Report - School of Physics

per star in order to sample the (potential) planet orbit, of the order of 300 stars
could be searched for planets. In that way, a catalogue of targets would be made
available for further studies. While the detection limit would not be as deep as
that of the full, completed OWL, objects discovered with the early, hypertelescope
version would have characteristics (separation and intrinsic magnitude) that would
permit further physical studies with the completed telescope.
In conclusion, it is stressed that there remain technical uncertainties in inferring
that OWL can indeed observe Earth-type planets: the major assumptions which
must be clarified over the coming years are that adaptive optics can work as needed,
producing the required Strehl ratios, and providing adequate S/N ratios in both
continuum and spectral diagnostic lines. Tracking is not seen as a comparable issue:
if the telescope can track to sub-arcsec accuracy (it does according to analysis, at
some 0.3 arcsec rms with 10 m s −1 wind and taking into account friction in drives,
and without field stabilisation with M6), then the problem of pointing stability is a
control loop issue comparable to adaptive optics but with a much lower frequency.
Even though smaller ELTs, such as the 30-m TMT (Thirty-Metre Telescope), will
not reach Earth-like planets, there is still a scientific niche for them between the
objectives of VLT-Planet Finder and OWL: notably, detection and characterisation
of giant planets, of all ages, and close to their parents stars. Evidently, their technical
challenges will be easier to reach than those for a 100-m aperture.
The astrometric capabilities of OWL remain to be assessed, but narrow-angle astrometric
prospects should be very good, and provision of an astrometric facility in
the instrument plan may be appropriate.
3.1.2 Observations at an Antarctic Site
Excellent ground-based astronomical sites in terms of telescope sensitivity at infrared
and submillimeter wavelengths are located on the Antarctic Plateau, where
high atmospheric transparency and low sky emission result from the extremely cold
and dry air; these benefits are well characterised at the South Pole station. The
relative advantages offered by three potentially superior sites, Dome C, Dome F,
and Dome A, located higher on the Antarctic Plateau, are quantified by Lawrence
(2004). In the near- to mid-infrared, sensitivity gains relative to the South Pole of
up to a factor of 10 are predicted at Dome A, and a factor of 2 for Dome C. In the
mid- to far-infrared, sensitivity gains relative to the South Pole up to a factor of
100 are predicted for Dome A and 10 for Dome C. These values correspond to even
larger gains (up to 3 orders of magnitude) compared to the best mid-latitude sites,
such as Mauna Kea and the Chajnantor Plateau.
Thus these Antarctic sites appear to offer extremely good conditions for future
interferometric projects and for (robotic) photometric surveys, yielding one long
observing ‘night’ per year (completely dark in June–July, decreasing to 7 hr darkness
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