Report - School of Physics

during March and October, although with an extended ‘twilight’ period due to the
fact that the Sun does not set very far below the horizon). Table 8 shows that
an Antarctic OWL might achieve comparable performances to Darwin/TPF in the
near-infrared; currently this is viewed as somewhat hypothetical, given the complex
logistics and formidable meteorological conditions for construction and operation.
Dome C (elevation 3250 m, latitude −75 ◦ ) is the site of the French-Italian Concordia
station, whose main characteristics have been studied over the last few years. It provides:
(i) an ambient temperature ranging from 195 K (winter) to 235 K (summer),
resulting in low, stable thermal emission; (ii) extremely dry air (250 µm precipitable
water vapour typical), resulting in enlarged and improved infrared transmission
windows; (iii) very low surface winds (median wind speed of 2.7 m s −1 , and below
5 m s −1 for more than 90% of the time), resulting in very low free-air turbulence,
with a quasi-absence of jet streams since Dome C is located inside the polar vortex;
(iv) some 80% of clear skies. The combination of coldness and dryness for the atmosphere
results in infrared photometric gains that peak at about a factor of 25 in
the K and L bands, i.e. an Antarctic 1.8-m telescope is more efficient than an 8-m
telescope at a temperate site. In the H and N bands the gain (of order 3) is also
notable although less dramatic.
Exceptional winter conditions were confirmed by teams from the University of Nice
(Aristidi et al., 2003) and Australia UNSW (Lawrence et al., 2004): over a 3-month
period, the median seeing was 0.27 arcsec, with 0.15 arcsec achieved for more than
25% of the time. As most of the turbulence is located near the ground, the isoplanatic
angle is enlarged (5.9 arcsec under normalised conditions, compared with
2.9 arcsec for Paranal) which improves the field of view (and the likelihood of finding
a reference star) for adaptive optics. This has consequences in the feasibility of
adaptive optics for ELTs, where multiconjugate systems and laser guide stars may
no longer be needed. Because the turbulence is generated by low-velocity winds, it is
slow, meaning better sensitivity and improved phase correction for adaptive optics
systems. There is still some debate about the normalised coherence time (whose
median value is 2.6 ms on Paranal): indirect measurements with the MASS scintillometer
indicate a median value of 5.9 ms, while direct DIMM measurements show
a correlation of the image motions beyond 250 ms (as expected from r 0 = 50 cm
and a typical 2.5 m s −1 wind speed).
Even taking the most pessimistic value (5.9 ms), the coherence volume that drives
the sensitivity of adaptive optics and interferometric systems is improved by a typical
factor of 20, if the AO wavefront sensor operates in the K-band. When combined
with the factor of 25 photometric gain, this results in an expected sensitivity for interferometers
and AO systems 500 times better than a similarly equipped temperate
site. For bright sources, higher dynamic range observations (either by coronography
or nulling) can be achieved due to the reduced phase and background noise.
Current adaptive optics mainly concerns the correction of the phase of the wavefront
to achieve diffraction-limited imaging. Even after a perfectly plane wavefront has
45