows the Earth's actual spectrum, sampled at the instrument's resolution, and a dashed line shows the spectrum of a blackbody at 300 K. This diagram clearly shows that DARWIN/TPF will be able to detect the presence of certain components of biological interest in the atmospheres of terrestrial-type planets, even at a low spectral resolution (20 in this image). H2O, CO2, and O3, in particular, are clearly visible relative to the blackbody at 300 K (After Mennesson and Mariotti, 1997)

The results expected from DARWIN/TPF-I utterly depend on the arrangement adopted for recombination, and in particular the number and size of the telescopes. Figure 8.27 shows the performance in terms of the signal-to-noise ratio for a linear arrangement of four telescopes, 3 m in diameter. (A study made for TPF-I.)

On the technical side, dark-fringe interferometry proves to be difficult because it requires extreme accuracy in terms of the coherence of the light-paths. In particular, to eliminate the extreme contrast between the star and the planet (typically 106 in the thermal infrared region), it is necessary for the interferometric extinction of the star be at least 105, with a relative stability of 1 in 10 000. Table 8.7 summarizes the principal characteristics of the DARWIN recombiner. By way of comparison, the characteristics of PEGASE, a possible predecessor to DARWIN (see the next section) are also given in this table.

Given the complexity of designing and constructing a dark-fringe interferometer recombiner, numerous studies are being undertaken all over the world to construct laboratory demonstration models (Ollivier et al., 2001; Brachet et al., 2003; Serabyn, 2003a; Flatscher et al., 2003; Serabyn, 2003b; Gondoin et al., 2003). The demonstrator of the Institut d'Astrophysique Spatiale at Orsay is shown in Fig. 8.28. We may finally mention that industrial research is being carried out to define the parameters and performance of the mission.

Precursors to DARWIN and TPF-I

Two missions have been proposed as precursors and are in the study phase, evaluating the instrumental concepts that will lead to DARWIN/TPF-I:

• NASA's FKSI mission (Fig. 8.29). This is a Bracewell interferometer with two telescopes mounted on a rigid base, with a fixed baseline.

• The PEGASE mission (Fig. 8.30), proposed to CNES in response to a call for proposals for a flight in formation, PEGASE is a Bracewell interferometer consisting of three satellites, two carrying siderostats, 30-40 cm in diameter, and one carrying the recombination laboratory and the detection chain.

TPF Signals in 105 sec

TPF Signals in 105 sec

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