Report - School of Physics

3.2.2 The Darwin Ground-Based Precursor: GENIE
GENIE is a nulling interferometer under development by ESA and ESO for the
VLTI. GENIE will allow the development and demonstration of the technology required
for nulling interferometry, allowing testing of the Darwin technology in an
integrated and operational system (amplitude control loops, high-accuracy optical
path difference control loop, dispersion control, polarisation compensation, background
subtraction, and internal modulation). GENIE is considered by the Darwin
project as necessary for demonstrating these technical concepts in advance of launch.
Two competitive instrument definition studies were due for completion by the end
of 2004, with the scientific case being prepared in collaboration between ESO and
ESA. GENIE will be considered by the ESO Council in April 2005, and could be
operational by mid-2008. If not accepted by ESO, a separate laboratory technology
demonstrator would be needed to validate the Darwin concepts.
GENIE is also considered mandatory as a specific instrumental configuration to
survey southern-hemisphere target candidates, in order to decide which targets are
most suitable for observation by Darwin, and specifically to characterise the level of
exo-zodiacal light, which must be below certain limits necessary for exo-planetary
detections. A corresponding programme is planned to be undertaken at the Keck
telescope in the northern hemisphere for preparations for TPF. Studies by den Hartog
et al. (2004) show that the capabilities of GENIE are sufficient to detect the
planet around τ Boo within 1 hour, and by inference some 5 other candidate ‘hot
Jupiter’ planets. GENIE could use either the VLT UTs or ATs, at a wavelength of
3.6 µm, and will require a significant number of observing nights (of order 50).
The main limiting factors for GENIE are the phase and thermal background stability
on Paranal, as well as system complexity issues involved with its integration
into the existing VLTI environment. If GENIE were located on the high Antarctic
plateau (Dome C) where the thermal background is lower, and the seeing both better
and slower (see Section 3.1.2), the performance required to perform its science
programme could perhaps be achieved using smaller (of order 1 m) telescopes and a
simpler, dedicated system architecture. This has to be balanced against the reduced
sky coverage at −75 ◦ latitude, and probably more complex logistics.
Table 9: Darwin: integration times for detection of Earth-like planets at S/N = 5, and spectroscopy
at S/N = 7 in the faintest part of the spectrum (in hours). See text for details.
Stellar type 10 pc 20 pc 30 pc
G2V 18–33 28–54 109–173
G5V 12–22 27–46 105–166
K2V 4–9 26–37 104–157
K5V 4–6 26–35 249–155
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